Stem- and Leaf- Feeding Insects

Cutworms

Cutworms are the larval stages of moths that are active at night. Several species of cutworms (Agrotis and Euxoa spp.) can damage sorghum. The moths prefer to lay eggs in grassy and weedy fields. They lay the eggs in the soil or on stems or leaves of sorghum or grassy weeds. They hatch in 2 to 14 days.

The typical cutworm larva attacking sorghum is plump and curls into a C shape when disturbed (Fig. 7). The larvae vary from grayish white to grayish black or brown, depending on the species. Fully grown larvae are 1 to 2 inches long. Some species overwinter in the soil as pupae, others as adults, but most overwinter as small larvae in cells in clumps of grass or in the soil under trash. They begin feeding in the spring and grow until early summer, when they pupate in the soil. The larvae of most species stay underground during the day and feed at night.

The most common cutworms in sorghum feed on the soil surface. The feeding cuts plants off at, slightly below, or above the soil surface. Some cutworms (climbing or army cutworms) feed on the aboveground plant parts; others are subterranean and feed on underground plant parts, including seedling roots.

Cultural controls for cutworms include destroying weeds, preparing the seedbed thoroughly at least 4 to 6 weeks before planting, and plowing under or using herbicides to reduce weeds in late summer or early fall. Cutworms are more severe in weedy fields.

When scouting for cutworms in sorghum, look for severed, dead, and dying plants. For surface-feeding and subterranean cutworms, calculate the number of damaged plants per foot of row. Base your decision to apply insecticide on the degree to which an adequate stand is threatened. For cutworms that feed on aboveground plant parts, significant losses occur when the grubs have eaten more than 30 percent of the leaf tissue.

Insecticide sprayed as a broadcast treatment on the ground and plants usually protects against cutworms (Table 5). Because cutworms spend the day hidden in the soil, insecticides are sometimes more effective if they are applied in late afternoon.

Although insecticidal baits are effective against some cutworms, they are expensive. Insecticide applied at planting can control subterranean cutworms. Apply it in a 6- to 7-inch band and incorporate it into the top 1 to 2 inches of soil. See the product label for specific directions. Aerial or ground applications of labeled insecticide can control cutworms in an established sorghum stand. However, foliar insecticides are more effective on climbing than on subterranean cutworms.

Table 5. Insecticides labeled for cutworms in grain sorghum. Follow label directions.

Active ingredient	Insecticide	Mode of action	Rate	Remarks	REI1	PHI2
At-planting treatment
chlorpyrifos	Lorsban 15G	1B	8 oz/1000 row ft	Apply in T-band and cover lightly with soil . See label . Restricted use	24H	Not listed
esfenvalerate	Asana XL, generics	3A	5 .8–9 .6 fl oz	May be applied broadcast at or immediately before planting . Restricted use	12H	21 days
Post-emergence treatment
alpha– cypermethrin	Fastac	3A	1 .3–3 .8 fl oz/A	Restricted use	—	14 days
beta-cyfluthrin	Baythroid XL	3A	1 .0–1 .3 fl oz/A	Application must be made before cutworms bore into plant . Restricted use	12H	14 days
chlorpyrifos	Lorsban 4E, Lorsban Advanced, Lorsban 75WG, Generics	1B	1–2 pt/A 1–2 pt/A 0 .67–1 .33 lb/A	Apply as a postemergence spray . To ensure complete coverage of plants, apply in at least 15 GPA for ground application and 2–5 GPA by aircraft . Restricted use	24H	30 days for 1 pint/A rate, 60 days for any rate above 1 pint/A
chlorpyrifos + gamma– cyhalothrin	Cobalt	1B, 3A	13–38 fl oz	Restricted use	24H	30 days for up to 26 oz/A and 60 days for more than 26 oz
chlorpyrifos + lambda– cyhalothrin	Cobalt Advanced	1B, 3A	13–38 fl oz	Restricted use	24H	30 days for up to 26 oz/A and 60 days for more than 26 oz
chlorpyrifos + zeta-cypermethrin	Stallion	1B, 3A	3 .75 –11 .75 oz/A	Direct spray to the base of plants with sufficient spray volume to penetrate the soil/ stem interface, leaf collars, and sheaths . Restricted use	24H	30 days
cyfluthrin	Tombstone	3A	1–1 .3 fl oz/A	Restricted use	12H	14 days
deltamethrin	Delta Gold 1 .5 EC	3A	1 .0–1 .5 fl oz/A	To ensure good spray coverage, apply in a minimum of 5 GPA by ground and 2 GPA by aircraft . Restricted use . Danger–Poison	12H	14 days
esfenvalerate	Asana XL	3A	5 .8–9 .6 fl oz	Restricted use	12H	21 days
gamma- cyhalothrin	Declare 1 .25, Proaxis 0 .5	3A	0 .77–1 .02 fl oz/A 1 .92–2 .56 fl oz/A	To ensure good spray coverage, apply at least 2 GPA by aircraft . Restricted use	24H	30 days
lambda– cyhalothrin	Warrior II with Zeon, Karate with Zeon, generics	3A	0 .96–1 .28 fl oz/A	To ensure good spray coverage, apply in a minimum of 2 GPA by aircraft . Restricted use	24H	30 days
lambda– cyhalotrhin + chlorantraniliprole	Besiege 1 .25 SC	3A, 28	5–6 oz/A	Do not exceed total of 18 fl oz/A/yr . Restricted use	24H	30 days
zeta-cypermethrin	Mustang Max	3A	1 .28–4 .0 fl oz/A	Restricted use	12H	14 days
1: REI = Restricted entry interval 2: PHI = Preharvest interval

Yellow Sugarcane Aphid

Yellow sugarcane aphids (Sipha flava) are usually lemon yellow (Fig. 8) but sometimes pale green. They are covered with small spines and have two double rows of dark spots on the back. There are winged and wingless forms.
This aphid feeds on many grasses, including johnsongrass and dallisgrass. The females give birth to living young for 28 days, averaging two nymphs a day for each female. The nymphs mature in 13 to 19 days; the adults live 25 to 30 days.

 

Description	% Loss /plant
No discoloration	0
Localized discoloration	8
< 1 entire leaf discolored	11
1 entire leaf discolored	31
> 1 leaf discolored	54
> 2 leaves discolored	77
Dying/dead plant	100

Yellow sugarcane aphids feed on sorghum and inject toxin into leaves of seedlings and older plants. Aphid feeding on seedlings turns the leaves purple (Fig. 8) and stunts their growth. By the time that the discoloration is visible, the aphids have injured the plants significantly (Tables 6 through 9). Damage often leads to delayed maturity and plant lodging, which may be increased by associated stalk rots. Many predators feed on yellow sugarcane aphids, but they are rarely parasitized.

Table 7. Economic injury levels for yellow sugarcane aphid based on percentage of seedling plants infested at the 1-true-leaf stage
Control cost ($) per acre	Crop market value ($) per acre
	100	150	200	250	300	400	500	600
	Percent infested plants
6	15	10	8	6	5	4	3	3
8	20	13	10	8	7	5	4	4
10	25	17	12	10	9	6	5	5
12	30	21	14	12	10	7	6	5

Table 8. Economic injury levels for yellow sugarcane aphid based on percentage of seedling plants infested at the 2-true-leaf stage
Control cost ($) per acre	Crop market value ($) per acre
	100	150	200	250	300	400	500	600
	Percent infested plants
6	26	18	13	11	10	7	6	5
8	35	24	17	14	13	9	7	7
10	43	29	22	17	16	11	9	8
12	51	35	26	20	18	13	10	9

Scout sorghum by inspecting plants beginning the first week of emergence and continuing twice weekly until they have at least five true leaves. Infestations of a week or less can significantly damage very small seedling sorghum plants (one to three true leaves). As the plants grow, they become more tolerant of aphid feeding.

The presence of purple seedlings can indicate an infestation of yellow sugarcane aphids. However, purple leaves can also develop in response to root injury, phosphorous deficiency, or cold, wet, or compacted soil. If aphids caused the discoloration, yield will decrease as damage increases (Table 6). Yellow sugarcane aphids can infest sorghum during later growth stages. The action thresholds for greenbug (Table 13) can serve as a guide for determining when to control yellow sugarcane on later growth stages of sorghum.

Insecticides can be applied as seed treatments or postemergence to control this aphid (Table 10). On plants at the 1, 2, or 3 true-leaf stage, apply a foliar insecticide when the percentage of plants infested with sugarcane aphids reaches or exceeds the economic injury levels in Tables 7 to 9. Do not count the two seed leaves that appear first.

Table 9. Economic injury levels for yellow sugarcane aphid based on percentage of seedling plants infested at the 3-true-leaf stage.
Control cost ($) per acre	Crop market value ($) per acre
	100	150	200	250	300	400	500	600
	Percent infested plants
6	67	44	33	27	24	17	14	12
8	89	60	44	36	32	22	18	16
10	*	76	55	44	39	28	22	20
12	*	92	66	53	44	33	27	22
* Do not treat .

Table 10. Insecticides labeled for control of yellow sugarcane aphid. Follow label directions.
Active ingredient	Insecticide	Mode of action	Rate	Remarks	REI1	PHI2
Seed treatment
clothianidin	Poncho 600, NipsIt Inside	4A	5 .1–6 .4 fl oz/100 lb seed		12H	—
imidacloprid	Gaucho 600, generics	4A	6 .4 fl oz/100 lb seed	Do not graze or harvest for forage within 45 days, and fodder or grain within 100 days of planting .	12H	45 days grazing
thiamethoxam	Cruiser 5FS	4A	5 .1–7 .6 fl oz/100 lb seed		12H	45 days
Postemergence treatment
chlorpyrifos	Lorsban 4E, Lorsban Advanced, Lorsban 75 WG, generics	1B	0 .5–1 .0 pt/A   0 .5–1 .0 pt/A 0 .33–0 .67 lb/A	Restricted use	24H	30 days at 1 pt, 60 days at 2 pt
chlorpyrifos	Cobalt	1B, 3A	6–13 fl oz/A	See label . Restricted use	24H	30 days for up
+ gamma–						to 26 oz/A; 60
cyhalothrin						days for
						> 26 oz
chlorpyrifos	Cobalt	1B,3A	6–13 fl oz/A	See label . Restricted use	24H	30 days for
+ lambda–	Advanced					up to 26 oz/A
cyhalothrin						and 60 days
						for > 26 oz
dimethoate	Dimethoate	1B	0 .5pt–1 .0 pt/A	Labeled for “aphids”	48H	28 days
	400,			Restricted use
	Dimethoate
	4EC,		0 .5pt-1 .0pt/A
	Dimethoate		0 .75-1 .5 pt/A
	2 .67
flupyradifurone	Sivanto Prime	4D	7 .0–14 .0 fl oz/A		4H	14 days
1: REI = Restricted entry interval 2: PHI = Preharvest interval

 

Sugarcane Aphid

Since the sugarcane aphid (Melanaphis sacchari) was first recorded feeding on sorghum in Texas in 2013, it has become one of the most damaging insect pests of grain and forage sorghum in much of the United States.

Sugarcane aphids are pale yellow, gray, or tan. The feet, antennae, and cornicles (“tailpipes”) are black (Figs. 9 and 10). In contrast:
• Greenbugs are light green with a dark green stripe running down the back.
• Corn leaf aphids are dark green and typically feed in the whorl of the sorghum plant.
• Yellow sugarcane aphids are bright yellow with rows of dark spots and short cornicles (Fig. 9). They do not produce honeydew.

The sugarcane aphid feeds primarily on the underside of sorghum leaves. Although the initial colonies consist of just a few aphids, they can increase rapidly until they cover much of the lower leaf surface. The aphids produce large amounts of honeydew, which collects on the tops of leaves below, making them sticky and shiny. Sugarcane aphids can also move into the grain head (panicle).

All are females and give birth to live young. In about 5 days, the immature aphids develop into adults, which live for about 4 weeks. Because sugarcane aphid populations can increase rapidly, monitor them 1 to 2 times a week once you find them in a field to determine if the infestations have reached treatment thresholds.

Planting date and hybrids maturity
One of the most effective ways to manage sugarcane aphids is to plant early in the normal planting period. Although the aphids can reduce yields up to the hard dough stage, infestations during preboot, boot, and head emergence are the most damaging. Early planting reduces the risk that the infestations will reach high numbers during these most sensitive stages. Early planting can also shift the peak infestations closer to harvest and reduce or avoid the need for insecticide applications. Another means of reducing the window for sugarcane aphid infestation potential is to select an early-maturing hybrid. However, when considering early planting and hybrid maturity, factor in the effect on yield potential.

Planting sugarcane aphid-resistant hybrids
Many seed companies market hybrids with resistance or “tolerance” to sugarcane aphids. Compared to susceptible hybrids, sugarcane aphid populations on resistant hybrids increase more slowly, cause less leaf damage, and are less likely to require an insecticide treatment. However, no current hybrids are immune to sugarcane aphid infestation and damage. Thus, you still need to monitor resistant hybrids for the presence of sugarcane aphids, and apply insecticides if the infestations exceed action thresholds. Also, stay-green sorghum hybrids tolerate drought better than do other hybrids and have withstood sugarcane aphid injury better under droughty conditions.

In addition to sugarcane aphid resistance, consider yield and other agronomic qualities when selecting hybrids. Those that are high yielding and well adapted yet susceptible to the aphids may be more profitable. Protect these hybrids from sugarcane aphid damage by scouting often and, if infestations exceed the action threshold, timing insecticide applications appropriately.

Insecticide seed treatments
The insecticide seed treatments clothianidin (NipsIt Inside, Poncho), imidacloprid (Gaucho and others), and thiamethoxam (Cruiser) are labeled for control of sugarcane aphids and other seedling pests. These treatments protect seedlings from aphid damage for about 4 to 6 weeks after planting. The value of seed treatment to control sugarcane aphids depends on how soon after planting that the aphids infest the field. Insecticide seed treatments tend to be more valuable for sugarcane aphid management in more southern areas and for late plantings.

Managing johnsongrass and volunteer sorghum
Infestations of sugarcane aphids can sometimes be associated with nearby stands of johnsongrass or volunteer sorghum. Sugarcane aphid infestations in a sorghum field often begin on field edges bordering ditch banks or in fields with johnsongrass or volunteer sorghum. To reduce the risk of infestation, eliminate these potential sources of these aphids.

Scouting for sugarcane aphid
Sampling procedures differ slightly according to the action threshold you choose (see below). Regardless of the threshold, begin scouting for sugarcane aphids soon after plant emergence and make it a part of a weekly field-monitoring program for all sorghum pests. Once you find sugarcane aphids, begin scouting twice a week. A suggested sampling protocol:

1. Once a week, walk at least 25 feet into the field and examine the plants along 50 feet of row.
2. Inspect the underside of leaves from the upper and lower canopy. The presence of winged aphids on the upper surface of upper leaves indicates that adults have recently migrated into the field. Inspect 15 to 20 plants per location.
3. Sample each side of the field as well as sites near johnsongrass and tall grain-sorghum plants.
4. Check at least four locations for a total of 60 to 80 plants per field.
5. If you see honeydew on a leaf, look on the underside of the leaves above it to confirm that the cause is sugarcane aphids. However, do not rely solely on obvious honeydew to detect infestations. You could easily overlook colonies that are too small to produce noticeable honeydew. Instead, look closely for small colonies on the underside of lower and upper leaves.
6. If no sugarcane aphids are present or if only a few wingless or winged aphids are on the upper leaves, continue scouting once a week.
7. If you find sugarcane aphids on lower or mid-canopy leaves, scout twice a week. Base your decision to treat primarily on one of the established thresholds listed below.

Treatment or action thresholds
There are two sampling procedures and action thresholds for sugarcane aphids, one based on the percentage of infested plants at different growth stages and the second based on the number of aphids per leaf. The choice of one action threshold over another varies by geographic region, personal preference, and degree of desired thoroughness. Use action thresholds as guides, considering the weather and your ability to make timely, effective insecticide applications.

Sugarcane aphid populations can double every 4 to 8 days, depending on the hybrid, environmental conditions, and density of the aphids’ natural enemies. Sample fields twice a week when the weather is warm and dry, which are the optimal conditions for aphid colony expansion. Yield loss is minimal once the grain reaches the hard dough stage. However, high infestations of sugarcane aphids may lead to lodging and harvest delays because of honeydew contamination. To determine if the infestation may impair harvest, scout sorghum 2 to 3 weeks before harvest. The best indicators that insecticide is needed to reduce honeydew contamination at harvest are the movement of aphids to the head late in the season and the presence of honeydew in the head.

1. Thresholds based on sorghum growth stages
The threshold based on sorghum growth stages, which is more conservative than that based on sugarcane aphids per leaf (see below), has been successful in parts of Texas and other states. The growth-stage threshold allows for nonuniform sugarcane infestations, problems caused by weather delaying insecticide treatment, the inability to scout the field more than once per week, and uncertainty about timing insecticide applications.

Field sampling protocol
Once you find sugarcane aphids in a field, use the following protocol to determine whether an insecticide treatment is needed:

1. Walk 25 feet into the field to look for aphids on the underside of green leaves from the lower canopy to the uppermost leaf. The presence of honeydew on top of a leaf will indicate that aphids are infesting the leaf above.
2. Record the numbers of plants that are not infested, plants infested with fewer than 50 aphids each, and plants with 50 or more aphids.
3. Check at least 10 plants at four locations across the field.
4. Calculate the percentage of plants infested with 50 or more aphids:
(Number of plants with 50 or more aphids ÷ Total number of plants inspected) × 100
5. Use the action thresholds in Table 11 to determine whether you need to take action.
In Table 11, the term localized area refers to a single plant or group of adjacent plants with sugarcane aphid colonies. Finding several of these areas indicates that the sugarcane aphid infestation is well established and increasing in the field.

In Table 11, the term localized area refers to a single plant or group of adjacent plants with sugarcane aphid colonies. Finding several of these areas indicates that the sugarcane aphid infestation is well established and increasing in the field.

Table 11. Action thresholds based on sorghum growth stages (Source: revised from thresholds created by Louisiana State University)
Growth stage	Action threshold
Preboot	20% of plants infested with 50 or more aphids
Boot	20% of plants infested with 50 or more aphids
Flowering–milk	30% of plants infested with 50 or more aphids
Soft dough	30% of plants infested with established aphid colonies and localized areas1 with heavy honeydew
Dough	30% of plants infested with established aphid colonies and localized areas1 with heavy honeydew
Black layer	Heavy honeydew and established aphid colonies.Treat only to prevent harvest problems.Observe preharvest intervals for insecticides.
1: A single plant or group of adjacent plants with sugarcane aphid colonies

2. Threshold based on sugarcane aphids per leaf
This threshold assumes that insecticide will be applied within 3 days of reaching or exceeding the action threshold. It applies to infestations during preboot through flowering and does not consider infestation during grain fill. This threshold is based on research conducted in the Texas Upper Gulf Coast, Louisiana, Arkansas, and Georgia.

Field sampling protocol

Once sugarcane aphids appear in a field, assess the infestation using the following steps:

1. Examine the underside of one completely green leaf from the lower canopy and one from the uppermost leaf (or the leaf below the flag leaf at boot to heading). Estimate and record the number of sugarcane aphids per leaf.
2. Examine two leaves (one upper and one lower) from each of five randomly selected plants for a total of 10 leaves per location.
3. Repeat at four locations for a total of 40 leaves per field.
4. Calculate the average number of aphids per leaf for the field:

Total aphids counted ÷ total leaves inspected

Counting exact numbers of aphids per leaf is often impractical, and estimating aphid number can be more efficient. Texas A&M AgriLife offers a card (https://tinyurl.com/ycapgkbl), that illustrates how to estimate sugarcane aphid abundance per leaf.

Treatment threshold: If a field averages 50 or more aphids per leaf (Fig. 12), apply an insecticide within 1 to 3 days. Evaluate control after 3 to 4 days and follow insecticide reentry intervals. Continue to monitor sugarcane aphid infestations until harvest to determine if additional insecticide applications are necessary.

Insecticides and applications

Always read and follow insecticide label directions; they change often. Only Sivanto Prime is currently labeled (24c) specifically to control sugarcane aphids in grain sorghum after planting (Table 12).

Since 2014, a Section 18 Emergency Exemption has allowed Transform WG insecticide to be used to control sugarcane aphids in sorghum. Availability is uncertain because continuation of this registration must be approved every year. Check the label for current status.

Regardless of the insecticide used, good coverage of the crop canopy, including the lower leaves, is necessary for acceptable control. Adjust the spray volume according to anticipated spray coverage. Generally, spray volumes of 10 to 15 gallons per acre are recommended for ground applications and at least 5 gallons per acre for aerial applications. Sivanto Prime can be applied by chemigation via overhead sprinkler irrigation systems. This application method is highly effective for managing sugarcane aphid infestations.

Adding a spray adjuvant usually does not increase sugarcane aphid control over the insecticide alone. Exceptions do occur, especially under hot, dry conditions such as those in the Texas High Plains, where much of the spray would evaporate before reaching the plants. Adjuvants may also be needed to adjust high-pH water.

Managing sugarcane aphids before harvest

Although sugarcane aphid infestations present after grain fill may have less effect on yield, they can continue to produce large amounts of honeydew, which can interfere with harvest. This risk increases when sugarcane aphids are present in the panicle. Rain can help wash honeydew from leaves. If it appears that aphids are likely to hinder harvest, consider applying an insecticide. Sivanto Prime can be applied up to 14 days before harvest. Harvest aid chemicals such as glyphosate and sodium chlorate have been used to kill sorghum leaves to reduce sugarcane aphid infestations before harvest. However, if the plants are slow to desiccate, the aphids may have time to move up into the sorghum panicle and continue to feed and produce honeydew. For this reason, you may need to use high rates of harvest aids that kill leaves quickly or include an effective insecticide with the harvest aid. An insecticide application is often more effective than harvest aids alone. Read and follow the label directions for the harvest aids, and observe preharvest intervals for both the harvest aids and the insecticides.

Managing sorghum midge, headworms, and stinkbugs when sugarcane aphid is present

Sivanto insecticide is effective against sugarcane aphids but is not labeled for midge, headworm, or stinkbug control in sorghum. Many insecticides labeled for these pests are toxic to beneficial insects, including lady beetles, syrphid flies, and parasitic wasps (Figs. 13 through 16), which help suppress sugarcane aphid infestations. Preserving these natural enemies whenever possible is an important goal in managing sugarcane aphids. All of the pyrethroid insecticides and methomyl and chlorpyrifos labeled for midge, headworms, and stinkbugs (Tables 18, 21, and 24) are broad-spectrum insecticides that are highly toxic to the natural enemies attacking sugarcane aphids. Thus, sugarcane aphid infestations can increase rapidly following their use, and treated fields should be closely monitored.

For control of midge, Blackhawk is less toxic to natural enemies than the other alternatives (Table 18). However, data for Blackhawk efficacy for midge control is limited; it is not recommended when midge pressure is high.

For corn earworm, fall armyworm, and sorghum webworm control, Prevathon and Blackhawk are less toxic to natural enemies than the alternatives while remaining effective on the headworm complex. Besiege is a premix of the same active ingredient as Prevathon plus a pyrethroid, and hence is not as safe for beneficial insects as Prevathon alone (Table 21). All of the insecticides labeled for stinkbugs are toxic to most aphid natural enemies.

Scout fields, and base insecticide applications on counts of midge, headworms, and stinkbugs. Do not treat based solely on crop growth stage; instead, use economic thresholds to make treatment decisions as described for each pest. Automatic insecticide applications for midge at bloom are not recommended. Midge, headworms, and stinkbugs at treatment thresholds present a certain threat to yield and profitability. While there is a risk of sugarcane aphid outbreaks after some insecticides have been applied, crop yield should not be jeopardized solely to spare the natural enemies.

Table 12. Insecticides labeled for controlling sugarcane aphid1,2. Follow label directions.
Active ingredient	Insecticide	Mode of action	Rate	Remarks	REI1	PHI2
Seed treatment
clothianidin	Poncho 600, NipsIt Inside 5	4A	5 .1–6 .4 fl oz/100 lb seed	2 ee label for TX	12H	—
imidacloprid	Gaucho 600	4A	6 .4 fl oz/100 lb seed	Label does not specify sugarcane aphid but lists “aphids” only .	12H	45 days grazing
thiamethoxam	Cruiser 5FS	4A	5 .1–7 .6 fl oz/100 lb seed	2 ee label for TX	12H	45 days
Postemergence treatment
flupyradifurone	Sivanto Prime	4D	4 .0–7 .0 oz/A (Section 2 ee)	Do not apply more than 28 oz/acre/season .	4H	14 days (Section 24(c))
1: Transform WG insecticide was approved under a Section 18 Emergency Exemption in 2018 . This exemption expires Nov 30, 2018 . Check label for current status .   2: Chlorpyrifos (Lorsban and generics) is not specifically labeled for sugarcane aphid but is labeled for “other aphids” in sorghum . University trials have shown that although chlorpyrifos can control sugarcane aphid short-term, it is not as effective as Sivanto Prime . Chlorpyrifos 4 at the rate of 1 pint per acre has a 30-day preharvest interval and higher rates have a 60-day preharvest interval . 3: REI = Restricted entry interval 4: PHI = Preharvest interval

Corn leaf aphid

Corn leaf aphids (Rhopalosiphum maidis) are oval and dark bluish-green and have black antennae, cornicles, and legs (Fig. 17). There are winged and wingless forms. These aphids are usually found deep in the whorl of preboot sorghum, but they also occur on the underside of leaves, on stems, or in grain heads. When abundant, corn leaf aphids within the whorl of sorghum plants are easy to see. To detect small aphid populations, pull the whorl leaf from the plant and unroll it.

When feeding, corn leaf aphids suck plant juices but do not inject toxin as do greenbugs and yellow sugarcane aphids. The most apparent feeding damage is yellow mottling of the leaves that unfold from the whorl.

Corn leaf aphids rarely cause economic loss to sorghum. In fact, they can be considered helpful. They attract beneficial insects such as lady beetles, which feed on the aphids. When corn leaf aphid numbers decline, the beneficial insects then can attack greenbug, sugarcane aphids, and other insect pests. Beneficials are also believed to move to adjacent crops, such as cotton, and feed on insect pests there.

Occasionally, corn leaf aphids will become so abundant on a few plants in a field that they hinder grain head exertion and development. These aphids are more likely to damage moisture-stressed than unstressed sorghum plants. Although very rare, infestations on seedling sorghum might cause stand loss, and grain head infestations might cause harvesting problems. The aphid also transmits the virus that causes maize dwarf mosaic.

Because corn leaf aphids prefer to live and feed in the whorl of sorghum plants, aphid numbers normally decline rapidly after the grain head emerges (exerts) from the boot.

Applying insecticide to control corn leaf aphids is rarely justified, but they can be suppressed with insecticides for greenbug control that are applied as seed treatments or postemergence as a foliar spray. Because it is rarely a pest, sampling procedures and damage assessment information for corn leaf aphid have not been developed.

Greenbug

The greenbug (Schizaphis graminum) is an aphid that sucks plant juices and injects toxin into sorghum plants. The adult greenbug is light green and about 1/16 inch long. It has a characteristic dark green stripe down the back (Fig. 18). The tips of the cornicles and leg segments farthest from the body are usually black. A colony can have winged and wingless forms.

Females produce living young (nymphs) without mating. Because there are no males, populations can develop quickly. Under optimum conditions, the life cycle is completed in 7 days. Each female produces about 80 offspring during a 25-day period.

Greenbugs usually feed in colonies on the underside of leaves and produce much honeydew. The greenbug may be a pest during the seedling, boot, and heading stages. To detect an infestation, look for reddish leaf spots caused by the toxin greenbugs inject into the plant. The reddened areas enlarge with increases in the number of greenbugs and amount of injury. Damaged leaves begin to die, turning yellow and then brown. Damage at the seedling stage may result in stand loss.

Although larger sorghum plants tolerate more greenbugs, these plants can also be heavily damaged. Yield reductions during the boot, flowering, and grain development stages depend on the number of greenbugs, the general health of the plants, and the length of time that the insects have infested the plants. Large populations of greenbugs on booting and older plants can reduce yields and weaken plants, which may lodge later.

Greenbug colonies usually begin on the underside of lower leaves and move up the plant. On most sorghum hybrids, you will need to examine the underside of the lower leaves only. However, in some cases, colonies may appear first on the underside of upper leaves. Do not confuse greenbugs with the bluish-green corn leaf aphid, often found with greenbugs in the plant whorl.

Once a week, scout for greenbugs by examining at least 40 randomly selected plants per field. Because greenbugs are seldom distributed evenly in an area, examine plants from all parts of the field, not just on the borders. Check more than 40 plants if the fields are larger than 80 acres or if making a control decision is difficult.

When determining whether to control greenbugs, consider several factors:
• Amount of leaf damage
• Number of greenbugs per plant
• Percentage of parasitized greenbugs (mummies)
• Number of greenbug predators (such as lady beetles) per plant
• Moisture conditions
• Plant size
• Plant growth stage
• Overall crop condition

Knowing whether greenbug numbers are increasing or decreasing from week to week can help you determine whether insecticide treatment is justified. For example, it would not be justified if the populations had reached the recommended treatment level (based on leaf damage) but had declined substantially from previous observations.

In seedling sorghum (less than 6 inches tall), greenbugs may infest any part of the plant, including the whorl, as well as in the soil at the base of the plant. When scouting seedling sorghum, examine the entire plant and the soil around its base. Note the presence or absence of greenbugs and any damage to the plants, such as yellowing or dead tissue.

Sorghum plants can lose about 30 percent of the leaves to greenbug feeding before yield is reduced. Control greenbug infestations before they kill more than two normal-sized leaves on 20 percent of the plants after sorghum flowering and before the hard-dough stage. When estimating leaf damage, consider any leaf to be dead if more than 75 percent of its surface is red, yellow, or brown. Do not mistake for greenbug damage the natural aging and gradual deterioration (senescence) of the small bottom leaves. Unless it is feasible to spot-treat some areas of the field, estimate the average leaf damage level for the entire field.

Action thresholds (Table 13) can help you determine when an insecticide treatment is justified at different stages of plant growth.

Table 13. Action threshold levels for greenbug on sorghum at different plant growth stages
Growth stage	When to treat
Emergence to about 6 in .	20% of plants visibly damaged (beginning to yellow), with greenbugs on the plants
Larger plant to boot	Greenbug colonies causing red spotting or yellowing of leaves and before any entire leaves on 20% of plants are killed
Boot to heading	At death of 1 functional leaf on 20% of plants
Heading to hard dough	When greenbug numbers are sufficient to cause the death of 2 normal-size leaves on 20% of plants

These guidelines assume that the greenbugs are increasing so rapidly that beneficial insects cannot control them. However, an insecticide treatment is usually unnecessary when more than 20 percent of the greenbugs appear brown and swollen from being parasitized. Also, the action levels in Table 13 should be lower when the plants are under drought or other stress.

Scout the fields more often when the weather is warm and dry—the conditions that favor rapid greenbug reproduction. Early in the season, rain as well as lady beetles and other predators suppress aphid abundance. However, the increase of natural enemies has a lag time of 1 to 2 weeks. In the spring, a common parasitic wasp usually causes the aphids to decline rapidly.

Plant sorghum hybrids that are resistant to greenbug. Because thresholds are based on plant damage, the action thresholds for resistant sorghums are the same as for susceptible sorghums.

Although the greenbug is usually susceptible to labeled insecticides (Table 14), it has developed resistance to organophosphorous insecticides (Mode of Action Group 1B) in several counties in the Texas Panhandle. If growers continue to use these insecticides extensively, the resistance problem could worsen. In areas where greenbug resistance has developed, apply an effective insecticide with a different mode of action from that of the previous insecticide and increase the application volume to ensure complete plant coverage.

Chinch Bug

Adult chinch bugs (Blissus leucopterus) are black with reddish-yellow legs and conspicuous, white, fully developed forewings, each with a black triangular spot at the middle of the outer edge (Fig. 19). Immature chinch bugs are shaped like the adults but lack wings.

Young nymphs are yellowish but later turn reddish with a white or pale yellow band across the front part of the abdomen. The older nymphs are black and gray with a conspicuous white spot on the back between the wing pads.

Chinch bugs are not a pest of sorghum in West Texas. However, false chinch bugs, discussed below, do occur there.

Chinch bugs lay eggs behind the lower leaf sheaths of sorghum plants, on roots or in the ground near the host plant. The life cycle is completed in 30 to 40 days, and there are at least two generations a year. Chinch bugs overwinter as adults in bunchgrass. They begin moving to sorghum when temperatures reach 70°F.

Cultural practices that stimulate dense, vigorous plant stands deter chinch bugs and usually reduce injury. Plant sorghum as early as practical.
Adult and immature chinch bugs suck juices from stems, leaves, and underground plant parts. Young plants are highly susceptible. Although older plants can better withstand attack, they too become reddened, weakened, and stunted. Chinch bugs proliferate in hot, dry weather, when many adult and immature bugs often migrate from wild bunchgrasses or small grains into sorghum.

To find chinch bugs, examine the lower leaves and stalks. Pull back the lower leaf sheaths to look for bugs on the stalk, and examine the surrounding soil at the plant base. Check at least five random areas per field.

Apply postemergent insecticide if you find two or more chinch bugs on 20 percent of seedlings less than 6 inches tall (Table 15). On taller plants, insecticide often is justified when chinch bugs infest 75 percent of the plants. Generally, one chinch bug per seedling sorghum plant reduces grain yield by 2 percent.

Apply postemergence insecticide with a ground rig using at least 20 gallons of water per acre through nozzles directed at the plant base. Control is seldom satisfactory on plants in the boot stage or later. Aerial application is seldom effective and not suggested.

Chinch bugs are sometimes difficult to control with foliar-applied insecticides. In fields with a history of economically damaging of chinch bug infestations, using insecticide seed treatments or applying insecticides applied at planting may be justified. Granular insecticides must receive about ½ inch of rainfall after application to effectively suppress early-season chinch bug infestations.

Corn earworm and fall armyworm (whorl worms)
(See later section for headworms)

Corn earworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda) infest the whorls and grain heads of sorghum plants. If there are no grain heads, the insects lay eggs on the leaves. Young larvae feed on tender, folded leaves in the whorl.
To find larvae in a sorghum whorl, pull the whorl leaf from the plant and unfold it. Larval excrement (frass) is present where larvae feed within the whorl. Damaged leaves unfolding from the whorl are ragged with “shot holes.” Although this may look dramatic, the damage usually does not significantly reduce yields, and controlling larvae during the whorl stage is seldom economically justified.

Insecticide application may be justified if:

• Larval feeding reduces leaf area by more than 30 percent, especially in drought-stressed fields
• Larvae are feeding on the developing grain head or growing point within the whorl or in high-value seed-production fields
• In seed production fields where the cost of the insecticide can often be justified due to the higher crop value

Once larvae are 1 to 1¼ inches long, they will complete feeding in 3 to 4 days and pupate. Treating these large larvae may provide little yield benefit. When considering treatment, check to see that larvae are still present in the whorls and less than 1 inch long. Control with insecticides is difficult because the larvae are protected deep within the whorl leaves. High volumes of spray applied by ground and directed into the whorl are needed to move insecticide into the whorl where larvae are feeding. Prevathon is labeled for corn earworm and fall armyworm. Pyrethroid insecticides are less effective on medium-size and large larvae and are toxic to beneficial insects. Also, chemigation (such as with Lorsban) can improve control by moving the insecticide into the whorl. See the section below for information on these insects as pests of sorghum grain heads.

Spider Mites

Banks grass mites (Oligonychus pratensis) and two spotted spider mites (Tetranychus urticae, Fig. 20) can infest sorghum, especially in drier areas of Texas. These mites are tiny, and females are larger than males. Each female lays about 50 eggs in webbing on the underside of sorghum leaves. The eggs are spherical, pearly white, and ¼ the size of the adults. They hatch in 2 to 3 days. Under favorable conditions, the life cycle takes about 5 to 7 days to complete.

Mite infestations begin along the midrib of the lower leaves. Spider mites suck juices from the underside of sorghum leaves, causing the infested areas to become pale yellow initially and reddish on the top later. The entire leaf may turn brown. As the mites become more abundant on the lower leaves, the infestation spreads upward through the plant. The undersides of heavily infested leaves have a dense deposit of fine webbing spun by the spider mites.
Spider mite abundance generally increases after the grain heads emerge. If many mites infest sorghum early in kernel development, the plants become less able to make and fill grain. Mites do not affect the grain after the kernels reach hard dough. However, if spider mites are very abundant, the sorghum plants may lodge, reducing yields.

Inspect the underside of the lower leaves carefully. Mite infestations usually begin along field borders and may spread quickly throughout the field. Hot, dry weather may favor a rapid increase in mites. They may also increase after an insecticide application for other pests such as greenbug or midge. If the increase occurs after such an application, the cause may be the tolerance of mites to some insecticides, the destruction of beneficial insects, or the dispersal of mites from colonies, which can increase their reproduction rate.

Natural enemies do not always control spider mites adequately. Because spider mites increase faster on moisture-stressed than nonstressed plants, time irrigation if possible to prevent plant stress. Rain can suppress spider mite population growth. Also, spider mites may move from small grains, especially wheat, to sorghum. To avoid this problem, plant sorghum away from small grains.

The use of miticides to control mites in sorghum has been erratic. Miticide application may be justified when damage appears on 30 percent of the leaf area of most sorghum plants in the field (Table 16). Thorough coverage is required; apply at least 3 to 5 gallons of spray mixture per acre by air and 10 to 20 gallons per acre by ground. Spider mites are often resistant to miticides.

Table 16. Miticides labeled for control of two-spotted and Banks grass mites. Follow label directions.
Active ingredient	Insecticide	Mode of action	Rate	Remarks	REI1	PHI2
hexythiazox	Onager 1.0EC	10A	10–24 oz/A	Apply Onager before mites build up.Use higher rates on moderate to high mite infestations or for larger plants with a dense canopy.15–20 GPA recommended for ground application, at least 5 GPA for aerial application.Chemigation is labeled.Do not apply it more than once a year.	12H	30 days
propargite	Comite II	12C	24–36 fl oz/A	Apply only to dry foliage.Do not tank-mix with other products or use oil-based surfactants.Use at least 20 GPA for ground applications and 5 GPA for aerial application.Treating a test plot is recommended to check for phytotoxicity before treating a large area.Restricted use	13 days	30 days for silage; 60 days for grain harvest
1: REI = Restricted entry interval 2: PHI = Preharvest interval