Insects that feed on developing grain in the head
Sorghum midge
The sorghum midge, Stenodiplosis (Contarinia) sorghicola, is one of the most damaging insects of sorghum in Texas, especially in the southern half of the state. The adult sorghum midge is a small, fragile-looking orange-red fly with a yellow head, brown antennae and legs, and gray, membranous wings (Fig. 21).
During the single day of adult life, each female lays about 50 yellow-white eggs in the flowering spikelets of sorghum. The eggs hatch in 2 to 3 days.
At first, the larvae are colorless; fully grown larvae are dark orange. They complete development in 9 to 11 days and pupate between the spikelet glumes. Shortly before the adult emerges, the pupa moves toward the upper tip of the spikelet. After the adult emerges, it leaves a clear or white pupal skin at the tip of the spikelet—a sure sign of sorghum midge damage (Fig 21).
Under favorable conditions, a generation is completed in 14 to 16 days, and midge numbers increase during the season with each subsequent generation. Thus, late-planted sorghum is at risk to large infestation of adults moving in from fields planted earlier.
Sorghum midges overwinter in cocoons inside spikelets of sorghum or johnsongrass that fall to the ground and become covered with litter. Adult sorghum midges emerge in the spring before flowering sorghum is available, and these adults infest johnsongrass. Sorghum midges developing in johnsongrass disperse to sorghum when it flowers.
Early-season infestations in sorghum are usually below damaging levels. As the season progresses, sorghum midge populations increase, especially when late planting makes flowering sorghum available in the area. Numbers often drop late in the season.
The larva damages sorghum by feeding on a newly fertilized ovary, which prevents normal kernel development. Grain loss can be extremely high. The glumes of an infested spikelet fit tightly together because no kernel develops. Typically, a sorghum grain head infested by sorghum midges has normal kernels scattered among spikelets that do not bear kernels, depending on the degree of damage.
The most effective cultural management practice is planting sorghum early and uniformly so that flowering occurs before the sorghum midges reach damaging levels. It is critical to plant hybrids early enough to prevent grain heads from flowering late (Fig. 22).
Cultural practices that promote uniform heading and flowering in a field are also important for reducing sorghum midge infestations. Use cultivation and/or herbicides to eliminate johnsongrass inside and outside the field. Where practical, disk and deep-plow the previous year’s sorghum crop to destroy overwintering midges. Late-planted and late-flowering sorghum are especially vulnerable to sorghum midge.
This midge lays eggs in spikelets when yellow anthers appear on the sorghum head. An individual grain head requires 7 to 9 days to complete flowering; it may take 2 to 3 weeks for all of the heads in a field to complete flowering. Thus, a field can remain susceptible to midge infestation for several weeks, depending on how uniformly the plants flower. Once the anthers turn reddish brown, they are no longer susceptible to midge infestation.
Begin scouting for sorghum midge soon after head emergence, when yellow blooms first appear in the field. Scout at midmorning when the temperature rises to about 85°F. The adult lives for 1 day, and each day a new brood of adults emerges. For this reason, you need to sample flowering fields almost daily.
Look for adults on the yellow blooms by inspecting carefully and at close range all sides of randomly selected flowering grain heads. The reddish, gnatlike adults crawl on or fly about the flowering heads. During inspection, handle the grain heads carefully to avoid disturbing the adult midges. Another sampling method is to gently but quickly slip a clear, 1-gallon plastic bag over the head. Tap the head to disturb the midges, which will fly up in the bag, where you can easily see and count them. A faster yet still efficient method is to turn the head downward into a white plastic bucket or pail and beat the head in the bucket to knock the midge from the head. Remove the head and count any sorghum midge in the pail or bucket. A 1-gallon milk jug with the bottom cut out also works well for this type of sampling.
Because they are relatively weak fliers and rely on wind currents to help them disperse, adult sorghum midges usually are most abundant along edges the of sorghum fields. For this reason, inspect plants along field borders first, particularly those downwind of earlier-flowering sorghum or johnsongrass. If the grain heads along the field edges have few or no sorghum midges, there should be little need to sample the entire field.
However, if you find more than one sorghum midge per flowering grain head in a field border area, inspect the rest of the field. Flowering heads are those with yellow blooms. Sample at least 20 flowering grain heads for every 20 acres in a field. For fields smaller than 20 acres, sample 40 flowering grain heads. Avoid plants within 150 feet of field borders. Record the number of sorghum midges for each flowering head sampled and then calculate the average number of midges per flowering head. Almost all of the sorghum midges seen on flowering sorghum heads are female.
Next, calculate the number of flowering heads (those with yellow blooms present) per acre. Record the number of flowering heads along a length of row equal to 1/1000 acre. As an example, for a row spacing of 40 inches, 13.1 row feet is equal to 1/1000 acre. Make counts in at least four areas of the field. If flowering (plant maturity) is highly variable across the field, sample additional sites. Average all counts and multiply by 1,000 to estimate the number of flowering heads per acre. If there is only one head per plant, the number of flowering heads per acre is the percentage of heads in bloom multiplied by the number of plants per acre.
Then calculate sorghum midge density per acre as the average number of midges per flowering head multiplied by the number of flowering heads per acre. For example, if there are 30,000 flowering heads per acre and scouting records show an average of 0.5 sorghum midge per flowering head, there are an estimated 15,000 sorghum midges per acre (0.5 sorghum midge per head × 30,000 flowering heads per acre). The percentage of flowering heads changes rapidly during bloom and should be calculated each time the field is sampled.
Studies have shown that the larvae from a single female sorghum midge will destroy an average of 45 grain sorghum kernels. The seed weight of sorghum hybrids averages 15,000 seeds (12,000 to 18,000, depending on the hybrid) per pound. A loss of 45 kernels per midge, therefore, represents 0.0030 pounds (1.364 grams) of grain.
Calculate the economic injury level for sorghum midge using the following equation:
Number of sorghum midges/flowering heads = Cost of control as $A × 33256 / Value of grain as $ per cwt × Number of flowering heads
In the equation above, the control cost is the total cost of applying an insecticide for sorghum midge control; the grain value is the expected price at harvest as dollars per 100 pounds. The value 33256 is a constant and results from solving the economic injury equation. Determine the number of flower heads per acre as described above.
For example, assume that field scouting yields an average of 1.1 sorghum midges per flowering head and that field sampling shows the number of flowering heads is 18,000 per acre. (This is equal to a plant population of 90,000 with 20 percent of the heads flowering and one head per plant). If the value of the crop is estimated to be $4.00 per 100 pounds and the cost of control is $5.00 per acre, the equation yields the injury level as:
$5.00 × 33,256 / $4.00 × 18,000 = 2.3 sorghum midge per flowering head
In this example, the field density of 1.1 sorghum midges per flowering head is below the injury level of 2.3 per head, and treatment would not be justified. If you scout the field 2 days later and the sorghum midge density is again 1.1 midges per flowering head, but the number of flowering heads has increased to 45,000 per acre (50 percent of the plants now have a flowering head in a plant density of 90,000 plants per acre), the economic injury level would be ($5.00 × 33256) ÷ ($4.00 × 45,000) = 0.9 sorghum midge per flowering head.
Now the average of 1.1 sorghum midges is above the economic injury level of 0.9 per flowering head, and treatment would be justified. These examples show the importance of considering the number of flowering heads (grain susceptible to midge damage) in estimating the economic injury level.
Table 17 lists the economic injury levels, as determined from the above equation, for a range of typical treatment costs per acre, market values per 100 pounds of grain, and numbers of flowering heads per acre. Use the equation to estimate injury levels for your control costs, crop value, and number of flowering heads per acre. These variables can be entered into an online calculator to determine the economic threshold for midge. The calculator is available at http://entomology.tamu.edu/extension/apps/.
Insecticide residues should kill the adults and prevent egg laying 1 to 2 days after treatment. However, if adults still are present 3 to 5 days after the first insecticide application, apply a second insecticide treatment immediately. Making several insecticide applications at 3-day intervals may be justified if the yield potential is high and sorghum midges exceed the economic injury level (Table 18).
| Table 17. Estimated economic injury levels for sorghum midge for a range of factors. (This table is only a guide. Use the equation in the text to estimate the economic injury level in your field.) | ||||
|
Economic injury level—mean number of midges/flowering head |
||||
|
Control cost, $/A |
Crop value, $100 lb |
Flowering heads = 18,000/A |
Flowering heads = 45,000/A |
Flowering heads = 67,500/A |
|
5 |
6 |
1.6 | 0.6 | 0.4 |
|
5 |
7 |
1.3 | 0.5 | 0.34 |
|
5 |
8 |
1.2 | 0.5 | 0.3 |
|
6 |
6 |
1.9 | 0.8 | 0.5 |
|
6 |
7 |
1.6 | 0.7 | 0.4 |
|
6 |
8 |
1.4 | 0.6 | 0.35 |
|
7 |
6 |
2.2 | 0.85 | 0.6 |
|
7 |
7 |
1.9 | 0.75 | 0.5 |
|
7 |
8 |
1.6 | 0.65 | 0.45 |
Table 18. Insecticides labeled for sorghum midge control in grain sorghum. Follow label directions.
| Active ingredient | Insecticide | Mode of action | Rate | Remarks | REI1 | PHI2 | |
| Post-emergence treatment | |||||||
| alpha– cypermethrin | Fastac | 3A | 1.3–3.8 fl oz/A | Restricted use.Danger | 12H | 14 days | |
| beta-cyfluthrin | Baythroid XL | 3A | 1.0–1.3 fl oz/A | Restricted use | 12H | 14 days | |
| chlorpyrifos | Lorsban 4, Lorsban Advanced, Lorsban 75WG, generics |
1B | 0.5 pt/A 0.5 pt/A 0.33 lb/A |
Do not use on sweet sorghum varieties.Restricted use | 24H | 30 days (1 pt) 60 days (> 1 pt) |
|
| chlorpyrifos + gamma– cyhalothrin | Cobalt | 1B, 3A | 7–13 fl oz/A | See label.Restricted use | 24H | 30 days for up to 26 oz/A and 60 days for > 26 oz | |
| chlorpyrifos + lambda– cyhalothrin | Cobalt Advanced | 1B, 3A | 6–13 fl oz/A | See label.Restricted use | 24H | 30 days for up to 26 oz/A and 60 days for > 26 oz | |
| chlorpyrifos + zeta-cypermethrin | Stallion | 1B, 3A | 3.75–11.75 oz/A | Restricted use | 24H | 30 days | |
| cyfluthrin | Tombstone | 3A | 1.0–1.3 fl oz/A | Restricted use | 12H | 14 days | |
| deltamethrin | Delta Gold 1.5EC | 3A | 1.3–1.9 fl oz/A | Restricted use.Danger–Poison | 12H | 14 days | |
| esfenvalerate | Asana XL, generics | 3A | 2.9–5.8 fl oz/A | 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 |
Restricted use | 24H | 30 days | |
| lambda- cyhalothrin | Warrior II Zeon, Karate with Zeon, generics |
3A | 0.96–1.28 fl oz/A | Restricted use | 24H | 30 days | |
| lambda-cyhalothrin + chlorantraniliprole | Besiege | 3A, 28 | 5–6 oz/A | Do not exceed total of 18 fl oz/A per year.Restricted use | 24H | 30 days | |
| spinosad | Blackhawk (2ee) | 5 | 1.5–3.3 oz/A | For low to moderate midge infestations | 4H | 21 days | |
| methomyl | Lannate LV, Lannate SP, generics |
1A | 0.75–1.0 pt/A 0.25–0.5 lb/A |
Do not use methomyl on sweet sorghum varieties.For SP, use a minimum of 10 gallons per acre by ground or 2 gallons per acre by air.Restricted use.Danger | 48H | 14 days | |
| zeta-cypermethrin | Mustang Maxx Respect | 3A | 1.28–4.0 fl oz/A | Restricted use | 12H | 14 days | |
| 1: REI = Restricted entry interval 2: PHI = Preharvest interval |
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Corn earworm and fall armyworm: Headworms
Corn earworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda) moths lay eggs on leaves or grain heads of sorghum. Newly hatched corn earworm larvae are pale and only 1/16 inch long. They grow rapidly, and older larvae range from pink, green, or yellow to almost black (Figs. 23 through 25). Each segment has several long bristles or hairs. Many larvae are conspicuously striped. Along the side is a pale stripe edged above with a dark stripe. Down the middle of the back is a dark stripe divided by a narrow white line that makes the dark stripe appear doubled. Fully grown larvae are robust and 1½ to 2 inches long.
Young fall armyworm larvae are greenish, have black heads, and lack bristles. Mature larvae vary from greenish to grayish brown and have a light-colored, inverted, Y-shaped suture on the front of the head (Figs. 24 through 26) and dorsal lines running lengthwise on the body. The tail end has four large black spots.
Corn earworm and fall armyworm larvae feed on the flowers and then on the developing grain and hollow out the kernels. The last two larval stages cause about 80 percent of the damage. Frass and fragments of grain kernels accumulate on top of the upper leaves and on the ground under plants where the larvae are feeding.
Many small corn earworm and fall armyworm larvae normally die because of predators, parasites, pathogens, and cannibalism. Infestations are less common in early-planted sorghum and in sorghum hybrids with loose (open) grain heads.
Begin sampling for headworms soon after the field finishes flowering and continue at 5-day intervals until the hard dough stage. To sample headworms, grasp the stalk just below the sorghum head, bend the head into a clean, white, 2- to 3-gallon bucket, and vigorously beat the head against the side of the bucket. Headworms will fall into the bucket, where you can see and count them. Sample at least 30 heads randomly selected across the field. In fields larger than 40 acres, sample at least one head per acre.
Record the number of small (less than ¼ inch long), medium (¼ to ½ inch long) and large (longer than ½ inch) headworms found in the samples. Divide the total number of medium or large headworms by the number of heads sampled to get the average number of headworms per head. Then multiply the average number of headworms per head by the number of heads per acre to calculate the number of headworms per acre. To estimate the number of plants or heads per acre, see the discussion on sorghum midges.
Studies have shown that a corn earworm larva will consume about 0.010252 pound (4.65 grams) of grain while developing in the sorghum head. However, estimating the economic injury level for headworms is complicated because the potential yield loss varies with the size of the larvae. That is why you need to record the number of small, medium-size, and large headworms.
Small larvae (up to ¼ inch) consume very little grain (about 10 percent of the total consumption), and about 80 percent of them die in this stage. Therefore, do not consider small larvae when determining the economic injury level. If most headworms are this size, sample the field again in 3 to 4 days. If at the later time, most of the larvae are longer than ¼ inch, determine which size (medium or large) is the most common and use the corresponding equation below to calculate the economic injury level.
As an example, if the cost of control is $8/acre and the grain value is $8/cwt, and if there are 50,000 grain heads per acre, the economic injury level equals an average of 0.2 large worm per head or 1.0 medium worm per head. In this example, an insecticide treatment should be considered if field scouting found an average of more than 0.2 large worm per head or 1 medium worm per head.
If the infestation consists of about equal numbers of medium-size and large headworms, use this equation:
Treatment is economically justified if the value of the economic loss (loss in pounds per acre multiplied by the dollars per pound of grain) exceeds the treatment cost per acre.
Most corn earworm larvae larger than ½ inch will survive to complete development. These large larvae account for 83 percent of the total grain consumed during larval development. About 19 percent of medium-size larvae (¼ to ½ inch long) survive beyond this stage. Thus, the potential grain loss from medium size larvae is only 19 percent of the potential loss from large larvae.
The above equations present the economic injury level as the number of larvae per head. The economic thresholds can also be calculated as the number of larvae per acre as shown in Tables 19 and 20. As an example, if the cost of control is $8 per acre and the grain value is $8 per 100 pounds, the economic injury level in the table below is 9,750 large larvae per acre. If there are 50,000 heads per acre, the economic injury level is 9750 ÷ 50,000 = 0.2 larvae per sorghum head.
These variables can be entered into a calculator available online to determine the economic injury levels given variable treatment costs and grain values. This calculator is available at http://entomology.tamu.edu/extension/apps/.
Table 19. Economic injury level for large (longer than ½ inch) corn earworm larvae shown as the number of larvae per acre. When the number of larvae per acre exceeds the number in the table at a given cost of control and value of grain per cwt, the value of the protected grain exceeds the cost of control.1
| Control cost, $/A | Grain value $/100 lb | ||||
| 6.00 | 7.00 | 8.00 | 10.00 | ||
| 6 | 9,750 | 8,500 | 7,250 | 5,750 | |
| 8 | 13,000 | 11,000 | 9,750 | 7,750 | |
| 10 | 16,250 | 14,000 | 12,250 | 9,750 | |
| 12 | 19,500 | 16,750 | 14,750 | 11,750 | |
| 1: This threshold table assumes all larvae will survive and complete development. | |||||
Table 20. Economic injury level for medium-size (¼–½-inch) corn earworm larvae shown as the number of larvae per acre. When the number of larvae per acre exceeds the number in the table at a given cost of control and value of grain per cwt, the value of the protected grain exceeds the cost of control.1
| Control cost, $/A | Grain value $/100 lb | |||||||
| 6.00 | 7.00 | 8.00 | 10.00 | |||||
| 6 | 51,500 | 44,750 | 38,250 | 31,250 | ||||
| 8 | 68,500 | 58,000 | 51,500 | 41,750 | ||||
| 10 | 87,750 | 73,750 | 64,500 | 51,500 | ||||
| 12 | 102,750 | 88,250 | 77,750 | 62,000 | ||||
| 1: This table assumes that 81% of the medium-size larvae will die in that stage and not contribute to additional yield loss. | ||||||||
Table 21. Suggested insecticides for controlling corn earworm and fall armyworm in grain sorghum. Resistance to pyrethroids (products with only mode of action 3A) in corn earworm has been reported from some areas. If resistance is present, applying pyrethroids can result in poor control of corn earworm, especially when the larvae are larger than ¼ inch (2nd instar). Also, pyrethroids are not recommended for fall armyworm larger than ¼ inch (2nd instar).
| Active ingredient | Insecticide | Mode of action | Rate | Remarks | REI1 | PHI2 for grain harvest |
| Postemergence treatment | ||||||
| chlorpyrifos + gamma– cyhalothrin | Cobalt | 1B, 3A | 19–38 oz/A | See label | 24H | Preharvest interval is 30 days for up to 26 oz/A and 60 days for > 26 oz |
| chlorpyrifos + lambda– cyhalothrin | Cobalt Advanced | 1B, 3A | 16–38 fl oz/A | See label | 24H | Preharvest interval is 30 days for up to 26 oz/A and 60 days for > 26 oz/A |
| chlorpyrifos + zeta–
cypermethrin |
Stallion | 1B, 3A | 9.25–11.75 fl oz/A | See label | 24H | 30 days to harvest |
| deltamethrin | Delta Gold 1.5EC | 3A | 1.0–1.5 fl oz/A | Apply at least 2 GPA by aircraft or 5 GPA by ground.Restricted use.Danger—Poison | 12H | 14 days |
| esfenvalerate | Asana XL, generics | 3A | 5.8–9.6 fl oz/A | Used for earworms on heads only.Restricted use | 12H | 21 days |
| gamma- cyhalothrin | Declare 1.25,
Proaxis 0.5 |
3A | 1.02–1.54 fl oz/A
2.56–3.84 fl oz/A |
Use higher rates for large larvae.Restricted use | 24H | 30 days |
| lambda– cyhalothrin | Warrior II Zeon, Karate with Zeon,
generics |
3A | 1.28–1.92 fl oz/A | Restricted use | 24H | 30 days |
| lambda-cyhalothrin+ chlorantraniliprole | Besiege 1.25 SC | 3A, 28 | 6–10oz/A | Use higher rate range for large larvae.Do not exceed total of 18 fl oz/A per year.Restricted use | 24H | 30 days |
| methomyl | Lannate LV,
Lannate SP |
1A | 0.75–1.5 pt/A
0.25–0.05 lb/A |
Do not use on sweet sorghum varieties.Restricted use.Danger | 48H | 14 days |
| novaluron | Diamond | 15 | 6–12 fl oz/A | Fall armyworm only.See label | 12H | 7 days for forage, 14 days for grain and stover |
| spinosad | Blackhawk | 5 | 1.7–3.3 oz/A | Apply to coincide with peak egg hatch or small larvae.Use a higher rate range for heavy infestations, advanced growth stages of target pests, or difficult spray coverage situations. | 4H | 21 days |
| zeta-cypermethrin | Mustang Maxx, generics | 3A | 1.76–4.0 fl oz/A | Restricted use | 12H | 14 days |
| 1: REI = Restricted entry interval
2: PHI = Preharvest interval |
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Sorghum Webworm
Sorghum webworms (Nola sorghiella) occasionally infest grain heads of sorghum planted 2 to 3 weeks later than normal. This insect occurs primarily in the more humid eastern half of Texas.
The adults are small white moths that are active at night. They lay about 100 eggs singly on flowering parts or kernels of sorghum. The eggs are round to broadly oval and are flattened from top to bottom.
Webworm larvae are somewhat flattened, yellowish or greenish brown, and marked with four lengthwise reddish to black dorsal stripes (Fig. 27). When mature, the larvae are about ½ inch long and covered with many spines and hairs. A silk cocoon encloses the reddish-brown pupal stage. A generation requires 1 month; as many as six generations may develop in a year. The larva overwinters in a cocoon on the host plant.
Young larvae feed on developing flower parts. Older larvae gnaw circular holes in and feed on the starchy contents of maturing kernels. Each larva may eat more than 12 kernels in 24 hours. The larvae do not spin webs (as the name might imply) over the sorghum grain head but, when disturbed, they often suspend themselves by spinning a thin silken thread.
Look for sorghum webworms when grain heads begin to flower and continue at 5-day intervals until the kernels are in the hard-dough stage. To examine grain heads for sorghum webworms, shake the grain heads vigorously into a 2- to 5-gallon white plastic bucket, where you can easily see and count even small larvae.
Inspect at least 30 plants from several areas of a field. In fields larger than 40 acres, sample at least one grain head per acre.
Insecticide application is economically justified when an average of five or more small larvae are found per grain head (Table 22).
Table 22. Insecticides labeled for sorghum webworm control in sorghum
| Active ingredient | Insecticide | Mode of action | Rate | Remarks | REI1 | PHI2 |
| Postemergence treatment | ||||||
| carbaryl | Carbaryl 4L, Sevin XLR Plus |
1A | 1–2 qt/A | Bee caution: Do not apply this product to target crops or weeds in bloom. | 12H | 21 days |
| methomyl | Lannate LV, Lannate SP | 1A | 1.5 pt/A 0.25-0.5 lb/A |
Danger–Poison | 48H | 14 days |
| spinosad | Blackhawk | 5 | 1.7–3.3 oz/A | Apply to coincide with peak egg hatch or the presence of small larvae.Use a higher rate range for heavy infestations, advanced growth stages of target pests, or difficult spray coverage situations. | 4H | 21 days |
| chlorantraniliprole | Prevathon | 28 | 14–20 fl oz/A | — | 4H | 1 day |
| lambda-cyhalothrin + chlorantraniliprole | Besiege 1.25 SC | 3A, 28 | 6–10oz/A | Use higher rate range for large larvae.Do not exceed total of 18 fl oz/A per year.Restricted use | 24H | 30 days |
| 1: REI = Restricted entry interval 2: PHI = Preharvest interval |
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Cultural practices to reduce sorghum webworm abundance include plowing sorghum residues after harvest to destroy overwintering pupae, planting as early as practical, and using sorghum hybrids with loose (open) grain heads.
Grain head-feeding bugs
During grain development, stink bugs, leaffooted bugs, false chinch bugs, and Lygus bugs can move from alternate host plants into sorghum. These bugs have piercing-sucking mouthparts and feed on developing grain kernels. Feeding can reduce grain weight, grain size, and seed germination. The bugs cause more damage during early kernel development and less as the grain develops to the hard-dough stage. Fungi often infect damaged kernels, causing them to turn black and further deteriorate in quality. Damaged kernels rarely develop fully and may be lost during harvest. The extent of damage depends on the species of bug, the number of bugs per grain head, and the stage of kernel development when the infestation occurs.
Grain head-feeding bugs tend to congregate on grain sorghum heads and sometimes within areas of a field. Use the beat-bucket technique to estimate abundance:
1. Shake grain sorghum heads vigorously into a 2.5- to 5-gallon bucket, where you can easily see and count the bugs.
2. Sample at least 30 plants from a field. In fields larger than 40 acres, take at least one sample per acre.
3. Calculate the average number of bugs per sorghum head.
Treatment thresholds vary according to species as discussed below.
Rice stink bug
The rice stink bug (Oebalus pugnax) is straw colored, shield shaped and ½ inch long (Fig. 28). The female lays about 10 to 40 short, cylindrical, light-green eggs in a cluster of two rows. The eggs hatch after 5 days. The nymphs require 15 to 28 days to become adults.
Sample for rice stink bugs using the beat bucket method as described above. Determine the average number of bugs per sorghum head, and use Table 23 to determine the treatment threshold based on the cost of control and grain value. For example, if the cost of control is $8 per acre and grain value is $8/cwt, the economic threshold is 30,500 rice stink bugs per acre. To determine the threshold as the number of rice stinkbugs per head, divide the threshold by the number of plants per acre with a grain head. As an example, if the plant population is 50,000 plants, each with 1 grain head, the threshold is 30,500 ÷ 50,000 = 0.6 rice stinkbug per head.
Table 23. Economic injury level, shown as the number of rice stink bugs per acre of sorghum at the milk stage
|
Control cost, $/A |
Grain value, $/100 lb |
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|
6.00 |
7.00 |
8.00 |
10.00 |
|||||
|
6 |
30,500 |
27,000 |
23,000 |
18,500 |
||||
|
8 |
40,500 |
35,000 |
30,500 |
24,500 |
||||
|
10 |
51,000 |
43,500 |
38,000 |
30,500 |
||||
|
12 |
62,000 |
52,500 |
46,000 |
36,500 |
||||
In this example, an insecticide treatment should be made if field scouting finds an average of 0.6 or more rice stinkbugs per head (Table 24).
Growers can also enter these variables into an online calculator to determine the economic threshold for rice stinkbugs (http://entomology.tamu.edu/extension/apps/).
False chinch bug
The false chinch bug (Nysius raphanus) resembles the chinch bug but is uniformly gray to brown (Fig. 29). False chinch bugs are 1/10 inch long. Large numbers of these bugs occasionally migrate from wild hosts, such as wild mustard, to sorghum. However, these insects usually concentrate in small areas of a field. Sample for false chinch bugs using the beat bucket method described above. The action level for false chinch bug is 140 bugs per grain head when infestation begins at the milk stage of grain development (Table 24).
Stink bugs, leaffooted bug, and Lygus bug
Like the rice stinkbug, other species of true bugs have piercing-sucking mouthparts and feed on developing grain in sorghum heads:
• Southern green stink bugs (Nezara viridula) are bright green, shield shaped, and slightly larger than ½ inch long.
• Conchuela stink bugs (Chlorochroa ligata) vary from dull olive or ash gray to green, purplish pink, or reddish brown. The most characteristic markings are orange-red bands along the lateral margins of the thorax and wings and a spot of the same color on the back at the base of the wings.
• Leaffooted bugs (Leptoglossus phyllopus) are brown, oblong, and just longer than ¾ inch. A white band extends across the front wings. The lower part of each hind leg is dilated or leaflike.
• Lygus bugs (Lygus spp. ) are oval, greenish insects that run and fly quickly when disturbed. The adults can fly into sorghum fields during grain fill and feed on the developing kernels, potentially reducing grain yield and quality.
The treatment threshold for the southern green stink bug and conchuela stink bug is an average of four or more bugs per grain head during the during the flowering, milk, or soft dough stages (Table 24, Fig. 30). Once the grain is in the hard dough stage, the threshold for these stink bugs and leaffooted bugs increases to an average of 16 or more per grain head. The threshold for leaffooted bugs is an average of six or more bugs per grain head during flowering, milk, or soft dough stages (Fig. 31).
No treatment threshold has been developed for Lygus bugs in grain sorghum (Fig. 32). Results of one experiment in the High Plains suggested a treatment threshold of 12 lygus bugs per head during the soft dough stage.
Stalk-boring insect pests
Lesser cornstalk borer
The larvae of lesser cornstalk borers (Elasmopalpus lignosellus) attack roots and bore into the stems of young plants of peanuts, corn, sorghum, and other crops. Damaging infestations of this insect rarely occur in sorghum. The larvae are light bluish green with prominent transverse reddish-brown bands (Fig. 33). They feed in silken tunnels covered with soil particles. The larvae pupate in silken cocoons under crop debris.
Infestations of lesser cornstalk borers usually are more severe during dry periods and in sandy soils. To discourage the insect, adopt cultural practices that preserve moisture and increase organic matter in the soil. Early planting and rotation with nonhost crops help avoid damage from lesser cornstalk borer. Insecticidal control rarely is justified, although formulations of lambda- cyhalothrin, gamma-cyhalothrin, deltamethrin and chlorpyrifos are labeled for control of lesser cornstalk borer in grain sorghum.
Other borers
The sugarcane borer (Diatraea saccharalis), southwestern corn borer (Diatraea grandiosella), European corn borer, (Ostrina nubilalis), Mexican rice borer (Eoreuma loftini), and neotropical borer (Diatraea lineolata) are closely related insects that tunnel in the stalks of sorghum, corn, and other crops.
The biology of these four species is similar. The moths are white to buff colored, and the females deposit clusters of flattened, elliptical to oval eggs that overlap like fish scales in a shingle-like arrangement on the host plant leaves. The eggs hatch in 3 to 7 days.
The larval stage lasts about 25 days and the pupal stage about 10. There are two to three generations a year. The larvae are creamy white and about 1 inch long when fully grown. Most of the body segments have conspicuous round brown or black spots (Fig. 34). The spots on mature overwintering larvae are lighter or absent. Most of these borers pass the winter as fully grown larvae in cells inside the stalks that remain after the crop is harvested.
Table 24. Insecticides labeled for stink bugs and false chinch bugs in grain sorghum. Follow label directions.
| Active ingredient | Insecticide | Mode of action | Rate | Remarks | REI1 | PHI2 |
| Postemergence treatment | ||||||
| alpha– cypermethrin | Fastac | 3A | 1.3–3.8 fl oz/A | Restricted use.Danger | 12H | 14 days |
| beta-cyfluthrin | Baythroid XL | 3A | 1.3–2.8 fl oz/A | Restricted use | 12H | 14 days |
| chlorpyrifos + gamma- cyhalothrin | Cobalt | 1B, 3A | 19–38 fl oz/A | See label.Restricted use | 24H | 30 days for up to 26 oz/A; 60 days for > 26 oz/A |
| chlorpyrifos + lambda– cyhalothrin | Cobalt Advanced | 1B, 3A | 16–38 fl oz/A | See label.Restricted use | 24H | 30 days for up to 26 oz/A and 60 days for > 26 oz/A |
| chlorpyrifos + zeta-cypermethrin | Stallion | 1B, 3A | 5.0–11.75 oz/A | Restricted use | 24H | 30 days |
| cyfluthrin | Tombstone 2 | 3A | 1.3–2.8 fl oz/A | Restricted use | 12H | 14 days |
| deltamethrin | Delta Gold | 3A | 1.5–1.9 fl oz/A | Not labeled for false chinch bug.Restricted use.Danger—Poison | 12H | 14 days |
| gamma– cyhalothrin | Proaxis Declare |
3A | 2.56–3.84 oz/A 1.02–1.54 oz/A |
Not labeled for false chinch bug.Restricted use | 24H | 30 days |
| lambda– cyhalothrin | Warrior II with Zeon, Karate with Zeon,generics |
3A | 1.28–1.92 oz/A | Apply no more than 6 oz of lambda-cyhalothrin-containing products once the crop has reached soft dough stage.Not labeled for false chinch bug.Restricted use | 24H | 30 days |
| lambda-cyhalotrhin + chlorantraniliprole | Besiege 1.25 SC | 3A, 28 | 6–10 oz/A | Do not exceed total of 18 fl oz/A per year.Apply no more than 6 oz of lambda-cyhalothrin-containing products once the crop has reached soft dough stage.Not labeled for false chinch bug.Restricted use | 24H | 30 days |
| zeta-cypermethrin | Mustang Maxx, generics | 3A | 1.76–4.0 oz/A | Restricted use | 12H | 14 days |
| 1: REI = Restricted entry interval 2: PHI = Preharvest interval |
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Young larvae feed for a few days on the leaves or leaf axes. Older larvae tunnel into the sorghum stalks; larvae bore up and down the pith of the stalk. Borer-infested stalks may be reduced in diameter and yield less. Larval tunneling just below the grain head can cause it to break and the grain head to fall. Injury by borers increases sorghum susceptibility to stalk rot diseases and lodging.
Plant sorghum early because borers are typically more abundant in late-planted sorghum. In northern Texas regions, shredding stalks very close to the ground or plowing and disking stubble destroys overwintering larvae of the southwestern corn borer by exposing them to cold temperatures. This practice reduces borer abundance the next year.
Check the plants carefully for stem borers. Look for small holes near the leaf axis, which indicate that a larva has entered the stalk. Once the larvae have entered the stalk, it must be split to see them. Inspect the leaves carefully—the eggs are hard to find. Clusters containing 10 to 20 individual eggs may be on the top or underside of leaves, depending on the borer species. Assess the abundance of eggs and small larvae before the larvae bore into stalks. Insecticidal control is effective only if applied before larvae bore into stalks.
Sugarcane rootstock weevil
The sugarcane rootstock weevil, Apinocis (Anacentrinus) deplanatus, infests sorghum sporadically, especially during dry years and in fields where johnsongrass is abundant. The adult weevil is dark brown or black, about 1/8 inch long and 1/16 inch wide (Fig. 35). It overwinters beneath plant residues on the ground. In early spring, the weevils infest wild grasses, such as johnsongrass, and later move to sorghum. The female uses its mouthparts to make a small puncture at the base of the plant, where the egg is deposited and concealed. It lays about 16 eggs, which hatch in 6 days. When fully grown, the larvae are white, legless grubs about 1/5 inch long. A generation is completed in about 40 days.
Adult weevils feed on young sorghum plants and create pinpoint holes in the leaves. The larvae cause the most damage as they tunnel into the sorghum stalk just above or below the soil surface. The larvae are often found at nodes and near the outer surfaces of the stalk. As a result of larval feeding, the plants appear drought stressed and may lodge. Pathogens can invade the plant through feeding tunnels. Although locally damaging populations may occur, economic thresholds for this pest have not been established, and control has usually not been required.