Many types of insects are found in alfalfa and clover. The beneficial insects include pollinators, parasites, and predators. Other insects, which feed on the leaves, stems, crowns, seed pods, and flowers, can cause economic loss when they become abundant.
Some alfalfa cultivars are resistant to certain aphids and diseases. For information on alfalfa varieties with resistance to disease and insect pests, along with other characteristics such as fall dormancy, see Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties, by the National Alfalfa and Forage Alliance, at http://alfalfa.org/pdf/2011NAFAVariety- Leaflet_small.pdf.
Field scouting: Each week during the growing season, check for insect activity in as much of the field as possible—at least four or five spots in each field. Pest populations can vary across fields, and your results may be inaccurate if you check only a limited area. Sample away from the field edges and avoid sampling when the fields are wet from rain or dew, which makes it difficult to detect insects.
Sweep net sampling: A standard 15-inch-diameter sweep net (Fig. 43) can be used to sample foliage-feeding caterpillars, alfalfa weevil larvae, and plant bugs in alfalfa:
- While walking through the field, take 10 consecutive 180-degree sweeps. Swing the net from side to side with each step. Hold the net so that you draw the lower half of the opening (7 to 8 inches) through the foliage. You should find some foliage and stems in the net; if not, you are not swinging it hard or deep
- For a good estimate of insect numbers, take five samples (each consisting of 10 sweeps), one from each quadrant of the field and one from near the Collect samples 30 to 50 feet or more from the field margin.
- Calculate the average number of alfalfa weevils, caterpillars, or plant bugs per sweep. Treatment thresholds based on the average number per sweep for each of these pests are presented
Beat bucket sampling: To determine the number of alfalfa weevil larvae or aphids per stem using the beat bucket method:
- Walk across the field and collect a single stem at 30 evenly spaced intervals. Cut each stem carefully at the base to avoid dislodging the larvae, and place the stem inside a small bucket. Collect each stem at random by cutting the first stem that your hand
- Vigorously beat the 30 stems against the side of the bucket for 20 to 30 seconds to dislodge all of the insects, which will accumulate in the bottom of the bucket. Small alfalfa weevil larvae, which feed between the folded terminal leaves, will not be dislodged. However, because they are small, these larvae do not pose an immediate threat of damage.
- Remove all of the stems from the bucket, count the stems and the insects, and determine the average number of weevil larvae and aphids per stem.
- For fields larger than 30 acres, take another 30 stem samples for each additional 30 acres, and average the results for the entire field.
The alfalfa weevil is primarily a pest of alfalfa but may also attack several species of clover. The larvae damage the plants by feeding on the leaves and buds, stunting plant growth and reducing forage yield and quality.
When small, the larvae are light yellow; large larvae are green with a white stripe down the back (Fig. 43). The head is a shiny black. Mature larvae are about ¼ inch long.
The adult alfalfa weevil is a snout beetle about ¼ inch long and brown with a dark brown band down the center of its back (Fig. 44).
Young larvae feed on the leaf buds and between folded leaflets in the plant terminal. Older larvae feed mostly on open leaflets, but they also feed on the terminal buds.
The larvae skeletonize the foliage (Fig. 45), which from a distance appears grayish to white.
Mature larvae then drop to the leaf litter and spin silken cocoons. The adult weevils emerge from the cocoons in 1 to 2 weeks.
Larvae can be present over a 6-to 8-week period from early February through March.
The new adults emerging in late spring feed for a short period and then leave the alfalfa field to go to nearby protected areas for a summer resting period, returning to the fields again in the fall.
The adults also feed on the leaves but cause less damage.
The first cutting is at the greatest risk for alfalfa weevil damage. However, in some years, the larvae and adults may threaten the second cutting and require treatment.
Begin sampling fields for young larvae in late February and early March. After a mild winter, the larvae may begin feeding earlier. Insecticide treatments may be necessary to protect crop yield if the number of larvae reaches the treatment thresholds (Tables 10 and 11). Sample larvae using a beat bucket or a sweep net as described above.
If the number of larvae is at or above the treatment threshold, and the crop is near harvest, consider harvesting early instead of applying an insecticide. When most of the plants are in the bud stage, cut the first crop as cleanly and closely as possible. Many larvae will die after exposure to the hot soil when the crop is cut. However, during cool, cloudy weather, they may survive under the windrows and damage the regrowth.
Scout for alfalfa weevil damage to re-growth, especially in the strips under the windrows. After cutting or if spring regrowth is short, a good way to sample alfalfa weevil larvae is to count the larvae per square foot.
Table 10. Treatment thresholds for alfalfa weevil larvae
|Plant height (inches)||Larvae per terminal/stem||Larvae per square foot||Larvae per sweep|
|Stubble2 (after cutting)||1||16||—|
1 In alfalfa within 1 to 2 weeks of cutting, it may be advisable to cut early rather than apply an insecticide.
2 Stubble treatment may be advisable if cloudy conditions and mild temperatures allow many weevils to survive on the stubble under the windrows.
Four species of aphids feed on alfalfa in Texas: the pea aphid, cowpea aphid, blue alfalfa aphid, and spotted alfalfa aphid.
Aphids are soft-bodied, slow-moving insects that live in colonies and suck plant sap from stems, leaves, and terminals. Infested plants turn yellow and wilt, and their growth can be stunted. Honeydew can accumulate on the leaves and stems.
Predatory insects and parasites, aphid diseases, and weather conditions often keep aphid numbers low. However, aphids have a high reproductive rate and can increase very rapidly under favorable conditions.
To sample aphid infestations, estimate the number of aphids per alfalfa stem by looking at the plants or using the beat bucket method as described above (page 15). Aphids dislodged by beating the stems into a beat bucket are easily viewed for estimating the number per stem.
Sample at least 30 stems at random for each 30-acre area within the field. Use the treatment thresholds to determine when an insecticide is justified to avoid crop loss from aphid feeding (Tables 12 and 13).
The most common aphid species in Texas alfalfa and clover crops is the pea aphid (Fig. 46). The adults are bright green with long legs. They are about 1/8 inch long, making them largest aphid species found in alfalfa.
The tip of each antennal segment is black, unlike those of blue alfalfa aphid, which has uniformly dark antennae.
Pea aphids congregate in dense colonies along the stems, terminal shoots, and leaves. Infested plants yellow and wilt. Often, crops are damaged the most in the spring. Honeydew is usually not abundant on infested plants.
Resistant cultivars are very helpful in reducing pea aphid damage. For varieties with some resistance, see Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties, by the National Alfalfa and Forage Alliance, at http://alfalfa.org/ pdf/2011NAFAVarietyLeaflet_small.pdf.
Use the treatment thresholds to determine when an insecticide is justified in established alfalfa to avoid crop loss (Tables 12 and 13). For seedling plants, the treatment threshold is an average of five pea aphids per plant.
Blue alfalfa aphid
The blue alfalfa aphid (Fig. 47) looks much like the pea aphid but is blue-green and slightly smaller, and the antennae are uniformly dark. The pea aphid is bright green and has a dark band at the tip of each antennal segment.
Blue aphid populations tend to build up in the early spring but decline when tempera- tures exceed 85°F. Feeding may severely stunt the plants and deform the leaves of new alfalfa regrowth less than 6 inches tall when temperatures are below 75°F. The leaves yellow as the plants die. Infestations often congregate at the top of each stem.
Several alfalfa cultivars are resistant to this pest. For varieties with some resistance, see Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties, by the National Alfalfa and Forage Alliance, at http://alfalfa.org/pdf/2011NAFAVariety- Leaflet_small.pdf.
Scout for blue alfalfa aphids in dormant alfalfa early in the spring. Use the treatment thresholds to determine when an insecticide is justified in fields of established alfalfa (Tables 12 and 13). For seedling plants, the treatment threshold is an average of one blue aphid per plant.
Cowpea aphids (Fig. 48) are easily distinguished from all other aphids found in alfalfa by their dark blue-black color. The adults are shiny, the nymphs dull. The legs of both stages are off-white with black tips.
This species injects a toxin while feeding that can stunt and kill plants. It also produces large amounts of sticky honeydew.
Cowpea aphids typically increase to damaging numbers in the early spring when the alfalfa is still dormant. When the plants break dormancy, those infested plants fail to grow, and some wilt and die.
Resistant cultivars can help reduce cowpea aphid damage. For varieties with some resistance, see Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties, by the National Alfalfa and Forage Alliance, at http://alfalfa.org/ pdf/2011NAFAVarietyLeaflet_small.pdf.
Scout for cowpea aphids in dormant alfalfa in the spring. Use the treatment thresholds to determine when an insecticide is justified to avoid crop loss (Tables 12 and 13).
Spotted alfalfa aphid
The spotted alfalfa aphid is 1/16 inch long and grayish yellow to yellow-green (Fig. 49). It has four to six rows of raised dark spots on the back.
These aphids are usually found on the undersides of the lower leaves. However, as the population increases, they infest all parts of the plant.
As it feeds, this insect causes a toxic reaction, injuring the plant and even killing seedlings. On established stands, the plants are stunted severely, and yellow (chlorotic) areas appear on the leaves. The veins of newly formed leaves often become discolored, a symptom known as vein banding.
Spotted alfalfa aphids secrete large amounts of honeydew, which interferes with cutting and baling and degrades hay quality. When disturbed, the aphids fall from the alfalfa plants. The insects can also increase during the fall on seedling alfalfa plants.
Resistant cultivars offer varying degrees of protection from yield and stand losses from the spotted alfalfa aphid. For varieties with resistance, see Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties, by the National Alfalfa and Forage Alliance, at http://alfalfa.org/ pdf/2011NAFAVarietyLeaflet_small.pdf.
Scout for spotted alfalfa aphids on the underside of the leaves, Also look for them near their honeydew, in stunted plants, and near the aphids’ characteristic vein banding injury. Use the treatment thresholds to determine when an insecticide is justified in fields of established alfalfa (Tables 12 and 13).
In the fall, look for increases of these aphids on alfalfa seedlings. For seedling plants, the treatment threshold is an average of one to three spotted aphids per plant.
Although blister beetles do not damage the crop, they contain a chemical, cantharidin, that when consumed is poisonous to livestock, especially horses and chickens. Harvest machinery can pick up live and dead blister beetle adults and incorporate them into hay bales.
Most species of adult blister beetles are narrow, cylindrical, and relatively soft-bodied with long antennae and legs. The head is always distinctly wider than the neck. This feature will easily separate blister beetles from the other beetles commonly found in alfalfa.
The striped blister beetle, Epicauta spp., is common in alfalfa. The beetle is yellow and ⅜ to ¾ inch long and has three dark stripes on each wing cover (Fig. 50). It is active on the plant in the morning and late afternoon. On hot days, it may shelter low in the canopy.
Other species of blister beetles are black, brown, gray, reddish, and metallic green and blue. (Fig. 51). Photos of many blister beetle species found in Texas are posted at http://texasento.net/TXMeloidae.html
Blister beetles feed primarily on the blooms and tender leaves of alfalfa and many other plants. The immature stage feeds on grasshopper eggs.
Monitor alfalfa fields for blister beetles before cutting hay, especially during June through September, when the beetles usually move into the fields. The risk of blister beetle infestation is generally lower in the first cutting. Blister beetles are best sampled using a sweep net (Figure 1).
Begin field monitoring at least a week before cutting and continue throughout the baling process. Pay close attention to blooming fields, as they are very attractive to blister beetles.
Adult beetles are mobile, and some species are prone to congregate in one or a few small spots within a field. Leave infested areas unharvested or spray them with an insecticide spot treatment (Table 14).
All of the insecticides labeled for blister beetle control are toxic to honey bees and other insect pollinators. Refer to the insecticide label for precautions and restrictions regarding pollinators.
Blister beetles are attractive to blooming fields, where they feed on flowers. Therefore, cutting hay before or at 5 percent bloom can reduce the risk of blister beetle infestations. Striped blister beetles typically congregate into groups or “swarms” of beetles. While cutting, closely watch for swarms and avoid cutting areas of the field
To reduce the risk of dead beetles being baled into the hay, remove the crimper device, which can crush the beetles into the hay, from the swathers. Cutting without a crimper allows the living beetles to leave the windrow before the hay is baled.
Also, raking the hay before bailing can help dislodge dead beetles from the hay. Because wheel traffic can crush beetles into the hay, avoid driving over standing or freshly cut hay.
Note that some insecticides have a 7- to 21-day waiting period before harvest, which allows time for the beetles to move back into the field. For this reason, inspect the fields again before harvest to determine if the beetles have reinfested treated fields.
Larvae of the alfalfa caterpillar, several armyworm species, the corn earworm, and webworms feed on the tender stems and leaves of alfalfa. The sweep net method (page 15) is used to sample most foliage-feeding caterpillars.
The alfalfa caterpillar is the larval stage of a yellow butterfly (Fig. 52) commonly seen in alfalfa fields during late spring and summer. The adult has a 2-inch wingspan and black margins on the wings.
Female alfalfa butterflies lay eggs singly on the underside of alfalfa leaves. The eggs hatch in a few days, and the larvae feed on the leaves for 12 to 15 days before pupating.
Mature larvae are about 1½ inches long and dark, velvety green with white stripes along each side (Fig. 53). They have large, rounded heads.
The alfalfa caterpillar is usually most abundant in mid to late summer.
Early cutting is an option for controlling alfalfa caterpillars. Sample using a sweep net (page 15). Consider applying an insecticide (Table 15) if sweep net samples average seven or more foliage-feeding caterpillars per sweep or if defoliation by these caterpillars exceeds 10 percent.
Armyworms are the immature stages of dull-colored moths (Fig. 54). The beet and fall armyworms (Figs. 55 and 56) are commonly found on alfalfa and clover crops; the yellow-striped armyworm is an occasional pest in alfalfa. Armyworm infestations are usually most severe in mid to late summer.
Armyworms lay masses of several hundred eggs each, which hatch in 2 to 3 days. The larvae feed in groups when they are young and disperse as they mature. Full-grown larvae are 1½ inches long. The larvae feed for about 3 weeks before crawling to the soil where they pupate.
Sample using a sweep net (page 15). Consider an insecticide treatment (Table 15) if the samples average seven or more foliage-feeding caterpillars per sweep or if defoliation by these caterpillars exceeds 10 percent. Early cutting is an option for controlling armyworm caterpillars.
Corn earworm larvae are greenish with light brown heads (Fig. 57). Fully developed worms are about 1½ inches long and pale green, pinkish, or brown. Unlike the fall armyworm, these caterpillars do not have an inverted “Y” pattern between their eyes (Fig. 58), and they lay eggs singly. The larvae are usually are most abundant from July through September.
The adult or moth stage is yellowish brown with a dark band near the wing margin (Fig. 59).
Consider an insecticide treatment (Table 15) if sweep net samples average seven or more foliage-feeding caterpillars per sweep or if defoliation by these caterpillars exceeds 10 percent. Early cutting is an option for controlling corn earworm caterpillars.
The alfalfa webworm and garden webworm (Fig. 60) feed on alfalfa, clover, cowpeas, peas, and similar crops as well as on several weed species, especially pigweed.
Webworm larvae create flimsy webs in the tops of plants and feed within the webs. They completely skeletonize the leaves. Webworms occasionally cause severe damage to alfalfa, primarily in East Texas.
Harvest early if the infested crop is near the cutting stage. The action level for applying an insecticide is when the crop is more than 2 weeks from cutting and 25 to
30 percent of the plant terminals are infested with webworms (Table 14). Insecticides can be less effective when the webs and larvae are large.
Army cutworm larvae are pale gray and up to 2 inches long (Fig. 62). Female moths (Fig. 62) lay eggs on the soil in the fall, and young larvae hibernate in the soil over the winter.
Army cutworms begin feeding on alfalfa leaves as the weather warms in late winter and early spring. Most damage occurs before the first cutting and to newly planted stands. They feed above the soil surface from late afternoon until daylight the next morning. During bright sunshine, they hide in the soil and under clods, alfalfa crowns, and field debris.
An indication that army cutworms may be damaging alfalfa is the failure of a field to begin growing in the spring. This cutworm can also severely damage newly planted alfalfa. Scout the fields closely for this pest.
and aphids when the alfalfa shoots start growing in late winter and early spring.
In seedling alfalfa, the threshold for control is two cutworms per square foot. In established alfalfa, it is three or more per square foot when the larvae are ½ inch long or less, and two or three larvae per square foot when they are longer than ½ inch long (Table 15).
The variegated cutworm larva is pale gray to light brown and has four to seven pale yellow diamond-shaped spots aligned along the center of its back (Fig. 63).
Full-grown larvae are 1½ to 2 inches long. When disturbed, the larvae curl into a C-shape.
These cutworms feed on alfalfa leaves and stems but may also cut plants off at the soil surface. They feed at night and hide beneath loose soil and plant debris during the day.
The forewings of the adults are grayish brown with pale, oval markings near the wing edge; the hind wings are white with brown markings (Fig. 64).
Damaging infestations typically occur during the spring.
Scout the fields closely for this pest and aphids when alfalfa shoots begin growing in early spring. They may also damage regrowth after the first cutting.
Look for larvae beneath loose soil and plant debris during the day. The larvae may also congregate under windrows of cut hay where they feed on the regrowth.
Consider an insecticide treatment (Table 15) if you find an average of two or more larvae per square foot and the larvae are 1½ inches long or less. Larger larvae have almost completed feeding and are less susceptible to insecticides.
Other alfalfa pests
Three-cornered alfalfa hopper
The adult three-cornered alfalfa hopper (Fig. 65) is a green, wedge-shaped insect that is about ¼ long and flies when disturbed. The nymphs (immatures) lack wings and have saw-toothed spines on the back (Fig. 66).
This insect feeds on plant sap by repeatedly puncturing the stem and creating a series of feeding wounds that girdles the stem just above the soil surface. Girdled stems become stunted and weakened and can break over (Fig. 67). The leaves on affected stems turn red underneath and light yellow-green on top. Damaged stems that develop these symptoms usually die. These leaf symptoms can mimic those of boron deficiency.
Most of the feeding damage is caused by the nymphs, the immature stage.
Three-cornered alfalfa hoppers also feed on soybeans, peanuts, and other legumes.
Look for girdled stems and examine crowns near the soil surface for nymphs feeding on stems and girdled stems that have not yet fallen over. Consider treat- ment when 10 percent of the stems are girdled.