Beet Armyworm Control: Information On Treating And Preventing Armyworms
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By: Jackie Carroll
Beet armyworms are green caterpillars that feed on a wide range of ornamental and vegetable plants. The young larvae feed in groups and usually don’t have any unique markings to distinguish them from other caterpillars. However, older larvae develop a yellow stripe that runs from head to tail, making it easy to identify them.
It is important to detect and treat a beet armyworm infestation early because these older caterpillars are resistant to most insecticides. Keep reading to learn more on identifying a beet armyworm infestation and preventing armyworms in the garden.
What are Beet Armyworms?
Beet armyworms (Spodoptera exigua) are caterpillars that feed on tender vegetable crops and a few ornamentals. They are normally found only in southern states and warm, coastal climates where the host plants survive through the winter.
The adult form is a medium-sized moth with mottled gray and brown upper wings and white or pale gray lower wings. They lay fluffy masses of up to 80 eggs on the crowns of seedlings or on the tender leaves of older plants where the young caterpillars will have plenty of food when they hatch. The larvae slowly move to the ground to pupate on the soil.
Identifying Beet Armyworm Damage
Beet armyworms eat irregular holes in foliage, eventually skeletonizing the leaves. They can eat tender young transplants to the ground and defoliate older plants. They burrow into heading vegetables, such as lettuce and cabbage. Beet armyworms also leave gouges in tender fruit, especially tomatoes.
Early detection aids in preventing armyworms. Watch for masses of eggs covered with fluff, small caterpillars feeding in groups, or single large caterpillars with a yellow stripe running down their sides.
Beet Armyworm Control
Beet armyworm control in the home garden begins with handpicking. Drop the caterpillars into a container of soapy water to kill them and then bag and discard the carcasses.
Bacillus thuringiensis (Bt-azaiwi strain) and spinosad are natural insecticides that are effective against young armyworms and don’t harm the environment.
These caterpillars are resistant to most chemical insecticides that are available to the home gardener, but neem oil products are sometimes effective. The eggs, which are covered by a cottony or fibrous mass, are susceptible to treatment with petroleum oils.
If you decide to try insecticides, carefully read and follow the label instructions. Pay particular attention to the length of time between treatment and harvest when treating beet armyworms on vegetable plants. Store all insecticides in their original container and keep out of the reach of children.
Now that you know more about what beet armyworms are and armyworm control, you can better manage or even prevent their presence in the garden.
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Controlling And Identifying Beet Armyworm Damage - garden
The beet armyworm originated in Southeast Asia. It was first discovered in North America about 1876, when it was found in Oregon, and it reached Florida in 1924. It rarely overwinters in areas where frost kills its host plants. Thus, overwintering is generally limited to Arizona, Florida, and Texas. Nevertheless, beet armyworm invades the southern half of the United States (Maryland to Colorado to northern California, and south) annually. Except in greenhouses, it rarely is a pest except in southern states.
Description and Life Cycle (Back to Top)
Seasonal activity varies considerably according to climate. In warm locations such as Florida, all stages can be found throughout the year, although development rate and overall abundance are reduced during the winter months (Tingle and Mitchell 1977). The life cycle can be completed in as few as 24 days, and six generations have been reared during five months of summer weather in Florida (Wilson 1934).
Figure 1. Newly hatched larvae of the beet armyworm, Spodoptera exigua (Hübner). Photograph by Lyle J. Buss, University of Florida.
Egg: Eggs are laid in clusters of 50 to 150 eggs per mass. Normal egg production is about 300 to 600 per female. Eggs are usually deposited on the lower surface of the leaf, and often near blossoms and the tip of the branch. The individual eggs are circular when viewed from above, but when examined from the side the egg is slightly peaked, tapering to a point. The eggs are greenish to white in color, and covered with a layer of whitish scales that gives the egg mass a fuzzy or cottony appearance. Eggs hatch in two to three days during warm weather.
Larva: There normally are five instars, although additional instars are sometimes reported. Duration of the instars under warm (summer) conditions is reported to be 2.3, 2.2, 1.8, 1.0, and 3.1 days, respectively (Wilson 1932), and at constant 30°C instar development time was reported by Fye and McAda (1972) to be 2.5, 1.5, 1.2, 1.5, and 3.0 days, respectively. Head capsule widths average 0.25, 0.45, 0.70, 1.12, and 1.80 mm, respectively.
The larvae are pale green or yellow in color during the first and second instars, but acquire pale stripes during the third instar. During the fourth instar, larvae are darker dorsally, and possess a dark lateral stripe. Larvae during the fifth instar are quite variable in appearance, tending to be green dorsally with pink or yellow color ventrally and a white stripe laterally. A series of dark spots or dashes is often present dorsally and dorsolaterally. Sometimes larvae are very dark in color, even black. The spiracles are white with a narrow black border. The body is practically devoid of hairs and spines. In the southern states, the larva of beet armyworm is easily confused with southern armyworm, Spodoptera eridania (Cramer), but southern armyworm can be distinguished by the presence of a large dark spot laterally on the first abdominal segment that disrupts the lateral stripe. Beet armyworm occasionally bears a spot laterally, but if present it occurs on the mesothorax, not on the first abdominal segment.
Figure 2. Partly grown larva of the beet armyworm, Spodoptera exigua (Hübner). Photograph by John Capinera, University of Florida.
Pupa: Pupation occurs in the soil. The chamber is constructed from sand and soil particles held together with an oral secretion that hardens when it dries. The pupa is light brown in color and measures about 15 to 20 mm in length. Duration of the pupal stage is six to seven days during warm weather.
Adult: The moths are moderately sized, the wing span measuring 25 to 30 mm. The forewings are mottled gray and brown, and normally with an irregular banding pattern and a light colored bean-shaped spot. The hind wings are a more uniform gray or white color, and trimmed with a dark line at the margin. Mating occurs soon after emergence of the moths, and oviposition begins within two to three days. Oviposition extends over a three to seven day period, and the moths usually perish within nine to 10 days of emergence. Heppner (1998) provides a key to the adults of North American Spodoptera.
Figure 3. Mature larva of beet armyworm, Spodoptera exigua (Hübner). Photograph by John Capinera, University of Florida.
Figure 4. Adult beet armyworm, Spodoptera exigua (Hübner). Photograph by Lyle J. Buss, University of Florida.
Host Plants (Back to Top)
The beet armyworm has a wide host range, occurring as a serious pest of vegetable, field, and flower crops. Among susceptible vegetable crops are asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip. Field crops damaged include alfalfa, corn, cotton, peanut, safflower, sorghum, soybean, sugarbeet, and tobacco. Weeds also are suitable for larval development, including such common plants as lambsquarters, Chenopodium album mullein, Verbascum sp. pigweed, Amaranthus spp. purslane, Portulaca spp. Russian thistle, Salsola kali parthenium, Parthenium sp. and tidestromia, Tidestromia sp.
As with many polyphagous insects, although the host range is potentially very broad, the insect has definite food preferences, and the suitabilities of the food plants vary as well. Greenberg et al. 2001 compared growth of beet armyworm larvae on three crops (cabbage, cotton, pepper) and two weeds (redroot pigweed, sunflower). They reported that the level of relative consumption was cabbage, followed by pepper, sunflower, cotton, and pigweed. Relative growth, judged by pupal weights, was greatest on pigweed, equal for cotton, cabbage, and pepper, and least on sunflower. The larvae also matured fastest on pigweed. The authors interpreted these results to indicate that pigweed was nutritionally superior, and that cabbage was the poorest host.
Damage (Back to Top)
Larvae feed on both foliage and fruit. In Florida it is regarded as a serious defoliator of flower crops and cotton, though much of the injury is induced by insecticide use that interferes with natural enemy activity. Young larvae feed gregariously and skeletonize foliage. As they mature, larvae become solitary and eat large irregular holes in foliage. They also burrow into the crown or center of the head on lettuce, or on the buds of cole crops. As a leaf feeder, beet armyworm consumes much more cabbage tissue than the diamondback moth, Plutella xylostella (Linnaeus), but is less damaging than the cabbage looper, Trichoplusia ni (Hübner) (East et al. 1989), as the latter grows to be a much larger caterpillar.
Tomato fruit is quite susceptible to injury, especially near fruit maturity, but beet armyworm is not considered to be as threatening to tomato as is the corn earworm, Helicoverpa zea (Boddie) (Zalom et al. 1986). In tomato crops, infestation early in the growing period is more damaging than later infestation, and as little as 1 caterpillar per 20 tomato plants can cause economic loss (Taylor and Riley 2008)
Sampling (Back to Top)
Pheromone traps can be used to detect the presence of adult beet armyworm. Visual sampling for damage and larvae, combined with an action threshold of 0.3 larvae per plant, was used successfully on cabbage in south Texas to determine the need for crop treatment with insecticides (Cartwright et al. 1987). Regular monitoring of crops, probably about twice per week, is recommended because adults frequently invade from surrounding crops or weeds.
Natural Enemies (Back to Top)
Numerous native natural enemies have adapted to this pest. Among the most common parasitoids are Chelonus insularis Cresson, Cotesia marginiventris (Cresson), and Meteorus autographae (Muesbeck) (all Hymenoptera: Braconidae), and the tachinid Lespesia archippivora (Riley) (Diptera: Tachinidae) (Oatman and Platner 1972, Ruberson et al. 1994). Predators frequently attack the eggs and small larvae among the most important are minute pirate bugs, Orius spp. (Hemiptera: Anthocoridae) bigeye bugs, Geocoris spp. (Hemiptera: Geocoridae) damsel bugs, Nabis spp. (Hemiptera: Nabidae) and a predatory stink bug, Podisus maculiventris (Say). Pupae are subject to attack, especially by the red imported fire ant, Solenopsis invicta Buren. Fungal diseases, Erynia sp. and Nomurea rileyi, and a nuclear polyhedrosis virus also inflict some mortality (Wilson 1933, 1934 Ruberson et al. 1994). The important mortality factors vary among crops, and among geographic regions. None except the nuclear polyhedrosis virus are highly specific to beet armyworm, which may explain why they are not especially effective. Virus is considered to be the most important mortality factor.
Insecticides (Back to Top)
In the Southeast and Southwest, the relatively high abundance of beet armyworm has stimulated frequent application of insecticides to foliage. Insecticide resistance is a major problem in management of this insect, possibly because it attacks crops such as flowers, cotton, and vegetables - crops that are treated frequently with insecticides. Beet armyworm abundance is favored by frequent insecticide use, and it is considered to be a secondary or induced pest in some crops (Eveleens et al. 1973). Also, intensive use of insecticides for beet armyworm control in vegetables such as celery has stimulated outbreaks of other pests, principally American serpentine leafminer, Liriomyza trifolii (Burgess).
Beet armyworm larvae are susceptible to neem products (Prabhaker et al. 1986). Eggs can be killed with petroleum oil, and both eggs and young larvae can be controlled with foliar applications of 5% cottonseed oil, but this concentration is damaging to some plants (Butler and Henneberry 1990).
Pheromones can also be used to disrupt mating and inhibit or eliminate reproduction. Saturation of the atmosphere around beet armyworm-susceptible crops has been estimated to reduce mating by 97% (Wakamura and Takai 1992).
Biological Control (Back to Top)
Several insect pathogens may prove to be useful for suppression of beet armyworm. A nuclear polyhedrosis virus isolated from beet armyworm is fairly effective as a bioinsecticide under greenhouse conditions, where inactivation by ultraviolet light in sunlight is not a severe problem. The fungus Beauveria bassiana has the same attributes and limitations. Entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae) successfully infect both larvae and adults of beet armyworm, though it is difficult to attain suppression of foliage-feeding insects under field conditions because the nematodes are favored by moisture. Thus, the nematodes are more effective at killing the larvae when they drop to the soil to pupate the prepual stage is more susceptible than the pupal stage.
Selected References (Back to Top)
- Butler Jr. GD, Henneberry TJ. 1990. Cottonseed oil and Safer insecticidal soap: Effects on cotton and vegetable pests and phytotoxicity. Southwestern Entomologist 15: 257-264.
- Capinera JL. 2001. Handbook of Vegetable Pests. Academic Press, San Diego. 729pp
- Cartwright B, Edelson JV, Chambers C. 1987. Composite action thresholds for the control of lepidopterous pests on fresh-market cabbage in the lower Rio Grande Valley of Texas. Journal of Economic Entomology 80: 175-181.
- East DA, JV Edelson, Cartwright B. 1989. Relative cabbage consumption by the cabbage looper (Lepidoptera: Noctuidae), beet armyworm (Lepidoptera: Noctuidae), and diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology 82: 1367-1369.
- Eveleens KG, van den Bosch R, Ehler LE. 1973. Secondary outbreak induction of beet armyworm by experimental insecticide applications in cotton in California. Environmental Entomology 2: 497-503.
- Fye RE, McAda WC. 1972. Laboratory studies on the development, longevity, and fecundity of six lepidopterous pests of cotton in Arizona. USDA Technical Bulletin 1454. 73 pp.
- Greenberg SM, Sappington TW, Legaspi Jr BC, Liu T-X, Sétamou M. 2001. Feeding and life history of Spodoptera exigua (Lepidoptera: Noctuidae) on different host plants. Annals of the Entomological Society of America 94: 566-575.
- Heppner JB. 1998. Spodoptera armyworms in Florida (Lepidoptera: Noctuidae). Florida Department of Agriculture and Consumer Services, Division of Plant Industry Entomological Circular 390. 5 p.
- Oatman ER, Platner GR. 1972. An ecological study of lepidopterous pests affecting lettuce in coastal southern California. Environmental Entomology 1: 202-204.
- Prabhaker N, Coudriet DL, Kishaba AN, Meyerdirk DE. 1986. Laboratory evaluation of neem-seed extract against larvae of the cabbage looper and beet armyworm (Lepidoptera: Noctuidae). Journal of Economic Entomology 79: 39-41.
- Ruberson JR, Herzog GA, Lambert WR, Lewis WJ. 1994. Management of the beet armyworm (Lepidoptera: Noctuidae) in cotton: Role of natural enemies. Florida Entomologist 77: 440-453.
- Taylor JE, Riley DG. 2008. Artificial infestations of beet armyworm, Spodoptera exigua (Lepidoptera: Noctidae) used to estimate an economic injury level in tomato. Crop Protection 27: 268-274.
- Tingle FC, Mitchell ER. 1977. Seasonal populations of armyworms and loopers at Hastings, Florida. Florida Entomologist 60:115-122.
- Wakamura S, Takai M. 1992. Control of the bet armyworm in open fields with sex pheromone. Pages 115-125 in N.S Talekar (ed.) Diamondback Moth and other Crucifer Pests. Asian Research and Development Center, Taipei, Taiwan.
- Wilson JW. 1932. Notes on the biology of Laphygma exigua Huebner. Florida Entomologist 16: 33-39.
- Wilson JW. 1933. The biology of parasites and predators of Laphygma exigua Hübner reared during the season of 1932. Florida Entomologist 17: 1-15.
- Wilson JW. 1934. The asparagus caterpillar: its life history and control. Florida Agricultural Experiment Station Bulletin 271: 1-26.
- Zalom FG, Wilson LT, Hoffmann MP. 1986. Impact of feeding by tomato fruitworm, Heliothis zea (Boddie) (Lepidoptera: Noctuidae), and beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on processing tomato fruit quality. Journal of Economic Entomology 79: 822-826.
Author: John L. Capinera, University of Florida
Photographs: John L. Capinera, University of Florida
Web Design: Don Wasik, Jane Medley
Publication Number: EENY-105
Publication Date: July 1999. Latest revision: May 2017. Reviewed: June 2020.
An Equal Opportunity Institution
Featured Creatures Editor and Coordinator: Dr. Elena Rhodes, University of Florida
Cut Cross-Striped Cabbageworms, Beet Armyworms, and Cutworms
Cutworms attack a variety of veggies in your garden, so you may be more familiar with them. Cross-striped cabbageworms and beet armyworms also feed on your cabbage, though. All three of these “worms” are actually caterpillars, the larval form of various moths. Eliminate them by drawing a line on cutting attacks by either the moths, the worms, or both.
- Cross-Striped Cabbageworms
- Identifying the Cross-Striped Cabbageworm and the Cross-Striped Cabbage Moth
- Damage Caused by the Cross-Striped Cabbageworm
Cross-striped cabbageworms may be less familiar to you because, originally, their range was limited to the southern United States. Over the past several years, though, this caterpillar has been eating its way up the East Coast into New England and also has been found in Illinois and along the West Coast.
Identifying the Cross-Striped Cabbageworm and the Cross-Striped Cabbage Moth
Both the cross-striped cabbageworm and the cross-striped cabbage moth have distinctive, bold markings.
The thick-bodied caterpillar grows to about 3/4 of an inch long. Their bodies are bluish-gray with contrasting black stripes across the width of their backs that end in the vivid back and yellow stripes along the length of their bodies.
The cocoons are small. You’ll find the cocoons just below the soil’s surface. The pupa inside will be yellowish-brown.
The moths have brown to yellowish brown wings with purplish-brown to olive markings and darker, narrow, zig-zagged stripes. The wings reach out to a wingspan of 1 inch.
Unlike the moths of some other species that feed on cabbage, the cross-striped cabbage moth lays its eggs in clusters of 20 to 30 on the underside of the cabbage leaves. The eggs are a flattened, yellow ovals, and they overlap each other, which gives them a fish-scale-like appearance.
Damage Caused by the Cross-Striped Cabbageworm
Cross-striped cabbageworms will feed on any tender part of your cabbage plants. They will eat holes in the leaves until nothing is left of the leaves of infested plants but the ribs. They also will bore through the heads. They much prefer the terminal buds. They are most prevalent late in the growing season, and they can leave infested plants with nothing but a skeleton of stems and veins while neighboring plants remain untouched.
Identifying Beet Armyworms
Beet armyworms are the caterpillar of the small mottled willow moth. They are usually found in warmer environments, such as the southern United States, and they are most prevalent during late harvests.
Beet armyworms pass through five or more growth stages with changes in coloration at each stage.
Newly hatched caterpillars feed as a group near the egg mass. They remain pale green during the first two stages of growth.
Fine, wavy lines appear crossing the back during the third stage of development.
The backs of the caterpillars become darker during the fourth stage of growth, and darker line appears along each side.
The fifth stage of growth introduces many variations.
- Usually, the backs of the caterpillars remain green while a white stripe may appear along each side below the dark stripe.
- A row of dark spots or dashes may also appear along each side above the dark stripe.
- A larger dark spot usually appears on each side of these caterpillars above the second pair of true legs.
- Below the white stripe, the caterpillars may show pink or yellow markings, or the underside may be colored pink or yellow.
- Others of these caterpillars may be much darker. Some are even black.
As these caterpillars feed, they trail silk threads behind them, creating webs of threads on your cabbage leaves.
As they grow, they become extremely mobile, wandering farther and farther from the egg mass and spreading to other plants. They also become more resistant to pesticides as they grow, so it is important to identify and deal with an infestation early.
The Pupa or Cocoon
You’ll find the pupae in the soil around your plants. It will be light brown and measure about 1/2 inch to 3/4 inch in length. However, beet armyworms are only able to overwinter as pupae in the southern states or in warmer coastal areas, which is their normal range. When they are found in other areas, it is because the moths have flown there. The first hard frost will kill the pupae.
The moth is gray or brown. The upper wings are mottled brown with a yellow dot at the center while the lower wings are white with brown veins and an inner brown border and outer white border. The wingspan ranges from 1 to 1-1/4 inches.
The eggs range from greenish to white. The moths lay clusters of 40-80 eggs, and then covers them with a layer of cotton-like white scales. You will usually find them near the tip of a stem on the underside of a leaf.
Damage Caused by Beet Armyworms
A cluster of young beet armyworms feeding together can leave only the stems and veins of your cabbage plants. As the more mature caterpillars separate to feed on their own, they may burrow into the cabbage heads or eat large, irregular holes in the leaves.
Cutworms are the nocturnal caterpillar form of several dark-winged, nocturnal, brown or gray moths that lay their eggs on cabbages, weeds, grass, and other plants.
Although there are several species, their smooth, gray skins tend to get them lumped together. Their greasy look, and the type of damage they do when feeding on foliage, also means that they can be mistaken for grubs.
Damage Done by Cutworms
During the day, cutworms burrow into underground tunnels where they feed on the roots and stems of your cabbage plants. They can kill plants if they cut through the stalk. At night, cutworms emerge to feed on young plants. Usually, they eat the stems, but they also sometimes feast on the foliage.
Drawing the Line on Cutting Attacks From Moth and Caterpillar Armies
Creating a less favorable habitat for moths and caterpillars, creating an inviting environment for natural predators, using organic pesticides, and surrounding cabbage with protective companion plants all work together to defeat attacks by cutworms, beet armyworms, and cross-striped cabbageworms.
Creating a Less Favorable Habitat
Creating a less favorable habitat for the next growing season begins with the growing season that precedes it.
During the growing season, immediately remove and discard any cabbage plants – or kale, broccoli, cauliflower, Brussels sprouts, or collard plants – that become diseased or infested with insects. These closely related veggies share most of the same pests and diseases. Eliminating diseased or infested plants prevents those pests and diseases from taking hold in your garden.
At the end of the growing season, either completely clear the roots and stems of these related plants from your garden and add them to your compost or plow them into the soil.
Tilling the soil in your garden at the end of the season also brings the pupae buried in the soil to the surface where they will be exposed to the cold temperatures of winter, killing them.
If you have an area of brush near your garden, many of the moths that lay the eggs that hatch into the caterpillars that attack your cabbage and other plants will feel you have provided them with a resort hotel for the winter. Mow the area down to eliminate this cozy winter habitat.
Each year, when you plan your garden for the next season, rotate cabbage and its sibling plants with unrelated plants. Rotating these plants to a different location each year makes it less likely that pests, fungal infections, and other diseases will become a recurrent problem.
Have row covers ready, and close them as soon as you transplant your seedlings in the spring. Be sure to bury the edges, because that is the only way to be certain that moths cannot crawl under them to lay their eggs.
Diatomaceous earth will puncture the bodies of any caterpillars or soft-bodied insects that crawl over it if it’s sprinkled around the base of your cabbage plants.
If you inspect your cabbages regularly when the caterpillars will be the most active, you have a good chance of stopping an infestation early. You can simply remove the eggs and caterpillars by hand. Dropping the caterpillars in a bucket of water and dishwashing detergent will kill them.
Creating an Inviting Environment for Natural Predators
The pests that prey on your cabbage have natural predators of their own that can help control them.
You can purchase trichogramma wasps to release in your garden. These parasitic wasps won’t sting humans, but their larva prey on caterpillars. Planting small, nectar-bearing flowers, such as sweet alyssum, provides food for the wasps and encourages them to remain.
Fireflies and birds prey on cutworms, but fireflies need shrubs or low trees to roost in during the day. Plant trees or shrubs around your garden to attract more fireflies, but keep them well-trimmed so that air and light can reach the inner branches to avoid creating a winter habitat for moths.
Selecting Organic Pesticides
Bacillus thuringiensis is a common pathogen that targets cutworms and other caterpillars when it is ingested. It remains active for 24-48 hours after application, and it’s safe for humans and pets. When applying it, though, avoid spraying flowers serve as sources of nectar for beneficial insects.
Various flours reportedly cause caterpillars to bloat and die. Try mixing 1cup of flour and 1/2 cup of table salt, and then sprinkle this powder on your cabbage leaves in the morning when they are still covered with dew.
Citrus spray is more of a repellant than a pesticide. Add the ground up seeds and rinds of any type of citrus fruit to 2 cups of water, and allow the mixture to sit overnight. Strain the pulp, and then add 2 teaspoons of dishwashing liquid. The bitter chemicals and citrus taste drive caterpillars away.
While garlic oil spray is a frequently mentioned organic pesticide, it is used more for aphids, cabbage loopers, and other pests than cutworms, beet armyworms, or cross-striped cabbageworms.
Surrounding Cabbage With Protective Plants
Companion plants can help repel cutworms, beet armyworms, and cross-striped cabbageworms away from your cabbages, or they can lure them to another area of your garden. As mentioned earlier, other plants that may not benefit cabbage directly provide food and habitat for beneficial insects.
Tansy, for example, repels cutworms.
Sweet alyssum and other plants with small flowers can be regarded as companion plants for cabbage because the draw trichogramma wasps to your garden.
Shrubs and trees that attract birds and fireflies also serve as companion plants for cabbage.
How to Get Rid of Armyworms in your Garden
It’s so exciting when you spot your first red tomato of the season. But when you see an armyworm caterpillar in your vegetable garden, there is no time to spare. They can devour a tomato plant in record time. They don’t discriminate. They will eat the leaves and the fruit, red or green. They don’t just attack tomatoes either — they can destroy most of your vegetable garden.
Armyworms can remain hidden from site because they hide in the shade of underside of the leaves during the day. Their larvae vary depending on the type of armyworm. There are three types of armyworms:
Western Yellowstriped Armyworm – These have bright yellow stripes running down the sides. Females lay eggs in clusters covered by a cottony-type material.
Beet Armyworm – These are pale green with light colored stripes that run down the length of the worm. The eggs are laid and covered with hair-like scales.
Yellowstriped Armyworm – These have two large and many smaller yellow stripes. The larvae are black.
Once you spot them there are five strategies for saving your garden, but act fast!
1.) Pull them off the plant and drop them in a bucket of soapy water, or stomp on them. Be sure they are dead, so they don’t just crawl right back into your garden. This can be a fun activity for kids in the garden.
2.) Spray your garden with Spinosad. This organic pest spray can kill a variety of worms and caterpillars and is safe to spray on food that will be eaten later. You can find spinosad based products at Amazon.
3.) Spread Bacillus Thuringiensis Dust (“BT”) – BT is not toxic to humans or animals and it may take a few days to work. Without giving you all of the gory details basically worms are no longer able to eat and they die. BT products are available on Amazon.
4.) Neem Oil – Neem oil comes from the neem tree, and will work for smaller worms and larvae but it won’t always work on larger worms. You can read more about neem oil in this article: Neem Oil General Fact Sheet. Neem oil can be irritating to skin and eyes, so be sure to wash any fruit that is sprayed. There are many neem sprays available on Amazon.
5.) Use natural predators in your garden. This can be a harder one to manage but birds and beneficial insects can destroy armyworms. Beneficial insects include ladybugs, lacewings and trichogramma wasps. The trichogramma wasps insert their eggs inside of pest eggs killing the larvae. See more about that in this article: Know Your Friends Trichogramma Wasps. You can purchase wasp eggs for your garden at Amazon.
Let us know if you have tried any of these and how they worked for you.
If you want to read more about armyworms see these articles below.
How to Manage Pests
Armyworm larvae feed in groups, which distinguishes them from other vegetable pests such as corn earworms and loopers. Markings on newly hatched armyworms are usually hard to distinguish from those of other caterpillars older larvae have distinct lengthwise stripes. The surface of the armyworm skin is smooth.
Armyworm eggs are laid in fluffy masses on the crowns of seedlings and leaves of older plants. Newly hatched larvae feed together near the egg cluster and gradually disperse as they grow. Older larvae chew irregular pieces from leaves and feed on green fruit. Larvae usually do not feed on fruit until the third or fourth instar, but in later generations, they may enter fruit soon after hatching. Armyworms pupate in a cell on or just below the soil surface. The adult is a gray and brown mottled moth. The life cycle takes about a month in warm weather and there are about three to five generations per year.
Armyworms may feed on the crowns of seedlings. On larger plants, armyworm caterpillars skeletonize leaves. In tomatoes, strawberries, and cucurbits they make shallow (occasionally deeper) gouges in fruit.
Handpick. Virus diseases, parasites (Hyposoter and Trichogramma), and general predators may be effective on caterpillars. Eggs are protected from parasites by fluff. Bacillus thuringiensis or other insecticides such as spinosad may be used against young caterpillars but are needed only when numbers are high on seedlings. Ignore armyworms in sweet corn, where they do not usually cause major damage.
Armyworm mature larva
Beet armyworm egg mass
Statewide IPM Program, Agriculture and Natural Resources, University of California
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Beet armyworms can destroy seedlings, consume large portions of leaves, and stunt growth by feeding on developing buds. Young worms feed on leaves but rarely cause substantial damage. However, larger larvae feed on petioles and can cause significant crop loss. Large larvae are quite mobile and have been observed to travel over 10 feet per night, feeding on several plants. Beet armyworms also cause damage by contaminating cilantro or parsley with their bodies and frass, reducing crop marketability. In the San Joaquin Valley, fall populations are more damaging than spring populations.