Description of the Pest
Beet armyworm caterpillars (larvae) vary in color, but are commonly olive green with a yellowish, lengthwise stripe on each side of the body. Often there is a dark spot on each side of the body above the second pair of true legs. The body surface is smooth and almost hairless. There are four pairs of prolegs (leglike appendages, or fleshy stubs) in the middle of the abdomen, on segments 3 to 6. Older larvae may be up to 1.5 inches long.
Adult moths lay their eggs in scale-covered, cottony masses on leaf surfaces. The first- and second-instar larvae tend to feed in groups near the egg mass, scraping the leaf surface, skeletonizing leaves and leaving veins largely intact, or completely consuming leaves. As they grow older, larvae disperse toward stems and the center of the plant and feed more individually.
Beet armyworms increase in numbers as weather warms and are most common on late-summer and fall crops.
Beet armyworms can kill seedlings, consume large portions of leaves, and stunt growth by feeding on buds. However, serious economic damage to cole crops is uncommon.
Use biological and cultural controls to suppress armyworm numbers. Monitor fields and field borders regularly from planting until heading.
Use the following cultural methods to control beet armyworms:
- Disc fields immediately following harvest to kill larvae and pupae.
- Destroy weeds along field borders; armyworms often migrate from these areas into newly planted fields.
Many natural enemies attack beet armyworms. Common parasites include the wasps Chelonus insularis and Hyposoter exiguae, and the tachinid fly Lespesia archippivora. Consider installing insectary plants to attract natural enemies.
Entomopathogenic viral diseases may provide some control of beet armyworm. Though these viruses may occur naturally, natural occurrence is not likely to keep this pest below damaging numbers.
Organically Acceptable Methods
Use biological and cultural controls in an organically certified crop. Sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad are also organically acceptable.
Monitoring and Treatment Decisions
Start monitoring for beet armyworm before seedlings emerge. Frequently monitor seedlings, which are very susceptible to armyworm damage.
- Check for egg masses and young larvae in pigweeds, lambsquarters, nettleleaf goosefoot, and other weeds surrounding the field. If numbers are high on weeds, watch carefully for infestations on crop seedlings.
- Consider placing pheromone traps to monitor adults and predict egg laying.
- Once seedlings emerge or right after transplanting, check plants at least twice per week for armyworm egg masses and young larvae. Sample for armyworms along with loopers and cabbageworms and include them in the total caterpillar count.
An insecticide application may be needed if one second- or third-instar beet armyworm is found for every 10 plants. However, it is usually not necessary to apply insecticide for armyworms on older plants (between thinning and heading). Apply insecticide just before heading if caterpillar numbers threaten to damage the heads.
Beet armyworms are more difficult to control with insecticides than loopers and cabbageworms, so note their presence in monitoring records. Additionally, beet armyworm has developed resistance to carbamate insecticides in some regions, such as the low desert.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Coragen)||3.5–7.5 fl oz||4||3|
|MODE-OF-ACTION GROUP NUMBER1: 28|
|COMMENTS: Foliar application; use with an effective adjuvant for best performance. Use higher application rates within this range for heavier infestations, larger or denser crops, or extreme environmental conditions such as rainy weather or high temperatures.|
|(Radiant SC)||5–10 fl oz||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: Not for yellowstriped armyworm and western yellowstriped armyworm control. Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days after. Control improved with addition of an adjuvant.|
|C.||BACILLUS THURINGIENSIS ssp. AIZAWAI|
|(Agree WG)#||0.5–2 lb||4||0|
|MODE-OF-ACTION GROUP NUMBER1: 11A|
|COMMENTS: Most effective against newly hatched larvae, so proper treatment timing is essential. Check with organic certifier to determine which products are organically acceptable.|
|(Proclaim)||2.4–4.8 oz||12||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 6|
|COMMENTS: Do not apply more than 2 sequential applications of this product without rotating to a chemical with a different mode-of-action group number. Preharvest interval is 7 days for head and stem cole crops, and 14 days for leafy vegetables.|
|MODE-OF-ACTION GROUP NUMBER1: 22A|
|COMMENTS: Add a wetting agent to improve coverage.|
|(Entrust SC)#||4–10 fl oz||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days afterward. Check with organic certifier to see which products are organically acceptable.|
|(Intrepid 2F)||See comments||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 18|
|COMMENTS: For early-season applications, application rate is 4–8 fl oz per acre. For mid- to late-season applications and heavy infestations, application rate is 8–10 fl oz per acre.|
|(Prokil Cryolite 96)||8–16 lb||12||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 8C|
|COMMENTS: Registered for use on broccoli, Brussels sprouts, cabbage, and cauliflower. Use on cabbage is allowed based on a supplemental label (EPA Reg. No. 10163-41). For broccoli, Brussels sprouts, and cauliflower, preharvest interval is 7 days. For cabbage, preharvest interval is 14 days. Must be ingested by the insect. Apply when young caterpillars are present. Can be useful in an insecticide resistance management program.|
|(Lannate LV)||1.5–3 pt||48||See label|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: Not registered for use on broccoflower (cavalo), kohlrabi, mizuna, mustard spinach, or rape greens. Adversely affects natural enemies. Add a wetting agent to improve coverage.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.|
|1||Rotate insecticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|
|#||Acceptable for use on organically grown produce.|
|*||Permit required from county agricultural commissioner for purchase or use.|