Description of the Pest
Beet armyworm is the most frequently encountered of the three species listed above. Adults are heavy bodied moths (wingspread is about 1 inch, or 25–28 mm) with a characteristic mustard or orange colored liver-shaped spot on the forewings that is narrowly ringed with white. The female lays egg masses on the undersides of leaves, covering the eggs with felt made from her body hairs. The first through third instar larvae often feed gregariously, skeletonizing the undersides of leaves, or they feed on the insides of buds. Later instar larvae disperse and feed individually, chewing entirely through leaves or flowers. Early instar larvae are small and green, while late instar larval color may be green, brown, black, or gray. In all instars, there are fine lines along the length of the body, with a more conspicuous lateral stripe and a black spot just dorsal to the lateral stripe behind the head.
Calculate degree-days for beet armyworm in your location.
Learn to use degree-days to time insecticide applications.
Beet armyworm continually develops during winter in mild areas and builds up on weeds and in cotton, lettuce, and tomato fields. One generation can take as little as 31 days at 75°F or 24 days at 80°F. Egg to adult generation times can be calculated using degree-days (DD) based on a lower developmental temperature of 54°F. The egg stage requires the accumulation of about 94 DD from the time they are first laid until egg hatch. The larval and pupal stages require the accumulation of 470 DD and 318 DD respectively for females and 540 DD and 344 DD for males.
The yellowstriped armyworm larvae have a pair of black triangles on the back of most segments. Some larvae appear nearly completely black when viewed from above. The lateral stripe is bright orange or yellow. The adult has a complex and highly contrasting pattern of brown, yellow, and white on the front wings, and wingspread measure is about 1.5 inches.
The variegated cutworm overwinters as a naked pupa in the soil. Adults have a distinct liver-shaped outline on the front wings and their wingspread can measure from 1.5 inches to a little over 2 inches. Larvae have yellow or orange spots or a broken longitudinal stripe at the top of the body, which is otherwise gray. Often there is a dark triangle or W-shaped mark on the top of the eighth body segment.
Armyworms and cutworms mostly are a concern because they directly damage flowers as well as leaves that would normally be marketed with the flowers. Presence of late-instar larvae in seedling flats can also cause tremendous plant loss. On the other hand, moderate early-season feeding by armyworms on gypsophila may actually increase tillering and yields.
A number of parasites, both tachinid flies and parasitic wasps, attack Lepidoptera larvae and reduce their population growth rate. However, even if armyworm or cutworm larvae are parasitized, they continue feeding through to the last instar and still damage crops. Viruses also do not usually kill the larvae until later instars. Applying insecticides other than Bacillus thuringiensis (Bt) products are likely to exclude parasites because their residues are lethal to these beneficial insects. For more information, see BIOLOGICAL CONTROL.
Because these pests feed on a large variety of plant species, keep production areas free of weeds, many of which serve as hosts to armyworms and cutworms . Exclusion of winged adults can be accomplished by covering openings to the greenhouses with screens. Screens are especially important when lights are used at night in greenhouses to control flowering because lights attract moths. Individual seedling flats may also be covered with screens to exclude adults and larvae. Row covers can be a practical measure to exclude moths in field production as long as the mesh prevents entry of adults and the row cover is held above the plant surface to eliminate oviposition (egg laying) through the fabric.
Monitoring and Treatment Decisions
If Bt sprays are planned, use pheromone traps to determine adult flight activity and mating. Once adults are caught in traps, it is very likely that larvae are present and Bt should be applied as soon as possible because it is most effective against young larvae. Use regular visual inspections of plants to detect larvae and their damage. For guidelines on when to treat, see ESTABLISHING ACTION THRESHOLDS.
Selected Materials Registered for Use on Greenhouse or Nursery Ornamentals
Read and follow the instructions on the label before using any pesticide. Before using a pesticide for the first time or on a new crop or cultivar, treat a few plants and check for phytotoxicity. Also consider pesticide resistance management and environmental impact.
|Manufacturer||REI1||Mode of action2||Comments|
|Valent||4||11||Most effective against early instar larvae; pheromone trapping recommended for timing applications.|
(PT Pyrethrum TR)
|Whitmire MicroGen||12||3/—||An aerosol.|
|insect growth regulator||A.||azadirachtin
|OHP||4||un||Must contact insect. Repeat applications as necessary. Label permits low-volume application.|
|Chemtura||12||15||Use no more than twice per year and do not exceed 52 oz/acre/year. Do not use on poinsettia.|
(Orthene T, T&O Spray)
|Valent||24||1B||A number of chrysanthemum varieties have exhibited phytotoxic reactions. In greenhouse only labeled for greenhouse use on anthurium, cacti, carnation, rose, orchids, some foliage plants, young poinsettia, and some varieties of chrysanthemum. Can stunt new growth in roses.|
(PT 1300 Orthene TR)
|Whitmire MicroGen||24||1B||An aerosol only for greenhouse use.|
|Whitmire MicroGen||12||3||Check label. A fogger for greenhouse use only.|
|FMC||12||3||Label permits low-volume application.|
|OHP||12||3||Label permits low-volume application.|
(Tame 2.4EC Spray)
|Valent||24||3||Label permits low-volume application.|
|Wellmark||12||3||Label permits low-volume application. Also labeled as a cutting dip at 5 fl oz/100 gal.|
|FMC||12||3||Direct application to blooms may cause browning of petals. Marginal leaf burn may occur on salvia, diffenbachia, and pteris fern. Label permits low-volume application. Do not apply more than 2 lb a.i./acre/year.|
|4||5||Do not apply more than 10 times in a 12-month period. Compatible with most beneficials, but highly toxic to bees and hymenopteran parasites. Direct contact can cause significant mortality to Phytoseiulus persimilis.|
|1||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.|
|2||Rotate chemicals 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; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|
|3||PBO = piperonyl butoxide|
|#||Acceptable for use on organically grown ornamentals.|
|*||Restricted use material. Permit required for purchase or use.|