Crawfish News Summer 2010 (Vol 3, No 5)

Charles Lutz, Romaire, Robert P., Baldwin, Jack L.  |  8/19/2010 1:05:32 AM

Controlling Armyworms in Late Planted Crawfish Forage Crops

Fall armyworm can present a substantial threat to production of a forage crop for crawfish, whether it is rice or sorghum-sudangrass. Large populations of fall armyworms may invade the forage crop for a variety of reasons, the primary one being that their previous host has been harvested. This movement into the crawfish forage crop has typically been observed in areas where corn, grain sorghum or milo or other grass-type plants have been harvested near a rice field. Armyworms are not afraid to march across soil in search of a new plant to devour.

Fall armyworms have a distinct Y-marking on their head capsule which can be used to distinguish them from other species of caterpillars (photo 1). Armyworms damage forage crops by cutting down seedlings or defoliating plants. This damage can be extensive enough to require replanting (photo 2). To scout for armyworm damage, look for cut and ragged leaves and then for the armyworm caterpillars on the plants or in the whorls (photo 3). You can also scout for armyworms using a sweep net to brush across the surface of plants and catch armyworms(photo 4). Another sign is the presence of cattle egrets or other birds foraging in the field (photo 5). These birds will often flock to fields with insect infestations.

Because the crop you are trying to protect will soon be food for crawfish, it is important to avoid applying chemicals to that crop that could potentially kill crawfish. Most synthetic insecticides are highly toxic to crawfish. This is true because crawfish have physiology that is similar to insects. At this time, there are limited options for control of fall armyworm attacking a crawfish forage crop. It is possible that the fall armyworms will be controlled by natural enemies – such as parasitoid wasps, bacterial infections or diseases. Although, oftentimes the natural controls will not control the population quickly enough to avoid extensive crop damage.

If you are experiencing crop defoliation or plant loss from an armyworm infestation, the first option is the application of Bacillus thuringiensis (Bt foliar spray). It is important that Bt be applied to young caterpillars. First and second instar caterpillars are most susceptible to Bt (photo 6). A second option for rice is to flood the field for two to three days to drive armyworms up onto plants where they will be more susceptible to attack by predators such as birds. The flooding may also drown the armyworms. Be sure to avoid completely submerging plants which may cause plant mortality from the flooding. Flooding in young sorghum-sudangrass is not an option in most cases because fields cannot be flooded and drained quick enough to prevent crop failure or set back from water-logged soils. Finally, try to avoid planting a forage crop near a host field (milo, sorghum, etc.) that will be harvested soon.

So is insecticide use for control of armyworms a potential option? Crawfish in capped or sealed burrows are probably not exposed to insecticides used in foliar applications for control of fall armyworms but the potential impact of residues on crawfish health is not known. The insecticides Karate Z, Mustang Max, Prolex and Proaxis, labeled for use in controlling fall armyworms in rice, have label restrictions that state "Do not use treated rice fields for the aquaculture of edible fish and crustaceans." Other insecticides used to control fall armyworms in pasture and forage crops, including sorghum-sudangrass, include Sevin, Lannate and Tracer. Although the labels for these chemicals do not specifically state that the fields cannot be used for the aquaculture of edible fish and crustaceans, most have specific warnings as to their toxicity to aquatic invertebrates, which, of course, includes crawfish. Contact your local county agent if fall armyworms attack your crawfish forage crop. They can provide you with the latest information and best course of action for fall armyworm control.

Contributed by Dr. Natalie Hummel and Dr. Jack Baldwin (e-mail; office: 225-578-2369), Professor and Extension Entomologist, LSU AgCenter, Department of Entomology, Baton Rouge, LA 70803-4505.

Crawfish News Summer 2010 (Vol 3, No 5)
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