Armyworm outbreaks are difficult to predict but infestations seem to occur in portions of the state every year, especially after early fall rains. Common species of armyworms present in Texas include the fall armyworm, Spodoptera frugiperda.
I have had some initial reports of armyworm activity after the heavy rains in various parts of the county.
The fall armyworm has four life stages: egg, larva, pupa and adult. The fall armyworm does overwinter in the southern regions of Texas in the pupal stage. The adult is a moth that migrates northward as temperatures increase in the spring.
Armyworm eggs and larvae are also sometimes transported from one part of the state to another on grass sod intended for residential and commercial turf. Several generations – a generation is the development from egg to adult stage – occur each year, and typically the life cycle from egg to adult takes 28 days. The life cycle can be extended if cooler temperatures occur and can last up to several months.
Armyworms in the spring and summer occur in more distinct groups than later in the season. Fall populations of larvae often blend together several generations and may appear to be continually occurring.
When feeding, larvae strip foliage and then move to the next available food. High populations appear to march side by side to the new food. Thus, the name armyworms has been applied.
Armyworms attack many different kinds of plants. When food is scarce, they will move to plants that are not normally attacked. Thus, armyworms can be found on nearly any plant as they migrate in search of edible foliage.
Although armyworm outbreaks are memorable when they occur, in reality, the outbreaks are usually small in scope. Weather and multiple natural enemies usually act together to keep populations under control.
Parasites such as wasps and flies are very effective against armyworms. Predators, such as ground beetles, are also effective in limiting outbreaks. Birds, skunks and rodents also consume large numbers of larvae and pupae.
Sometimes weather conditions occur that favor armyworms. High egg survival of fall armyworms is favored by above-average rains in August and September. Because armyworm moths are strong fliers, outbreaks can also occur when storms move the moths and allow them to escape natural enemies.
Armyworms should be controlled when they occur in large numbers or plant damage is becoming excessive. This will be apparent in turfgrass by examining the grass blades. Damaged areas of lawns appear off-color and eventually turn brown as damage progresses from small windowpane strips of damaged leaf tissue to destruction of entire leaves. Armyworms feed any time of the day or night but are most active early in the morning or late in the evening.
Treat with a labeled insecticide when leaf damage becomes evident and large numbers of caterpillars are visible. Effective, low-impact insecticides include halofenozide (small caterpillars only) and spinosad. Bacillus thuringiensis (Bt) products are widely available and will selectively control small armyworm larvae without harm to beneficial insects; however, Bt residues does not last on turf for more than 1-2 days. Conventional insecticide choices for armyworms in lawns include bifenthrin, carbaryl, esfenvalerate, permethrin and others.
Call the Victoria County Extension Office at 361-575-4581 if you have questions or need more information.
SOURCE: Chris Sansone, Rick Minzemayer, and Mike Merchant, Extension entomologists, Texas A&M AgriLife Extension, City Bugs Fact Sheet