Saturday, August 30, 2014

Attack of the Armyworms

The edge of the battlefield.

Spodoptera frugiperda (its species name meaning "fruit ruiner") was always a fun name to say when learning insect trivia for Linnaean games. The common name, fall armyworm, was also visually interesting, evoking marching hordes of munching, wriggling larvae. Apparently those descriptions aren't too far off. This year looks to have been a good year for them and a bad year for homeowners with lawns and crop producers. As evidence, there are now many reports about these pests from across the country (TX, OK, ALPA, and more).

Fall armyworms feed on a variety of plants - at least 80 species. Hosts like apples, strawberries, flowers and many weeds may be eaten by these caterpillars. They are even common in many field crops like alfalfa and soybeans. However, fall armyworms prefer to feed on various grasses (Poaceae), including some of our favorite foods (corn, wheat) and turf grasses (fescue, Bermudagrass, etc.). The larvae grow rapidly by chewing holes in leaves, or completely devouring entire grass blades. At first, larvae are less than a centimeter long and may be a bright green:

Young fall armyworms are more green than brown, but even older ones can vary in color. Specific traits other than color are best used to identify this species.

As they age through six instars (larval "stages"), they normally change to a darker green-brown color:

Mature fall armyworms can be close to 1.5" long and olive green-brown, giving them a military appearance further adding to the "army" moniker.

It takes about 16-30 days for the caterpillars to mature (depending on temperature) at which time they burrow into the ground where they pupate. Pupae are reddish brown and may be found in high densities in the soil of infested areas:

Pupae of fall armyworms are typical of many moths - reddish-brown, wiggly and buried in the soil.

After a little more than a week in warm weather, adult moths emerge from the pupae, take flight, and mate to make more armyworms. Eggs are laid on vegetation around grassy areas. Adults are readily sexed due to their strong color dimorphism. Females are a drab brown with subtle markings, while males have much more bold patterns and are actually pretty attractive:

An adult female fall armyworm, exemplifying the typical brown moth appearance (length ~2 cm).
Male fall armyworms are pretty nice looking for a pest (length ~2 cm).

The best way to tell larvae of fall armyworms from other armyworms in the genus Spodoptera (as well as other owlet caterpillars in the family Noctuidae) is through a combination of traits. The most often cited characteristic is the inverted yellow "Y" on the head (extending up the pronotum) as seen here:

Mature caterpillar's head and thorax, showing the inverted "Y" found on this species.

However, some other members of the genus have a similar "Y" and some fall armyworm instars lack it (later ones show it best). Thus Wagner et al. in Owlet Caterpillars of Eastern North America have a key that includes a character that is more reliable and separates out S. frugiperda from its relatives in the first couplet: the dorsal pinnacula (top-most black spots with an associated seta) are as big or bigger than the spiracles:

The black dots (pinnacula) on the tail end reveal the fall armyworm's identity: the dorsal ones (square of four shown by orange arrow) are each larger than the spiracles (blue arrow).

Fall armyworms can be found during warm parts of the year throughout the eastern United States (east of the Rocky Mountains). However, they can only survive winters in places like Texas and Florida (as well as Mexico and the Caribbean). Adults are strong fliers and migrate up through the states, sometimes with help from storms that blow them part of the way. Here in NC they may be present for several generations from spring through fall, while there is usually only one generation up north. The generations around late summer and early fall are usually the largest, thus the "fall" in the armyworm's name.

From Sparks (1979)

So what can be done and why is this year worse? Fortunately, most years do not see mass amounts of these caterpillars. Parasitoids and pathogens kill many of the armyworms in the overwintering areas, reducing the size of the resulting northern migrants. However, as Sparks (1979) describes below, some weather conditions in their year-round range can cause mass outbreaks:

From Sparks (1979)

If you fear you may have armyworms in your lawn or corn crop, monitoring early can help detect the caterpillars. For homeowners, the presence of birds in a lawn can indicate an abundance of larvae. However, by the time large, mature larvae are seen, control may not be helpful as they will soon pupate. Thresholds for crops and potential control methods can be found through the links below.

Helpful Resources: