Armyworm look-alike in wheat

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click image to zoomSawfly larva on wheat. Small, caterpillar-like larvae have been reported in East Central Indiana wheat fields just before fungicides were to be applied. These are not caterpillars at all, but are sawfly larvae, probably grass sawfly (Pachynematus sp.). Adult sawflies are insects with four wings that are distant relatives of the honey bee, but they do not have a stinger. The larvae are usually plant feeders and a few are pests of home gardens (rose sawfly, for example).

The sighting of foliage feeding sawfly larvae in wheat fields is infrequent in the Midwest. And although this species was feeding on the leaves, it was minor and economic damage is not expected. This is more of a curiosity than anything else, and an opportunity to learn the differences between different insect larval types – always fun! Full size larvae, about 1.25 inches, may feed on stems causing head clipping much like armyworm. Another closely related insect, the wheat stem sawfly, occasionally causes problems in northern regions of the United States. This species, as the name implies, feeds only in the stem causing lodging of small grains.

click image to zoomSawfly and armyworm differentiation. Unlike armyworms, which feed at night or on dark, cloudy days, sawfly larvae feed throughout the day. Another difference takes close inspection – caterpillars have three pairs of true legs just behind the head and another five pairs of prolegs on their abdomen. Sawflies also have the three pairs of true legs up front, but also have six or more pairs of prolegs further back on the abdomen. Because sawflies’ coloration blends into the vegetation, a sweep net would be useful to determine their presence. If populations of 10 or more larvae per 100 sweeps are found, plants should be examined more closely. By shaking undisturbed plants in multiple locations in the field, count and calculate the number of larvae per foot of row that fall to the ground. Populations as low as 0.4 larvae/foot of row have been suggested by the University of Delaware as the treatment threshold. That is not many, but it would be very surprising if any Indiana fields topped that threshold – it’s extremely unlikely that treatment will be required.

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