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Wednesday, May 23, 2012

Do it yourself: An update on distinguishing SWD larvae from other insects in strawberries

Early this spring, I wrote a post discussing how to distinguish spotted wing drosophila (SWD) larvae from other internally feed insects.  That post was focused primarily on blueberries.  This spring and early summer, however, I received several calls about SWD in strawberries, and they appeared in our research plots for the first time.

Fortunately, the majority of grower calls I received about larvae in strawberries ended up being sap beetle larvae rather than SWD larvae.  Sap beetles or picnic beetles are actually a complex of beetle pests which include at least three species in North Carolina (Carpophilus lugubris, Stelidota geminata, Glischrochilus quadrisignatus, and others) and are attracted to rotting, not sound fruit.  If rotting fruit are present near sound fruit, as can commonly happen when you-pickers aren't thorough or rain prevents picking, sap beetle adults and larvae can also attack sound, otherwise marketable fruit.

While sap beetle larvae are also not desirable in strawberries, they are more easily controlled than SWD.  Good sanitation, meaning removal of overripe or rotting fruit, is usually enough to keep sap beetle populations in check.

How, then, can you distinguish between sap beetle larvae and SWD?

Host fruit
Sap beetles will first attack overripe, rotting, or otherwise damaged fruit.  SWD will be found in fruit that appears otherwise marketable until cut open.  As SWD grow older, fruit condition may deteriorate, but fly larvae found in sound fruit are more likely to be SWD than those found in rotting fruit.

Young sap beetle larvae may be similar in size to large SWD larvae, but in general they will be larger than SWD larvae.  Size on its own is not a good determinant, but it can be a good initial indicator.

Sap beetle larva on knife blade. Photo from a commerical strawberry farm in eastern NC, 2012.
SWD larvae lack legs, have no distinct head, lack hairs or bristles, and are tapered on both ends (see here for more images).  Sap beetle larvae have distinctive head capsules, three pairs of legs, and bristles along their bodies.  Adult sap beetles may also be present along with larvae.
Sap beetle larva. Note the three pairs of legs, the distinctive brown to black head capsule and the bristles along the body.  Sap beetle larvae will be mostly white or cream colored and lack patterns on their body. Photo from a commerical strawberry farm in eastern NC, 2012.

Two spotted wing drosophils larvae (center) inside a day neutral strawberry, Upper Mountain Research Station, October 2011. Photo: HJB
What other insects might be present inside strawberries?
Corn earworm (Helicoverpa zea) are occasional pests of strawberries, and the larvae can tunnel into berries.  Corn earworms are caterpillars, which means they have a distinctive head capsule and three pairs of front legs like sap beetle larvae, but they also have short, grippy prolegs along their abdomen and have a pattern of stripes along their body (although their appearance can be highly variable as larvae age).

Corn earworm larvae feeding on strawberries. Photo via UC IPM.

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