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.

Size
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.

 Appearance
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.

First strawberry preplant meeting - July 28th

The first strawberry preplant meeting of the season (is it that time already?!) will be next Thursday, July 28th at the Mountain Horticultural Research Station in Mills River, NC.  Area horticulture agent, Sue Colucci has organized this meeting, and interested growers can find more details here.  I will be sharing results from our 2010 twospotted spider mite trials, discussing new insecticide options and IPM strategies for sap beetles in strawberries, and sharing information on unusual caterpillar issues from this year's harvest season.
Hope to see the mountain strawberry growers there!


Update
Sue Colucci has just posted an announcement for the meeting on her blog, Western NC Vegetable and Small Fruits News.

What does the short strawberry season mean for pest management?

There has been a lot of news lately about the fast, furious, and fantastic North Carolina strawberry crop this spring.  I have posted about the almost overwhelming harvest at the Central Crops Research Station, which looks to be slowing a bit in the next few weeks.  In fact, most of the heavy strawberry picking in NC should be tailing off by the end of May.  This is good for my research program, since we're moving on to blueberries, blackberries, and tobacco this time of year, but is not necessarily music to the ears of growers or consumers.  We are not seeing lower yields in 2010, just a more concentrated crop.

This short season has some direct pest management implications that growers should be aware of.  First for our key arthropod pest of strawberries, twospotted spider mites.  I have only gotten phone calls from one grower with spider mite issues this spring!  Either you are all comfortable with your management programs, or this was a mild mite year.  My money is on the later, as we did not have a single spider mite in our research plots at Central Crops until mid April, and even then, we had to manually infest to generate the number that were needed for our threshold and efficacy trials.  The low spider mite numbers are due to several factors:  Cold winter weather decreased mite populations and produced very hardy, healthy plants.  Healthy plants do not harbor as high mite populations as stressed plants, and they can better tolerate the populations they do have.  The fast fruiting season also reduced the importance of TSSM.  Fruiting strawberry plants can tolerate higher numbers of mites than young plants.  Finally, the rapid harvest season has likely rendered the use of miticides unnecessary.  Even the most effective conventional miticides take 2-3 week for full efficacy because they act on all mite life stages.   When it appears our season will be winding down in that same amount of time, a miticide treatment is probably not advised.  Of course, there are always exceptions to this rule, and I encourage growers and agents to contact me with questions.

Next, thrips: 2010 has been, as expected, a late thrips year with relatively low populations.  Thrips treatments are likely not justified.  I address other thrips related issues in posts here and here.

Lygus bugs are of the same ilk as thrips this season.  We will be out of strawberry harvest before they are likely to be an issue.

Finally, sap beetles.  These late season pests may rear their ugly heads in 2010 not because we have a long field season which will stretch into to heat of summer but instead because the sheer volume of fruit in a short period of time may result in less thorough picking by both employees and customers.  Sanitation is crucial for sap beetle control and is often sufficient by itself.  Chemical controls should be our last option, but they are available.

Just some of the news stories from throughout the state:
WBTV Charlotte at Hall Berry Farm
WRAL's Go Ask Mom blog on organic strawberries near the triangle 
WRAL and Dr. Barclay Poling at our research plots at the Central Crops Research Station, Clayton, NC
WECT's Strawberry Festival Coverage
WWAY on Columbus County strawberries
Sampson Independent on the large crop
In the Field on strawberries in NCDA's Food to School Program