Larvae in fruit: distinguishing between spotted wing drosophila and other internally feeding fruit pests

It finally feels like winter in North Carolina, which may seem like a strange time to talk about pest of ripe fruit.  However, this morning I answer what I expect to be the first several questions from a homeowner who had "lots of maggots" (we prefer to use the term larvae here) in their fruit last summer and wanted to be prevent the same issue from occurring this year.  In this case, the homeowner was growing blueberries and was searching for tools to monitor and manage blueberry maggot.

Before 2010, it would be reasonable to assume that blueberry maggot (Rhagoletis mendax) larvae would be the most likely insect to be present in infested ripe blueberries grown in North Carolina.  However, the establishment of spotted wing drosophila (SWD) throughout the eastern US means that it is no longer safe to assume that fly larvae found in blueberries (or blackberries, raspberries, strawberries and other soft skinned fruit) are native insects and not SWD.  It is important to correctly identify which pests are present in commercially or home grown fruit because the management strategies for pest species will differ.  If you suspect that you have either of these insects in your fruit, contact your county cooperative extension agent or myself. 

I posted this summer about insects that might be present in "funny looking" blueberries and included information and links about two caterpillar pests of blueberries, cranberry and cherry fruitworms.  I'm not going to review the distinctions between these pests and fly larvae except to reiterate that if a larvae has legs (even if they are tiny), it is not one of our fly pests.

In order to determine if you have fly larvae present in your fruit, collect a minimum of 30 ripe, sound fruit.  If you are growing more than one variety or species of fruit, collect a separate sample from each.  It is essential that only sound fruit be sampled. If you would not eat a fruit, don't sample it!  Lots of non pest insects will feed on overripe, damaged, or rotting fruit, and sampling this fruit may lead you to conclude that you have a problem when you actually do not.  You should never make management decisions based on a crop you wouldn't eat or sell.

Once you have collected your sample, there are several ways you can determine whether the fruit are infested.  These include simply dissecting or crushing the fruit and looking for live larvae.  If you have a relatively small number of samples and good eyes (or a hand lens), this is the easiest method.  However, fruit dissection can be time consuming when you have a large number of samples, and it is possible to miss larvae if you do not look carefully. SWD researchers at Oregon State University have posted a video detailing the fruit dunk sampling method here. This method involve freezing the samples, which causing larvae to exit fruit and then floating the frozen fruit away from the larvae, which can be observed in the bottom of your container.  Finally, salt or sugar solutions can be poured over gently crushed fruit, which encourages the larvae to exit.  When conducted in over a dark surface, it's easy to see larvae moving.  Protect US, an invasive species detection and monitoring program headquartered out of the University of Florida, has a posted videos describing the salt and sugar tests here.

All of these tests will help you determine if you have larvae in your sample but not what the larvae are. If you find fly larvae in sound, otherwise harvestable blueberries, they are most likely either blueberry maggot or SWD.  These two different pests can be distinguished from one and other based on shape and size.  Shape is the most reliable differentiator between these two pests.  Blueberry maggot larvae are "carrot-shaped". Their head end, with dark mouth parts often visible, is tapered to a point and their rear end, with six, light brown spiracles (breathing holes; in two rows of three) is flattened. Larvae of all Drosophila spp., including SWD, are tapered on both ends, with their breathing tubes coming to a point at their rear end.

Larva of "true" fruit flies (Tephritids), which include the blueberry maggot fly, cherry fruit fly, and many others (top); larva of "vinegar flies" (Drosophilids), which include spotted wing drosophila and many native species (bottom). 

SWD and blueberry maggot larvae may also be distinguished by size.  A full grown blueberry maggot larva will be roughly twice as large as a full grown drosophila larva.  However, a middle aged blueberry maggot larva will be similar in size to a SWD larva, so shape is a more reliable tool.

Spotted wing drosophila larvae in ripe yellow raspberry, a closer detail in upper left hand corner. Photo: HJB

Blueberry maggot larva inside a blueberry.  Note that the larva is in motion and not always curled in a c-shape. Photo: Shawn Banks, Johnston County Cooperative Extension.

You can trap adult blueberry maggot and SWD as well.  If you are trapping before fruit are ripe, catching adults may provide you with a warning that these insects are present before you find larvae in fruit.  I provided DIY instructions for blueberry maggot and SWD trapping last summer.


More information
Do it yourself - Spotted wing drosophila monitoring
Do it yourself - Blueberry maggot monitoring
Protect US - YouTube Channel
Oregon State University Spotted Wing Drosophila Site
Michigan State University Spotted Wing Drosophila Site