Identifying "worms" in plums and peaches

By the time wormy fruit shows up around harvest, it's too late to do anything. The damage is already done and the larvae are well protected from any revenge spray efforts one might want to apply against them. However, identifying the culprits is key to understanding what went wrong with pest management strategies so that steps can be taken to reduce the problem next year. Management strategies including monitoring, product choice and timing all differ depending on the pest in question.

Two common larvae found in plums and peach fruit are plum curculio (PC) and oriental fruit moth (OFM). While they do look similar to the untrained eye, there are a few simple ways to tell them apart. A good hand lens or a dissecting microscope will help.

Make sure to collect 40-50 infested fruit so that sampling is representative, as it is possible more than one pest is causing damage. Keep in mind that the pest that caused the damage may no longer be in the fruit: Part of the identification process involves looking at what's been left behind. Examine the outside of the fruit for damage symptoms. Do you see any frass sticking out of the fruit? What about crescent-shaped scars or clean holes? When PC females lay they eggs in fruit, a scar forms at the oviposition site. Over time, this scar develops a distinct crescent shape (Figure 1). When the PC larvae is finished devouring the fruit, it leaves by a clean hole in the side of the fruit (Figure 2). Oriental fruit moth larvae often leave the fruit by the same route they used to enter the fruit, and frass is often present at these sites.

Figure 1 - scar with distince crescent shape

Figure 2 - note hole on left and scar on right

Carefully cut the fruit open and collect any larvae for examination. Larvae can be placed in alcohol to preserve them in the event you can't look at them right away or if you wish to submit them to the Pest Diagnostic Centre or to a qualified consultant for expert confirmation.

Both OFM and PC larvae have well-developed head capsules. Plum curculio larvae are legless, creamy white and have a slight "C" shape when disturbed (Figure 3). Because they do not have well-developed legs, they will simply wriggle and roll about helplessly when removed from the fruit. In contrast, mature OFM larvae have a pinkish tone and have well-developed legs which they will use in their attempt to get away from you. They also have a structure called an "anal comb" at the rear end that is easy to see in mature larvae under magnification (Figure 4), a feature which will help to distinguish them from codling moth in pome fruit.

Figure 3 - Plum curculio larvae are legless, creamy white and have a slight "C" shape when disturbed

Figure 4 - OFM have a structure called an "anal comb" at the rear end seen under magnification

Once you know what pest you're dealing with, review your management strategies for next year and make appropriate changes..