Drosophila suzukii, or spotted wing drosophila (SWD) is mother nature’s loving way of reminding us how much she sometimes hates us. SWD can grow on anything. Some growers were hit hard in June of this year in Santa Maria and Oxnard with Drosophila. Processers were rejecting truckloads and growers were upset. Andrew Molinar of the California Strawberry Commission and I were approached to do an examination of samples from these fields for SWD larvae to confirm their presence.

We confirmed the presence of several Drosophila larvae at the Strawberry Center. However, we did not rear them out to confirm that it was indeed SWD. Processers do not discern between SWD or Drosophila melanogaster, vinegar flies.

Figure 1: A Dall-E generated image by the author’s input of white worms in a red liquid smiling in 2D cartoon anime style and being angry.

We used two different methods for monitoring Drosophila larvae. We used a salt and vinegar solution and a sugar and vinegar solution (which goes really bad after 48 hours) pictured below. The sugar solution keeps them alive, while the salt “forces” them out more quickly but kills them. Colin Koubek, my research associate, went through 6 samples, 5 from Santa Maria from growers, and 1 from Watsonville. They all had larvae.

Figure 2: These nausea inducing bins contain macerated strawberries placed in either a salt and vinegar solution (left) or a sugar and vinegar solution (right). This photo was taken 48 hours after samples were prepared in the lab. The minimum time to leave samples in either bath is 15 minutes and up to 24 hours.

I inquired about the harvest schedule, and it turns out these fields were on a weekly schedule. What remains unclear is how long the berries were held prior to processing. Also, how large were the larvae when they were detected by the processers?

This situation seemed to me to be a classic clash of countering incentives and biological opportunism. The berries were picked on a weekly schedule vs. twice weekly because of labor costs. That means more overripe berries. Overripe berries attract vinegar flies. Now we have two fruit fly species feasting on both underripe and overripe berries. This increases the risk of positive detection at the processor. If the processor is at capacity and cannot process in time, then it is almost certain that there will be a detection and rejection. For the processor, the species of maggot found makes no difference. (NOTE: it might make a difference in terms of management but not in terms of the result).

Figure 3: A Dall-E generated image by the author’s input of angry strawberry dressed as a 1960s NASA scientist pointing cartoon anime style and being angry. This is how the author imagines the attitude of processors and growers towards Drosophila larvae.

I emphasize the issue of overripe berries because as a PCA or grower you may be controlling your SWD but exposing your Achilles heel to vinegar flies to come and ruin your day. I did not inquire about how many sprays were going out. To be sure about the type and abundance of each fruit fly species, I will have to either use DNA analysis or rear the larvae out on diet and observe the female and male morphological features (spotted wings on males and serrated ovipositors on females).

Regardless of fruit fly species, there is no way around sanitation as the main cultural control for fruit flies. Overripe fruit need to be picked and discarded. It will reduce the likelihood of rejections and keep SWD populations low. For curative options, you have Malathion, Dibrom, and Radiant. Timing that spray with proper coverage is key. Hopefully in the future we will have SWD sterile insect technique, bait and kill trapping, and biological control as reliable options to rotate with chemistry and sanitation.

When it comes to SWD, the “I” in Integrated Pest Management is key. All options are on the table given the high stakes. However, there is no substitute for keeping a field clean. As always, the economics will make the choice for you.