by Sonja Heyck-Merlin
According to Dr. Juliet Carroll, fruit IPM coordinator at Cornell, the economic impact of spotted wing drosophila (SWD) is a billion dollars a year, and $7 million alone in New York State. “The impact of this insect is broad and wide,” said Carroll. “Crop loss is severe in seasons when ripening and harvests correspond with high SWD populations.” Some of Carroll’s current research involves using ruby-throated hummingbirds – the only hummingbird species in the Northeast – to help control SWD in raspberries.
SWD was first found in New York in autumn 2011. It currently arrives in May, with multiple generations per year, and builds up to high populations through August, September and November. In the one-month life span of a female, she can lay, on average, 380 eggs, using her saw-like ovipositor to pierce fruit skin and lay eggs inside the fruit. It only takes eight days from egg to adult in warm, optimum conditions. In the lab, according to Carroll, the optimum temperature found was 77º F, with a maximum of 91º and a minimum of 28º.
SWD attacks nearly all soft-skinned fruits if they are ripening when SWD populations are building. Raspberries are known to be the most susceptible crop, with blackberries a close second, followed by blueberries and cherries (tart and sweet). Strawberries, plums, peaches and grapes can also be infested. SWD infestation destroys fruit intended for fresh eating; however, processors will also reject the fruit because there is typically zero tolerance for any kind of white worms – SWD larvae, for example – in processing fruit.
SWD management depends on which type of raspberries are grown. Floricanes (summer raspberries) fruit on canes that overwinter. They flower and fruit in the summer. “For this type of raspberry, insecticides may not be required until the end of the harvest season. So, monitoring the insect with traps makes a big difference in knowing whether the insect has shown up and its population size,” Carroll said.
Primocanes (autumn raspberries) get mowed in fall. The canes grow in spring and then they flower and fruit in the late summer through early autumn. Because the timing of the fruit coincides with high levels of SWD, insecticide protection is a requirement with primocanes. Insecticides are effective for controlling SWD, but they must be applied weekly when the insect is present and the fruit is ripening. A rotating supply of active ingredients is critically important, as insecticide resistance is, Carroll said, “rearing its ugly head in some of our active ingredients.”
Other than insecticides, there are few controls for SWD. Exclusion netting can be an effective control, assuming that there are no holes in the netting and no SWD trapped inside. Pruning and weed control increase sun penetration, which has been shown to be detrimental to the insects. Removing and destroying dropped and overripe fruit is helpful but labor intensive. Refrigerating fruit at 32º to 33º F can also help protect the value of the harvest because it can kill and inactivate eggs and larvae in the fruit.
In search of alternative solutions for this destructive insect, Carroll visited the 1967 research of ornithologist, Walter Scheithauer, who showed that arthropods form an essential part of the white-eared hummingbird’s diet. “Hummingbirds may feed mainly on or entirely on arthropods when floral resources are scarce. They actually evolved from insectivorous birds. Hummingbirds eat arthropods daily and at intervals as frequent as 10 minutes. The frequency of arthropod foraging increases during breeding, nesting, fledging and before migration. They use the same vegetation strata for arthropod foraging as they do for nectar feeding,” Carroll said. She also noted that once hummingbirds discover a rich feeding ground, they will return to the site in subsequent years.
Drawing on this former research and trait behavior, Carroll designed a multi-year study to test the efficacy of using feeders to attract ruby-throated hummingbirds to raspberry plantings. Her hypothesis was that the feeders would result in greater presence of hummingbirds, lower the trap catch of SWD and lower fruit infestation in the locations of the field that had the feeders compared to those without.
The feeders were filled with a cane sugar and water solution (one part cane sugar to four parts tap water), which were filled twice per week. Carroll suggested obtaining two sets of feeders: one for active use in the field and a clean, sanitized (she uses a 10% bleach solution) set that’s waiting to be filled. The feeder solution should be changed once or twice per week, especially during hot weather. Carroll also said to make sure to use the ant dam to prevent ants from finding the nectar.
In her trial, the feeders were placed in different patterns at a rate of about 25 feeders per acre. The plots were observed for the presence of hummingbirds, and jar traps and lures were used to quantify the presence of SWD. “In conclusion,” Carroll said, “feeders will attract ruby-throated hummingbirds to spend time in raspberry plantings. They were most abundant in August, and they were more numerous the second year. This indicates they would be present when SWD populations built to damaging levels in August and would become more numerous over time.”
With growers having suffered devastating losses in the past decade due to SWD, Carroll said this is the first year she’s heard growers discussing new raspberry plantings. “This is a testament to the advances the research community has made on managing this invasive insect in North America,” she said.