by Katie Navarra
In 2008 the Spotted Wing Drosophila (SWD) was first spotted in California, Since then it has emerged as one of the most devastating pests of small and stone fruit in nearly every state and worldwide. The pest, of Asian origin, can multiply its population by 200 times within one to two weeks.
“In 2014, $718 million in damaged crops were reported and the cost of control increased by $129 million,” said Ash Sial, PhD. Sial is a blueberry entomologist at the University of Georgia.
Sial is the Project Director for the SWD Organic Management grant, an initiative to find sustainable and effective methods of control for SWD in organic production. This multi-regional project was funded by USDA-NIFA through Organic Agriculture Research and Extension Initiative (OREI) to develop, evaluate and implement systems-based organic management programs for SWD.
In February, Sial and his colleagues highlighted results from 2016, the project’s first year. The webinar was hosted by eOrganic webinar and provides a comprehensive update on organic management of SWD and highlights the findings of the research conducted related to behavioral, cultural and chemical strategies.
Behavioral strategies
Understanding how SWD behaves provides critical insight that can be used to develop effective control strategies. Researcher, Matthew Grieshop, assistant professor of entomology at Michigan State University led the effort to determine when attractants are most appealing to SWD.
“We found that attractants are most appealing to hungry flies and flies that have not yet mated,” he said.
Then different types of attractants were tested for efficacy. Raspberry chemicals, yeast/flour/apple cider vinegar, scenting commercial lure and some of these in combination were all included in the trials. Traditionally, a yeast/sugar attractant is used. Study results show that this is still the most effective and may be slightly more so when used in combination with a scenting commercial lure.
“The yeast /sugar remains a solid attractant. When used with the scenting commercial lure we did see a slight increase in attraction,” he said.
Finding a combination that smells and tastes good to SWD is only part of the equation. Mode of delivery is equally important. A modified apple maggot trap shows the most promise. The red sphere is a contoured wax with a pesticide/attractant cap. Dew and rain release the mixture. As it drips down the sides of the sphere, SWD feast on the sphere.
“Once SWD ingest the attractant it’s systemic and it quickly kills,” he said.
These spheres are complex to manufacture and can cost a dollar, or more, each to make. Hoping to find a more cost effective alternative, Grieshop tested insecticide treated fabric pouches. A cotton and a polyester fabric pouch were impregnated with the same pesticide/attractant mixture used in the spheres. Unfortunately, the fabric didn’t yield as high mortality rates.
“The mortality significantly decreased with time, only three weeks, as compared with six weeks of the other,” he said. “We’re going back to the drawing board since it’s not a viable option.”
Cultural control
Cultural methods used in combination with other strategies often provide effective methods for control. Researchers tested the effectives of pruning, floor management and exclusion to determine if any of these approaches lessen SWD populations.
First, they experimented with pruning. At test sites in diverse geographic regions, canopies were left dense, moderately pruned and aggressively pruned. On average, the more aggressively a canopy was pruned, the lower SWD populations.
“When the temperature rises above 86 degrees F, the development of SWD larvae decreases and mortality increases,” said Dalila Rendon, research associate (Post Doc) at Oregon State University.
With medium and light canopies and thus less shading, the temperature is warmer and naturally controls SWD populations better than dense canopies. While SWD populations were lower, so too were the marketable yields meaning this approach is not likely to be practical.
Second, researchers looked at floor management and evaluated whether or not the use of weed barrier impacted SWD populations. Results show that temperatures above the mulch are hotter than temperatures below the mulch. As the temperature reaches or exceeds 86 degrees, SWD look for a cooler environment, mainly below the ground’s surface.
“There was no effect with the weed mat on reducing the survivability of SWD. However, the weed mats provide an effective barrier to prevent the larvae from getting under the soil where they have a higher chance of survivability,” Rendon said.
Third, and potentially most promising, exclusion methods were tested. The exclusion materials included a poly cover, an 80 gram fine tech mesh netted tunnel and mini-tunnels. Data showed that exclusion significantly reduced the number of eggs and larvae in the fruit and the adults.
Mary Rogers, PhD, an assistant Professor at the University of Minnesota and co-project director for the OREI project, explains that raspberries in poly tunnels in Minnesota had only 2 percent infestation compared with 35 percent in netting and 81 percent infestation in the field. In Arkansas, zero infestation was reported. The exception was the day after a windstorm which blew open a door on the mini-tunnel and provided an entry. “It’s important to be aware of this when you go in to harvest and to check for small holes,” she said.
The team hypothesizes that the better results in Arkansas may be related to the overall warmer temperatures in the region already nearing and crossing 86 degrees. Further work is needed to determine if higher temperatures that accompany poly exclusions has any impact on the crop.
The exclusion method produced the same total crop yields, as crops without exclusion, but the marketable yield was higher.
Rogers cautions that exclusions block pollinator access to fruit and that this factor must be considered. In the study, the exclusion materials were not installed until after green fruit had developed to provide ample pollinator access.
“This can be great for small farmers, but we’re not sure how it will scale up. There’s a need for further investigation into the economics,” she said.
Chemical strategies
Chemical control is still a vital control strategy in organic operations. Spinosad 22.5 percent, commonly known as Entrust, is the most widely used pesticide. As part of the project, researchers wanted to determine if SWD populations are developing a resistance to this important chemical.
The good news is that trials using Entrust at fairly low concentration levels still provide good control and indicate there is not a resistance developing. However, labeling restrictions and concerns about future resistance make it necessary to find other products for rotational use. In the last two weeks of a five-week trial, Grandivo used in rotation with Entrust, appeared to offer the best control.
“The (Entrust) label only allows for two back to back applications,” said Phillip Fanning, research associate at Michigan State University.
The project team is also working to determine the viability of adding feeding stimulants to chemical applications to encourage SWD to ingest more insecticide. In lab tests when sweetened materials such as sucrose or sugar were added to Entrust, SWD began feeding immediately.
Fanning cautions that there is still a lot of work to be done before he recommends mixing such additives with insecticides. “There is a concern about if and how this would affect pollinators so more research is needed,” he said.
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