Dec 5, 2016

Learning curve steep for controlling spread of SWD

Michigan researchers are on the forefront of work being done to offset the continuing onslaught of spotted wing drosophila (SWD) on commercial blueberries. But the relative newness of the pest – and its complex behaviors – create numerous challenges in determining the best control strategies.

During a fall research field day at Michigan State University’s (MSU) Trevor Nichols Research Center near Fennville, Michigan, MSU entomologists John Wise and Rufus Isaacs shared findings from several trials that sought to pinpoint the behavior and control mechanisms for SWD.

A 4-acre blueberry plot initiated several years ago, prior to SWD’s arrival as a persistent pest in soft fruit crops, was the focus.

“One thing we didn’t plan for is the need for varieties that ripen late, when all the SWD action is taking place,” Isaacs said. “If we do expand (to an adjacent property) we will go for more later-season varieties.”

“When we made choices, there wasn’t SWD yet,” said Wise, Trevor Nichols’ research coordinator. “If we knew (SWD) would dominate our pest management lives, we would do more than this Bluecrop, which is a really nice cultivar for studying blueberry maggot and Japanese beetle, but no one talks about that anymore.”

Wise said the field has become a good planting for research, although a fabric cloth to control weeds inhibits SWD infestation.

“It’s great for weed control, but the black fabric creates temperatures that kills the weed,” Isaacs said. “The good news is if you have a small farm, it’s a cultural way to help reduce pressure.”

In pesticide treatment studies, SWD patterns have been a moving target, Wise said.

“That’s been the case, especially with a newer pest,” he said. “If we’re talking about codling moth in apples, there’s decades of data – some of it’s field trials, some are laboratory studies, and you end up knowing real well how each compound works and the mode of action required.

“SWD is still new enough that, sure, we have a couple of years of data, and other people do,” Wise said. “There’s really a lot to learn. Field trials are a really important piece of the puzzle. There is no single field trial that I would ever bank everything on. There’s a lot of noise in the system. You need bioassay work, especially if you’re starting to look at new products.”

Wise produced preliminary findings that showed the efficacy of several treatments when compared to untreated checks.

While Captiva, Imidan and Delegate and Apta were seen as effective in curtailing SWD infestation, Wise said he also was intrigued regarding the potential of an unregistered product from Michigan- based Vestaron using a derivative of spider venom – taken from one of the most poisonous spider species in Australia.

Other biological products from Marrone, Grandevo and Entrust, showed positive results, Wise said.

He said unseasonably hot conditions in Michigan this past summer curtailed the full collection of data, but one theme continues to emerge.

“In combatting SWD, because of how many generations it can go to, you want to rotate products so you don’t end up with resistance from any one mode of action,” Wise said.

“Most (treatments) significantly reduced populations over the untreated check,” Wise said.

“In the last two weeks before harvest, the good thing is a lot of programs here looked quite good compared to untreated,” he said. “It looked good considering the amount of pressure on small plots. The encouraging piece of this is if resistance management is likely to be a real problem – and I think it will as we go on – then it’s really good to have not just one chemistry, but also be able to have some biological products that are working. That’s the most encouraging piece of this study.

“With a lot of treatments that have a lot of different adjuvants, we didn’t shake out a lot of differences,” Wise said.

Isaacs presented information regarding a 2015 and 2016 evaluation of Mustang Max and Assail 30SG for control of SWD in blueberry.

In 2015, he said, treatment plots consisted of six-bush plots and were sprayed using an FMC 1029 airblast sprayer calibrated to deliver 50 gallons of water per acre. Treatments were applied Aug. 4 and Aug. 11, 2015. In 2016, treatment plots consisted of three-bush plots and were applied using a C02-powered backpack sprayer operating at 50 psi in a volume of water equivalent to 160 gallons per acre. Treatments were applied on Aug. 31.

Isaacs said in 2015 there were visual, but not significant, differences among treatments in the number of SWD per ounce of fruit, primarily at Aug. 12 and 17 sampling dates. Fewer SWD were found in insecticide- treated berries, with berries from Mustang Maxx+Assail 30SG treatment containing the fewest SWD eggs and larvae.

The 2016 trial detected higher fly mortality and lower fruit infestation in the containers with shoots from the combined treatment than either of the individual treatments, although this was again not significantly better than the individual insecticide treatments.

“As far as insecticide control, we’re not getting down to zero (SWD infestation) – that’s what growers want,” Isaacs said.

While early research on SWD indicated use of neonicotinoid sprays was not particularly active on SWD, Isaacs said he worked with Wise to show “that even when larvae were in berries, with some of the neonics you could spray the outside of those berries and the level of infestation would go down, offering some after infestation control of the larvae.”

“Obviously there is a resistance management issue here,” Isaacs said.

SWD activity periods

Isaacs presented some findings from a study of SWD activity on blueberry and raspberry crops, and the potential implications for controlling the pest.

“There are some pretty strong patterns to talk about where in the early morning and early evening are the times when we can stand and see individual (SWD) flies buzzing around on the bushes,” Isaacs said. “They’re on the fruit, they’re mating, advertising to each other. When it comes to the middle of the day, it’s really hard to find a fly in these fields, even though we know that they’re there and there’s infestation. There is clearly a daily pattern.”

Isaacs said with 2015 having been a cooler season, more activity was observed in the middle of day.

“This summer it was a blazing hot July and August,” he said. “There was a lot less activity during the day, and you see them shifting more in the early morning. When we came back in the evening, it was still really hot, in the 80s, and we were not finding as much. They’re obviously tracking the weather. We’re not not sure where they’re hiding, but they’re obviously getting down into shady parts of the environment and not up on the bushes.”

An effort was made to determine whether spraying at different SWD activity periods affects the level of control.

“The question that comes up from the behavioral observations is would it make a difference on what time you spray,” Isaacs said.

In August and early September 2015 and 2016, observations were made in blueberry bushes and in raspberry bushes to compare the level of activity of SWD. Bushes were observed for 2.5 minutes at four times of the day, and the number of flies observed was recorded.

The field trial was run in blueberries, with Mustang Maxx applied at two different times of day, either at noon or in the evening.

“Although both timings reduced infestation relative to the untreated control, we found no significant difference in the level of control provided by this insecticide when applied at the two times of day,” Isaacs said.

“This indicates that growers can select the optimal timing for spray application against SWD based on weather conditions and other issues, without needing to time their application to the time when SWD are active,” Isaacs said. “However, the typical early morning or late evening timings typically used by growers are likely to provide the greatest overlap of maximum residues from the initial applications when the flies are most active. In subsequent days, as the residue decline, the timing of application will be less and less important.”

“Pretty consistently in the number of samples, there was really no difference in the time of day. We were hoping, while the flies were out on the bushes, if you hit them then, they would be more likely to be controlled,” Isaacs said.

“With the noontime application, when it’s really hot, as well in the evening when they come out, we had almost exactly the same results,” Isaacs said. “From a growers’ perspective, that’s probably good news. If I would stand up and say, ‘well, you’ve got to spray between 6 and 8 p.m., that’s the time it’s going to work best,’ that might not fit everyone’s schedule. The time of day doesn’t matter. It worked every time. The fly is very adaptable. They’re trying to avoid sunlight, heat, humidity. You will see an early morning, early afternoon period of activity.”

— Gary Pullano, associate editor