by Courtney Llewellyn

How to dispose of fruit wastes without attracting the dreaded spotted wing drosophila? That is the question being tackled by researchers at Michigan State University – and their answers (so far) were divulged at the most recent Great Lakes Expo.

Matthew Grieshop, who was associate professor of organic pest management and director of the Sustainable Farming and Food Systems academic program at MSU, noted that the most susceptible fruits for SWD infestation are berries, cherries and rotting fruit, especially pomace waste.

The current options for post-harvest management of fruit wastes to manage SWD are solarization, crushing, burial and composting. Grieshop said solarizing infested raspberries for 32 hours in plastic bags can reduce larval survival by 99% – but this practice is difficult to translate to non-berry crops and is impractical for the large quantities of fruit wastes typical of post-sorting or processing operations. Before going the crushing route, growers first need to determine the potential of crushing their fruit as an effective method of fruit waste disposal. Waste tart cherries were crushed with a golf cart and evaluated in an MSU lab over time. “A pretty big population [of SWD] came from uncrushed cherries over nine days,” he said. “You can probably really reduce incidence if you crush them in the field.”

When burying waste, the research team used organic apple pomace (from cider pressings) as a fruit waste. After exposing the waste to SWD, they buried the pomace at 12, 24, 36 and 48 centimeters deep and then placed traps over the pits to catch any flies that emerged. The 24 cm depth achieved a 97% reduction (in fairly sandy soil). “For flies, it’s the equivalent of a human digging the height of the Eiffel Tower,” Grieshop commented.

The MSU team also used organic apple pomace as a fruit waste in composting, along with organic chicken manure as additional feedstock. “Composting alone saw a dilution effect, but with chicken manure we saw an exponential decrease in emergence,” he said. They found mixing apple pomace with 5% or more chicken manure was deleterious to SWD – and incorporating greater than 25% chicken manure eliminated apple pomace as an SWD reproductive source. (It was noted manure type may impact these results.)

“Why does chicken poop work?” Grieshop pondered. It’s likely it’s because it’s high in nitrogen (4% by volume). Chicken manure probably alters waste pH too. They’ll next be playing with synthetic nitrogen to look for a similar response from SWD.

Researcher Charlotte Schuttler picked up where Grieshop left off, stating, “We know from previous studies that organic poultry manure will affect SWD reproduction when mixed with fruit waste (apple pomace) at a ratio of 80:20. We wanted to determine if nitrogen content specifically will affect SWD reproduction.” Nitrogen-based fertilizers might provide a readily available, cost-effective option for growers which could be added to a regular spray plan to manage SWD on post-harvest waste.

For their tests, apple pomace was mixed with nitrogen-based fertilizers (urea, calcium nitrate, potassium nitrate, magnesium nitrate or ammonium sulfate). The apple pomace was placed inside individual arenas and exposed to 10 previously mated SWD females. Once offspring emergence began, the arenas were vacuumed daily to collect all adult SWD until no emergence was detected for seven consecutive days. Only the control with no fertilizer had emerging adults after 10 days.

Schuttler said they also wanted to determine if the pH of fruit waste can be manipulated to affect SWD reproduction. She noted that previous research on tephritid fruit flies has shown that raising pH negatively impacts larval performance – but there is little to no information available for drosophilid flies like SWD. Five treatments were tested, with the control having a pH of 4.5; the others were buffered to pHs of 3, 5.5, 6.75 and 8 (using glacial acetic acid to lower the pH). Again, 10 pre-mated female SWD were added and then removed. Emerging flies were removed and counted daily.

The results from the lab: The 5.5 pH was similar to the control; the pH of 3 had few SWD, and pH 6.75 and 8 produced a similar number of SWD (slightly lower than the control). In 2021, the team tested a field-size trial with cherries, and they’re still processing hundreds of samples.

“Lowering the pH appears to reduce SWD reproduction in waste fruit,” Schuttler said.

MSU’s nitrogen fertilizer experiments are ongoing. pH levels will be recorded throughout the fertilizer experiments to gather more data related to pH. They will evaluate other low pH buffers to determine if the SWD response was to the pH or to vinegar (acetic acid) volatiles. The field studies extending the lab findings will continue this year.

But while they determine what treatments work best, Grieshop also offered these management suggestions: Harvest all fruit from plants and collect dropped fruit to keep SWD at bay. Crush dropped fruit to accelerate its decomposition. Solarize waste for 48 hours in clear plastic bags if you can. Bury waste more than 10 inches below the ground – or compost the waste with more than 25% chicken manure.