Spotted Wing Drosophila — not your average fruit fly

By Anne-Marie Oomen
Sun contributor

I am grossed out. On the video, a creature reminiscent of horror flicks, B-movies, an almost pornographic monster, except it’s not a monster, except it’s real and it is a monster. Sortof. The winged thing trembles on a flesh-like surface. The film reveals in full detail the tail-end of the monster’s abdomen, where a serrated ovipositor descends, and a double row of “teeth” pierces the surface. Slowly, with mesmerizing tenacity, she saws into the thin-skinned softness, dipping ever deeper into the flesh. Then, and this is where I feel sick, out of that same organ she forces a single small white egg, deposits it firmly into the hole. The ovipositor closes, lifts like a machine, revealing a tiny filament still extending from the hole—the breathing tube of the egg. The egg’s breathing tube!?! The creature turns; huge red eyes stare straight into the camera, and after all that, the darn thing starts the process all over. Hundreds of times. I am not kidding.

I am watching a fruit fly, and we could all heave a sigh of relief if it were a regular fruit fly, if it were one of many frequent summer populations, a nuisance, but something we recognize and can manage. But this one, called Spotted Wing Drosophila (Drosophila Suzuki) because the males have a single dark spot near the tip of each wing, this invasive has the potential to destroy soft fruit harvests throughout this county.

Spotted Wing Drosophila (SWD) showed up on the west coast in 2008, (probably carried on canister ships from Asia), in the South and Northeast in the next two years, and in Michigan in 2010. Then, in 2012, it hit Leelanau County in late summer. Researchers hoped it would be a late summer pest. If it arrived late in the growing season, summer’s soft fruits were safe. Then our winters would kill it off. But each year, researchers and farmers discovered this fly earlier in both traps with lures and fruit. Last year, SWD showed up in traps on June 29. That early showing means its presence now overlaps with cherry season. It loves cherries. And that means it’s also here for blueberries, blackberries and many other beloved soft-skinned fruits, including a favorite of mine, raspberries.

That’s how I encountered it. Last August, I was joyously picking raspberries in my sister’s patch in Oceana County, eating as many as I dropped into my basket, and noticing some of the beauties tasted vaguely bitter, not as sweet as the first bearing in June, and some were soft in a peculiarly melty way. It didn’t stop me; I grabbed for the plump and ripe, noting smaller ones coming on, another picking. Picking and eating sun-warmed fruit is happy summer work. Apparently, SWD thinks so too. My beloved brother-in-law looked up from yard work, hollered over to the patch. “You might want to check those.” At my frown, he added, “Just open one and look inside.” When I did, I found a single white worm wiggling in a raspberry lobe. I picked another. Opened it. Three worms. Then I picked a soft one. Crawling with them. How many had I eaten? Eeuuww.

That’s the way the SWD works. According to Michigan State Extension agent and specialist at Northwest Michigan Horticulture Research Center, Dr. Nikki Rothwell, they are about as insidious as insects come because “unlike other drosophila that like soft or over ripened fruit, this one, because of the ovipositor, can infest unripe fruit. It can invade cherries in the straw color phase of ripening.” The straw color phase is the yellow color just after cherries lose the green. That early. SWD invades fruit before it’s ripe, taking whole harvests without giving fruit a chance to be fruit.

Normally, for insect management, commercial farmers use a carefully calibrated system called Integrated Pest Management (IPM) to make decisions about controlling insects and diseases. Farmers dislike using any more spray than they have to because, besides the high cost of each application, they are no dummies about insect resistance. IPM allows them to identify appropriate treatment, reduce and rotate the number and kinds of treatments, and to target application for specific pests at specific times in their lifecycle. But this process could be complicated by climate change: our warmer and longer growing seasons, the 50 additional above 50-degree days of the last summers offer better reproductive opportunity for the SWD. Milder winters offer the “winter morphs” (still an unknown) a chance to live until spring and rebuild population. What do farmers do in the face of this? Dr. Rothwell says, “Farmers want to be good stewards. They want to take care of land and maintain healthy farms. Most Leelanau growers seem especially thoughtful about this process and are responsive to good science. They try to be fully informed, use the research, and make smart decisions. But SWD, it’s a game-changer. We need to know more.” She looks thoughtful.

Good science is key. Research is moving quickly, as becomes evident when I visit the Research Center on Center Road, where I am escorted into the lab. Karen Powers (research technician) pulls a tray: dozens of carefully labeled vials from a growth chamber (refrigerator size but warm), the beginnings of a test colony which, when mature, will number 3,200 — the number of flies necessary to conduct the research for 2016. She uses CO2 to gas one vial, then pours out a couple dozen tiny stunned flies, sorts the older generation from the younger because “We get more consistent results if flies in a test colony are the same age.” She flicks them apart with a tiny fly brush, brushing the new ones into a fresh vial with diet of cornmeal, yeast, water, preservatives. She sets this vial on its side “…until the flies wake up — so they don’t drown in the diet at the bottom of the vial.” Close work. That process doesn’t even compare to work Karen and others do when the lures arrive and they turn on the microscopes. It’s painstaking, made more challenging by how hard it is to identify the sneaky females — they don’t have the identifying wing spots but are distinguished by a subtle golden color and dark leg bands, harder to see.

The researchers match lab hours with long hours in the field. They identify sentinel fruit trees, hang lures, collect and count. They will measure infestation speed, distance density and reproductive cycles. Rothwell says, “We need to establish all these variables to make wise decisions about management.” Another researcher at MSU figured out how to spray SWD with atomized dye. Researchers could then track range by the dyed flies, released at specified color/distance intervals, as they showed up in distant lures. A year ago, Dr. Rothwell hypothesized, “Fruit from sweet cherries infests the adjacent but later ripening tart cherry orchard.” The team now knows that previously infested fruit remaining on or under trees after early harvests may infest later harvests. She says, “Now we need to know how fast, how many? Might there be a known level of SWD population which we can use to make informed decisions — as there is with the codling moth?” Other questions: does this mean farmers must spray after harvests? And a follow-up concern: to what degree does SWD rise from wild hosts — a particularly prickly question. If it does, what does that mean for wild berries? Rothwell is reassuring and positive, “The more we know, the more likely we are to manage it.” The Horticulture Station has more than 35 research projects going; many centered on SWD. They are trying to get the science right so they can give Leelanau farmers real information to help them make informed management decisions, and not give up on their harvest. Or their farms.

Which is how I end up walking a Leelanau orchard with Rothwell, Emily Pochubay (Fruit IPM Educator) and Karen Powers (research technician) on a blustery morning in late April — with Sadie, Rothwell’s three-year old daughter toddling along. The team demonstrates how they do transect research. The Richter farm grows a tart cherry orchard right up against a sweet cherry orchard. Nikki, Emily, Karen and I walk long rows, tying orange tape on trees, counting off five, hanging a dummy lure (the real ones won’t be hung until the end of May, but they are establishing grids, learning territory). They also note orchard perimeter, what’s there, what’s wild. They’ve already asked farmers to remove potential wild hosts, but now they need examples to test — so they search for wild honeysuckle, mulberries, wild raspberries. They dedicate long hours checking indicators, counting, collecting, communicating. They are observers of the best sort. We come to the last tree in a long row. I look onto the rough meadow; I look back down the row, see lures set every sixth tree, and see hope that we will learn the solution quickly. I ask Rothwell if there are bright spots in this dire situation. She says yes, that the greatest hope lies in “… how many people have come together around this threat. As an agricultural community, we are listening more deeply, learning more quickly, sharing more broadly, and making sure everyone is in the loop so we can find solutions that work.”

This story was produced for Nature Change, an online magazine that chronicles conversations about conservation and climate change in Northern Michigan. To watch a video featuring Dr. Nikki Rothwell and Anne-Marie Oomen discussing Spotted Wing Drosophila, click here.

In the next installment, Oomen talks to growers about this invasive species.