One good way to lose income is to have an entire crop rejected at the point of sale. There is zero tolerance for spotted wing drosophila (SWD) infestation in fresh marketed fruit. Infested fruit looks yucky and is flat-out unmarketable.
Because of the economic impact of the pest, researchers are becoming more concerned about the damage done to crops like blueberries, blackberries, raspberries, strawberries and cherries by SWD.
“This pest is here to stay and will remain at economically significant levels,” said Hannah Burrack, associate professor and extension specialist in entomology and plant pathology at North Carolina State University. Unlike other pests – say brown marmorated stink bug – she says SWD shows no sign of dropping off soon.
“You will have to manage SWD,” she added. To that end, researchers are trying to identify better monitoring tools to track risk, and discover when to treat, what to use for treatment, and how to minimize insecticide use by integrating non-chemical tools.
So serious is the challenge to fresh fruit producers that the U.S. Department of Agriculture Research Service made a $6.7 million grant to learn more about SWD (Drosophila suzukii). SWD even has its own website at http://swdmanagement.org. The research, which involves a host of colleges across the country, is being conducted at Cornell University, the University of California campuses at both Davis and Berkeley, University of Georgia, University of Maine, Michigan State University, North Carolina State University, University of Notre Dame, Oregon State University and Rutgers University. The four-year project is in its second year.
From that project comes hope in the form of a natural parasite whose home range is China and South Korea. “We are working on classical biological control,” said Kent Daane, at the University of California-Berkeley. He is in extension with offices both at Berkeley at Kearney Agricultural Research and Extension Center in Parlier, California.
Right now, those agents are in quarantine by USDA. “If it does come out of quarantine, it will have a positive impact on SWD,” Daane said. “Proactive prevention is important,” agreed Burrack.
Buyers reject SWD-infested fruit. “If they find a single larva in a fruit, the entire load from that grower will be rejected,” Burrack said. Nationally, she estimates the economic loss to growers on an annual basis is over $700 million.
If fruit appear otherwise undamaged – having no holes, rotted areas, or disease damage – except for a soft spot with larvae underneath, it is still likely that those larvae are SWD.
Ten years ago, SWD did not even exist in this country. Since it was first detected in California in 2008, SWD spread rapidly across the country. It made it as far east as Georgia by 2010. “Since then, this small vinegar fly has impacted the $255 million Georgia blueberry industry with crop losses of up to 20 percent annually,” stated Phil Brannen, professor of plant pathology at the University of Georgia in Athens.
SWD has even been detected in later-season crops, including strawberries in North Carolina. This, Burrack said, is likely due to the warmer springs and milder winters of late. It is important for growers to scout for SWD.
KNOW YOUR FOE
SWD flies have brownish-yellow thorax, black stripes across the abdomen, and distinct red eyes. Males have dark spots on the wingtips and black combs on the forelegs. Female SWDs lack the spots and black combs. They do have a large serrated ovipositor. The adult female punctures the skin of intact fruit using its serrated ovipositor and deposits white eggs just under the fruit skin, laying as many as 500 or 600 in her lifetime.
SWD larvae eat the fruit pulp, causing the fruit to collapse. This often occurs within days of egg-laying. “If SWD is not managed properly, fruit infested with SWD larvae may be harvested, which will result in either downgrading or rejection of the entire shipment,” Brannen warned.
Growers of SWD hosts should begin preventative treatment when fruit becomes susceptible, which is when it starts to change color. Both Brannen and Burrack recommend harvesting frequently and removing leftover fruit from the orchard to reduce fly feeding and breeding resources.
While growers are well aware of the economic impact of SWD, there is a lot that we do not know about the pest. Burrack ticks off a list:
We cannot say for sure at this time where SWD spend most of their time and when during the day they are most likely to be present in fields.
We do not know how many flies it takes to cause a significant infestation, so we don’t know how many we need to kill.
We do know that when they come into our fields, the females are attracted to and interested in laying eggs in fruit.
Therefore, the best known opportunity to impact flies is when they land on fruit, whatever time that might be.
Burrack encouraged growers to think about protecting fruit, rather than thinking about killing flies at the time of treatment. With this strategy, growers should treat when it is safest for bees and other beneficial insects but focus on achieving and maintaining good coverage of fruit.
“Evening or night treatments generally have the least potential impact on pollinators and other beneficial insects because they are not active at these times and there is as long a period as possible for materials to dry before bees become active the next day,” Burrack said. “The goal of SWD treatments should be to make sure that the fruit has insecticide sufficient to kill flies when they land on it.”
For large plants, treating every row is likely necessary to achieve good coverage, Burrack said. “When in doubt, growers should check coverage with water-sensitive spray cards. They may be surprised that alternate row spray, even in dormant plants, may not result in acceptable coverage.” she said.
Remove SWD habitat in fence rows and field borders. Wild plants that may be hosts for SWD include any wild plant with berries: grapes, beautyberry, elderberry, pokeweed, pokeberry, honeysuckle, nightshade, dogwood, spicebush and autumn olive. Get rid of them.
TEST WORTH ITS SALT
To see if SWD is a problem in your fields, sample fruit each harvest by either cutting them open and looking for larvae or soaking them in salt water. An excellent video (with foot-tapping music) illustrates how to perform a salt test. It can be found at the North Carolina State University Entomology web site.
The key to minimizing the likelihood of a SWD infestation is to practice excellent sanitation at all stages of the process. Thoroughly harvest all ripe fruit and sell or destroy the fruit if it is deemed unmarketable. Whatever you do, do not discard culls in the field.
Have a crew clean up after rain and after pick-your-own customers leave the field. Unpicked fruit is a reservoir for SWD larvae. Burrack recommends storing fruit cold post-harvest for as long as possible. Most eggs and larvae do not develop further at temperatures below 40F, and some may die. If fruit is kept cold, any eggs that may have been laid will not hatch, and some may die. In the field, begin or resume aggressive spraying. This generally requires a spray once a week if it does not rain, with a reapplication after any rainstorm. Rotating between at least two modes of chemical action will reduce the likelihood of resistance development. In a typical program, this might mean rotating Entrust with Pyganic to reduce the risk of resistance.
Insecticide use in crops that are susceptible to SWD has grown by at least 30 percent in response. For this season, then, be aggressive both with monitoring and proactive prevention. Stay tuned next year for the possibility of a new weapon in the arsenal against SWD.