Spotted Wing Drosophila and Small Fruit Production in Kentucky

Last year was a disastrous year for many growers of blackberries and fall raspberries. The problem was small larvae in the berries. Frequently berries looked fine at the time of harvest, but had no shelf life and began to ooze and fall apart within a day or two of picking. This was the result of spotted wing drosophila (SWD), which is a new pest to Kentucky. It is a pest of thin- and soft-skinned fruit. While blackberries and raspberries were hard hit, some grapes and blueberries were also attacked last year. Last year (2013) was the first year with widespread problems in Kentucky. We don’t know if this will again be the case in 2014, so growers are advised to watch for SWD in strawberries, blueberries, blackberries, and raspberries.

Figure 1. Spotted wing drosophila adults on grapes.

Figure 1. Spotted wing drosophila adults on grapes.

SWD is a serious threat to small fruit production; one agent called it a ‘game changer.’ While fruit flies are common, this fruit fly is different. SWD can lay eggs under the skin of otherwise sound fruit just as fruit begins to soften and ripen. This can result in numerous ¼-inch white larvae in the fruit at harvest. In addition to damaging a large percentage of a crop, this pest also has the potential to upset customers and ruin the reputation of markets. Blackberries, raspberries, late-maturing blueberries, and some thin-skinned grapes are very susceptible to this invasive pest. However, based on the 2013 situation in Kentucky and over a long period of time in other parts of the southeastern U.S., June-bearing strawberries have not experienced these large-scale losses and have lower risk to SWD than other later ripening small fruit crops.

Trapping and spraying versus preventive sprays without trapping

When SWD is active, commercial producers of susceptible crops must spray on a weekly basis, beginning when the fruit begins to turn color and soften. If it rains, treatments need to be reapplied. Weekly sprays are continued until harvest is finished.  The difficult question is determining when SWD first becomes active. Small fruit growers producing for wholesale markets must meet a zero-tolerance level for insect contamination. Some of these growers in other southeastern states have found that beginning SWD sprays after trapping adults does not provide the consistency for their zero-tolerance markets. These southeastern growers begin sprays for blackberries, fall raspberries, and blueberries when the crop reaches the susceptible stage. Growers producing berries for local markets do not necessarily have a zero tolerance standard and are encouraged to use SWD traps and to begin sprays when the first adult is trapped. For example, one commercial KY farm with blueberries, blackberries, and raspberries used SWD traps in 2013 beginning in early summer. As soon as the traps indicated SWD was active, SWD sprays began on susceptible crops with SWD recommended insecticides on a weekly basis. Results were satisfactory. I think the keys to this success were:

  • Monitoring for SWD with traps.
  • Using recommended SWD insecticides when SWD was detected (See ENTFACT 230).
  • Getting thorough spray coverage inside the canopy; in this case the grower used an air blast sprayer.
  • Reapplying sprays to susceptible crops on a 5- to 7-day interval during the harvest period (shorter interval after rains).
  • Observing pre-harvest intervals carefully.

It is notable that the grower did not wait until he found infested fruit before he began SWD sprays. Had the grower waited to begin spraying, the results may have been different. My recommendation for strawberry, blueberry, blackberry, and raspberry growers this year is to use traps to monitor for the first instance of the adults and be prepared to spray as soon as SWD is detected.

SWD traps

We are recommending a 1-quart, clear deli container with filled with about 1 inch of bait solution to which one drop of liquid dish soap has been added (otherwise the flies can walk on the surface of the liquid). While sixteen ¼-inch holes are punched below the rim to allow SWD to enter the trap, we prefer a pair of 3½-inch by 1½-inch windows covered with 1/8-inch screen (Figure 2). Traps must be checked weekly and bait solution changed on each visit. Do not dispose of the bait solution in the planting as this will attract SWD away from traps. Last year we used an apple cider vinegar bait, but research has shown a yeast-sugar bait catches flies 1 to 2 weeks earlier. Yeast-sugar baits are made by combining 2 tablespoon yeast, 4 tablespoons sugar, and 1 quart of water. Fresh bait must be prepared each week.

Figure 2. SWD trap with screen windows on either side. Traps should be placed inside the canopy.

Figure 2. SWD trap with screen windows on either side. Traps should be placed inside the canopy.

Other management tools

Cool berries

Another very important practice is to immediately cool the harvested berries. This suspends the development of eggs and larvae. Prolonged cooling below 35°F for 3 or more days has been shown to provide upwards of 90% control of eggs and larger larvae. Freezing fruit will kill SWD. While washing berries is a good practice for other reasons, it will not rid berries of eggs or the larvae inside.

Clean harvest

Producers should practice clean harvest and not leave overripe, damaged, or unmarketable fruit in the field. At harvest, if the stem end of the berry looks watery, it is likely infested with SWD. Sound fruit with a dry stem end may still be infested with eggs and should be refrigerated immediately. Ideally, producers should pick using two containers; one for sale and the other for disposal.  Don’t bury fruit to dispose of SWD-infested berries as the insect can emerge from fruit buried up to 3 feet underground. Fruit to be disposed of should be placed in a clear plastic bag. The bag needs to be sealed and left in the sun to solarize and kill the SWD.

Mechanical exclusion

Small commercial plantings and home gardeners may chose to use mechanical exclusion using fine mesh or spun-bond materials. To exclude SWD, the fine openings need to be less than 0.9 mm, the netting needs to be in place at least 2 to 3 weeks prior to the start of harvest, and the net needs to be secured at the bottom.  The netting should be left in place while picking.  SWD is generally less active during the middle of the day, so that is a good time to pick.

Backyard fruit

Home gardeners are less likely to spray regularly and do not have the same options available.  I do recommend that backyard crops be harvested carefully and soft berries be discarded. Home gardeners should refrigerate fruit immediately as this will arrest the development of eggs and any small larvae that may be present inside the fruit.

Additional information

There are three factsheets on the UK entomology Web site that can provide help with SWD:

 

By Ric Bessin, Extension Entomologist

Posted in Fruit