How to Manage Pests
UC Pest Management Guidelines
Spotted wing drosophila has recently been found in many California counties infesting ripening cherry, raspberry, blackberry, blueberry, and strawberry crops; it has also been observed attacking other soft-flesh fruit such as boysenberry, varieties of Japanese plums, plumcots, and nectarines. Adults and maggots closely resemble the common vinegar fly, Drosophila melanogaster, and other Drosophila species that primarily attack rotting or fermenting fruit. The spotted wing drosophila, however, readily attacks undamaged fruit.
Adults are small (2-3 mm) flies with red eyes and a pale brown thorax and abdomen with black stripes on the abdomen. The most distinguishable trait of the adult is that the males have a black spot towards the tip of each wing. The female does not have spots on its wings, but their ovipositor is very large and serrated, unlike other common Drosophila species. Larvae are tiny (up to 3.5 mm), white cylindrical maggots that are found feeding in fruit. One to many larvae may be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.
At this point not much is known about its life cycle in California; however, like other vinegar flies it appears to have a short life cycle (one to several weeks depending on temperature), and may have as many as ten generations per year. This rapid developmental rate allows it to quickly develop large populations and inflict severe damage to a crop.
In Japan and in coastal California the adult flies may be captured throughout much of the year. They are most active at 68°F; activity decreases at temperatures above 86°F, and egg laying stops at about 91°F.
Unlike other vinegar flies that occur in California, spotted wing drosophila attacks healthy ripening fruit as well as damaged or rotting fruit. The female's serrated ovipositor is very large and able to penetrate the skin of soft-skinned fruit and lay eggs just under the skin, creating a small depression ("sting") on the fruit surface. Each clutch of eggs is from one to three, and the female will oviposit on many fruit. Multiples of larvae within a single fruit are quite possible because many females may visit the same fruit to oviposit. As fruit integrity is compromised by spotted wing drosophila's activities, common vinegar flies (i.e., Drosophila melanogaster) may also oviposit in the damaged fruit.
Eggs hatch and the maggots develop and feed inside the fruit, causing the flesh of the fruit to turn brown and soft; sunken areas that exude fluid often appear on the fruit surface. Damage can provide an entry site for infection by secondary fungal and bacterial pathogens, but this is not always the case.
Spotted wing drosophila may be monitored with a variety of traps. In the berry production districts of the Central Coast of California, one of the most successful trapping methods has been a yeast-sugar-water mix in a jar or bottle trap. Beyond the capability to consistently trap spotted wing drosophila, this mix is sufficiently clear to easily distinguish the flies, and it can be used for several weeks without changing the liquid.
To make the bait solution, mix 12 oz of water with 0.25 oz of baker's yeast (e.g., Fleischmann's) and 4 teaspoons of sugar. Allow the solution to ferment for a day or so (in an open or loose-lidded container–quite a bit of gas is formed during the fermentation process) then transfer 3 to 4 fl oz of the liquid to a 500-ml Nalgene bottle or other container of low height that has four or more 7/16-inch diameter holes drilled into the lid. (The idea is to use a container that is low enough that the opening is well below the plant canopy.) Flies enter the bottle through these holes and while there is the possibility of flies escaping back out through the holes, most eventually land in the liquid and drown.
While some jars or bottles can be hung with a wire in the shady, cooler areas of the field or farm, others should be placed directly in the strawberry field itself. It is important that the traps be placed in the shady canopies of the strawberry plants. Check traps at least weekly and count and remove the flies.
While no set management program has yet been determined for spotted wing drosophila, a successful one will need to focus on controlling flies before they lay eggs and reducing breeding sites. There are no effective tools for controlling maggots within fruit. Three essential parts of a management program will likely include:
1. Attractant bait sprays and pesticides. Pesticides such as malathion, pyrethroids, and spinosyns have been shown to be very effective in reducing numbers of spotted wing drosophila. Horticultural oils do not show much promise as a control agent. As always, applicators should be aware of restrictions in using pesticides, especially the effects that they may have on nontarget organisms such as predators, parasites, and honeybees. Growers should consult with their processors concerning any export restrictions for specific products.
2. The fruit seems most susceptible to attack after it has colored and developed some sugar. If monitoring indicates pest presence, apply a spray to protect the fruit. If monitoring indicates a high population early in the season, an earlier spray to lower populations may be warranted in addition to harvest applications.
3. Attractant-based bait sprays targeting adult flies such as GF-120 (bait plus insecticide) can be applied and could prove effective, but data on this approach are lacking for strawberries. Because it is a bait, coverage is not as important as keeping the bait attractive. Applications made at greater product concentration and with very large droplet size (4-5 mm diameter) across the field and border areas can be useful in reducing fly populations while minimizing effects on predators, parasites, and honeybees. However, because the efficacy of any bait and toxicant decreases over time, this material need to be re-applied, perhaps at weekly or bi-weekly intervals to be effective. Traps will need to be monitored to assure that adult fly suppression has been achieved.
4. Sanitation. Infested fruit that remains in the field or orchard serves as a food source and allows eggs and larvae to fully develop and serves as a source of more flies. When feasible, removing ripe, overripe, and rotten fruit from the crop site and destroying, either by burial or disposal in a closed container can help to reduce populations of this pest. This can be especially important if a nearby susceptible crop will soon be ripening.
5. Harvest intervals. Even though spotted wing drosophila oviposits in newly ripening fruit, they readily infest older, over ripe fruit as well. Extending harvest intervals, as may occur for the processing crop, will results in larger populations, more fruit damage, and a greater risk for future infestations of the new crop.
6. Area-wide management. In looking at other successful programs of fruit or vinegar fly management, it is clear that using the above practices over a wide area is essential. It is important for every grower within a fly-infested area to participate, because a single, unmanaged field or orchard will serve as a source of infestation to nearby susceptible crops.
UC IPM Pest Management Guidelines: Strawberry
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