First, I would urge you to contact Neil Miller, who works with the USDA's Agricultural Research Service (ARS) researching fruit fly management. He has been working with many organic growers on the big island to help them manage fruit flies. His contact information is:
Agricultural Science Research Technician (insects)
Phone: (808) 959-4302
Fax: (808) 959-5470
920 Stainback Highway
Hilo, HI 96720
As you are aware, fruit fly control is very difficult, although there are some new products that can be effective. A first step in any pest-management program is to correctly identify the pest, so please collect some adults and have them correctly identified, or collect some infested fruit and rear out the adults to obtain a positive identification. Once this is done, insights into possible management strategies can be gleaned from examining the insect's life cycle. There are dozens of species of fruit flies, each with a range of preferred hosts and a unique set of behaviors. For example, one of the reasons that a new commercial fruit fly bait (GF 120) is effective against the melon fruit fly is the fact that the adult fly overnights and forages away from the host crop in which it lays its eggs. This can be seen as a "weak link" in its behavior that a management strategy can take advantage of. In this case, a border crop can be planted and sprayed with the bait, and if the border is wide enough, the melon fly will feed on the bait and perish before it has a chance to lay eggs on a host crop. Knowing the host plants and crops of the pest will influence management decisions with respect to where a future crop is planted and what types of crop rotations are used. If applied control measures are needed, proper identification is critical; some control measures work well with some species of fruit fly and less well with others.
Life Cycle and Ecology
The description that follows is a generalized life cycle of fruit flies. The adult female will pierce a thin-skinned fruit (a tomato, for example) and lay one to several eggs. Other females may follow suit, depositing eggs in the same hole. A single female generally has the capacity to lay several hundred eggs over the course of her life (1). The eggs hatch in 2 to 20 days (depending on temperature) and the larvae burrow into the fruit to feed for 10 to 40 days. At this time, the larvae will be roughly ¼-inch long and will leave the fruit and pupate in the top 1 to 2 inches of soil. Adult flies will emerge in 10 to 50 days. These times are very temperature-dependent, and the whole life cycle can be completed in two and a half weeks under ideal conditions.
Fruit flies are very mobile as adults and some species are strong fliers (e.g., Bactrocera cucurbitae (Coquillett), the melon fly). Sterile males of the medfly, Ceratitus capitata, have been found 24 miles from their release point.
Likely Species of Fruit Flies
It’s likely that one of three species is attacking your crop. The melon fly, Bactrocera cucurbitae (Coquillett), has more than 80 hosts and is a major pest of beans, bittermelon, Chinese wax gourd, cucumbers, edible gourds, eggplant, green beans, hyotan, luffa, melons, peppers, pumpkins, squashes, togan, tomatoes, watermelon, and zucchini (2). The oriental fruit fly, Bactrocera dorsalis (Hendel), attacks more than 300 cultivated and wild fruits including Annona (cherimoya, atemoya, sugar apple), avocado, banana, bittermelon, citrus, coffee, guava, macadamia, mango, papaya, passion fruit, peppers, persimmon, and tomato (3). This pest will apparently breed in all fleshy fruits. On Oahu it is estimated that 95% of the oriental fruit flies develop on guava, Psidium guajava L. (4). They do not attack cucurbit crops such as cucumber and squash. The last fruit fly suspect is the Mediterranean fruit fly, Ceratitis capitata (Wiedemann). This pest attacks more than 260 different fruits, flowers, vegetables, and nuts, with a preference for thin-skinned, ripe, succulent fruits (5).
Once the female fruit fly has deposited eggs in the fruit, it’s too late for managing the pest. Therefore, focus should be directed at preventing egg-laying, either by mechanical means such as bagging the fruit, or by use of processed kaolin clay (known as particle film barriers), or by removing sources of adult flies (sanitation practices). Use of insecticidal baits to kill the adult female before she deposits her eggs is discussed below, but may not be acceptable under organic production guidelines.
Sanitation is important in the control of any fruit fly. All dropped and prematurely ripe fruit, as well as small fruit suspected of being infested, should be destroyed to prevent the larvae from developing into adult fruit flies. The fruit may be fed to livestock (but must be eaten before any larvae have a chance to emerge), composted in a well-managed compost pile (so the heat will destroy the larvae), or buried 2 feet below the soil surface so that adult flies will not be able to emerge.
Bagging should begin when the fruit is small, shortly after the flower parts have fallen. This method of control is more adapted to small plantings (1 to 25 plants) than to large ones (quarter-acre or more). Although bagging the fruit is the most certain method of control it is a laborious process and requires attention at regular intervals (10 to 14 days) to keep the young fruit covered. Also, this procedure will injure some of the fruit unless they are handled carefully.
For more information about Surround, contact:
Marketing manager, Crop Protectants
There are some new pesticides developed by Dow Ag-Chemicals, based on spinosad, that are approved for use in organic systems. Spinosad is the common name of a mixture of spinosyn A and spinosyn D, two molecules derived naturally from a bacterium through fermentation (9). Entrust is an organically approved formulation of spinosad. There is also a fruit fly bait based on the same pesticide now being marketed under the name GF120. A researcher at Washington State University found that the cherry fruit fly could be effectively controlled using a combination of a wettable powder formulation of Entrust applied at 10-day intervals, combined with use of the GF 120 bait spray. GF-120 bait treatments were applied weekly, and reapplied after rain on some of the sites.
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