Cherry Fruit Fly

Introduction

Western cherry fruit fly is native to North America, and has been found in the Pacific Northwest states since the 1940's. While native cherry species such as "Bitter Cherry" (Prunus emarginata) have been reported as hosts, it is uncommon that this pest is found on any tree other than sweet or tart cherry. Cherry fruit fly is considered the primary insect pest of sweet cherries in the region. Quarantine agreements between the region and other states or countries result in a zero tolerance for cherry fruit fly larvae in packed fruit. Washington State Department of Agriculture inspectors are stationed at each cherry packing facility during the harvest season to check fruit for infestation as it comes to the packinghouse, prior to acceptance, and again after packing. Fruit may be inspected again prior to entering California. When a larva is found, the entire load of infested fruit is rejected, and all other fruit from that grower is intensively inspected for signs of cherry fruit fly. It is quite rare that subsequent inspections of the "infested" fruit result in discovery of other larvae.

Life Cycle

This fly has a single generation per season, emerging from the soil under the host tree for about eight weeks, with emergence peaking around sweet cherry harvest time. During the peak emergence period, about 15 to 20 percent of the population emerges each week. The adult emerges from the pupae which has over-wintered about 2 to 10 cm below the soil surface. Almost all emerging adults stay on the closest host, but will disperse if there is no fruit on the tree, or if the host has been removed. The adults live on the host tree, consuming micro-organisms and pollen grains from the leaf surfaces, aphid excretions, and cherry fruit wounds. Bird droppings are an important food source, as nitrogen and protein is not easily found in other foraged food items. Adults are most active on relatively warm days with light or no wind. They can fly several hundred meters searching for a new host, but most remain near their emergence site. After about five to ten days feeding, maturation and mating, they begin to lay eggs. Most egg laying occurs after the early to mid-season cultivar fruit begins to turn yellow-green. Egg deposition starts in green fruit about the same time on the later varieties (Frick, 1954), so fruit development stage should not be used as a timing method for first spray covers.

Each female may deposit 100 to 300 eggs under the fruit skin over a period of thirty days, with highest activity during the first fourteen days after mating (Frick, 1954). During this most active time, ten to twenty eggs may be oviposited each day. Usually, only one egg is inserted into each fruit, unless the population on the tree is very high, and no alternative hosts are near. The eggs hatch five to eight days after deposition.

The first instar larva is a typical legless and headless fly maggot, about 1 to 2 mm in length. After a one to three day feeding period near the surface, the first instar maggot mines to the center of the fruit, where it remains, near the seed, for most of its development. About four days after egg hatch, the larva molts into the 2 - 4.5 mm second instar. This stage lasts about another four days, after which the larva converts into the rapidly growing third instar (pictured at right), which increases to about 8 mm over the next eight days. About three days prior to leaving the fruit, the larva burrows to the fruit surface, where it cuts one to three 1 mm holes in the skin. When nearing the end of its third instar, the maggot emerges from the fruit and drops to the orchard soil surface. This entire egg to emergence of larvae takes about 21-25 days, but in cooler conditions may be extended to 35 days. Few cherry fruit fly can emerge from the fruit to complete their life cycle prior to harvest of most cherry varieties that ripen with or before "Bing." The greatest percentage will emerge from the fruit from one to three weeks after fruit turns red. After dropping out of the fruit, the larvae rapidly seek out a place to burrow into the soil, penetrate to a depth of 1 to 6 inches, and pupate. They remain in this state through the winter, emerging the next growing season. A low percentage remain in the pupal state until the second growing season after pupation.

Management

Most cherry fruit fly in the Pacific Northwest USA are found in non-commercial sweet or tart cherry trees planted in home orchards. Pest populations can be greatly reduced in a region by organized efforts to identify and treat or remove these wild or neglected host trees.

The two most commonly used trap for detection is a yellow, two sided, 8 x 12 inch rectangle covered with a sticky substance and baited with ammonium carbonate (Mayer, et al., 2000). These traps are used to monitor first emergence of adults on infested trees.

Pest populations can be suppressed or eliminated by careful attention to harvest. Few cherry fruit flies will complete their life cycle if all fruit is picked and removed from the orchard each season early in the traditional harvest period. If they are not controlled early in the fruit ripening period, some of the most advanced larvae will emerge and drop to the soil by the time "Bing" cherries are fully ripe. If all of the fruit is removed from an isolated tree or orchard for two consecutive seasons prior to the time larvae emerge from the fruit, the trees become free of the pest until reintroduction. If fruit is abandoned due to rain cracking, and post harvest pest management is neglected, cherry fruit fly populations often greatly increase and control is more difficult the next several seasons.

Chemical and Biological Control

Effective biological control agents have been not been identified.

The zero quarantine tolerance of this pest has forced growers into intensive control programs to achieve perfect control. Commercial growers begin spraying when first fly emergence is detected on infested sentinel trees, or when temperature-driven phenology models (Jones, et al.,1991) indicate emergence has commenced in the region. Growers continue to spray every week to ten days, depending on product used, until harvest is completed. Usually, one or two sprays are applied post harvest to disrupt the attack on unharvested fruit.

Control materials fall into three categories: "knock-down" ,"residual"and "bait." Knock-down products kill the adults only if the substance contacts them during or very soon after application. Included in this group are pyrethrums and malathion ultra low volume applications. Recent research indicates spinosad (Success, Spintor, Entrust), works well when applied at 10 day intervals. These products kill both by contact and by residual action, probably through ingestion as the adult cleans itself or feeds on the treated tree surface. Products in this group include imidacloprid (Provado), spinetoram (Delegate), carbaryl (Sevin), diazinon, azinphos-methyl (Guthion), and dimethoate. Most residual products also have "knock-down" activity.

A bait (GF-120NF) should be considered residual, as it is an attractive substance with spinosad as the active ingredient. It is lethal to flies that feed on it while "grazing" on the tree. This bait is "squirted" and spattered on the trees weekly at recommended rates.  Baits have no immediate action on the cherry fruit fly on the tree, so should be first applied a few days prior to predicted emergence and maintained on the foliage continuously. Foraging young flies find and eat the bait before they mature and lay eggs. Baits cannot prevent at least some egg deposition by a mature female cherry fruit fly that migrates into the orchard from near-by infested trees. That may be why the bait works best in regions where few infested host trees are near the treated orchard.  The GF-120 bait has proven very effective and practical as a control for CFF on home garden cherry trees.