Overview:

Western Cherry Fruit Fly is native to North America, and has been found in the Pacific Northwest states since the 1940's. This pest lives only on cherry, has a single generation per season. It may emerge from the soil over an eight week period, with peak emergence occurring about the time of harvest. Though they are rarely found in commercial orchards, cherry fruit fly is 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. This zero tolerance has forced growers into intensive control programs to achieve perfect control. The advent of the US Federal Food Quality Protection Act has caused special concern to sweet cherry growers, as the two products most commonly used to control this insect, azinphos-methyl and carbaryl, are receiving special regulatory attention during the evaluation process, due to their very common usage on high-profile crops. It is likely that adjustments in pre-harvest interval would greatly restrict the usefulness of these products for pest control in sweet cherries, as the target pest is most present near the harvest period. Alternative, effective, and environmentally acceptable cherry fruit fly control materials and methods are being researched and adopted.

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 have been reported as hosts, it is uncommon that this pest is found on any tree other than sweet or tart cherry. Most Pacific Northwest growers have never seen a cherry fruit fly, and have no method to determine if they exist in their orchard in low numbers. Even though they are rarely found in commercial orchards, cherry fruit fly is 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 is rarely found to be infested. However, single larvae are found during inspections of fruit produced in Washington State's 12,000 hectares of sweet cherries from one to ten times per season. 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 is usually 2 to 10 cm below the soil surface, and most fly to the closest host. They live on the host tree, consuming sugars from extra-floral nectaries on the leaf petiole, aphid excretions, and cherry fruit wounds. Bird droppings are an important food source, as nitrogen and protien is not easily found in other forraged food items. Female cherry fruit fly often create feeding sites by wounding the fruit with their ovipositors, without inserting an egg. 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 short 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. After about 4 days, the larva molts into the 2 - 4.5 mm second instar. This stage lasts about another 4 days, after which the larva converts into the rapidly growing third instar, 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. 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 very few remain in the pupal state until the second growing season after pupation.

Management

Cultural Management

Most cherry fruit fly in the Pacific Northwest USA are found in non-commercial sweet or tart cherry trees planted in home orchards. Few hobby orchardists have the ambition, knowledge or equipment necessary to control this pest. Pest populations can be greatly reduced in a region by organized efforts to identify and remove these wild or neglected host trees.

Traps are not an effective control or monitoring device in commercial orchards, as they are for many other pests. The available traps do not attract flies well, as they rely on visual cues to attract the pest, rather than the far more effective pheromones. Cherry fruit fly have no sexual attractant pheromones. The only pheromone the female cherry fruit fly produces is a repellant, most likely used to indicate to others that an egg has been deposited into a specific fruit. The two most commonly used traps are yellow, two sided, 8 x 12 inch rectangles or red 4 inch spheres, 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. Attempts by the author to control populations of cherry fruit fly by the placement of numerous traps in infested trees have not been successful.

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 mid-harvest. The author determined that from 10,000 to 20,000 fruit/acre remain in the average well-picked orchard after harvest. These fruit are sufficient to maintain endemic populations of this pest in the orchard, unless control is continued post-harvest. If all of the fruit is removed from an isolated tree or orchard prior to the time larvae emerge from the fruit, the trees become free of the pest until reintroduction. This re-establishment may occur rapidly, but often may not occur for several years. 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.

Biological Control

Effective biological control agents have been not been identified. There is a current research effort to develop control programs aimed at the over-wintering pupal stage, utilizing specific nematodes known to attack insect pupae. Many isolated cherry trees remain free of infestation due to the complete removal of fruit each season by birds.

Chemical Control

The advent of the US Federal Food Quality Protection Act has caused special concern to sweet cherry growers, as the two products most commonly used to control cherry fruit fly, azinphos-methyl and carbaryl, are receiving special regulatory attention during the evaluation process, due to their very common usage on high-consumption crops. It is likely that adjustments in pre-harvest interval would greatly restrict the usefulness of these products for pest control in sweet cherries, as the target pest is most present near the harvest period. Alternative, effective, and environmentally acceptable cherry fruit fly control materials and methods continue to be researched and developed.

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. The phenology model has predicted first trap catch of adults on heavily infested trees very accurately during eight seasons of use in Washington State.
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, malathion ultra low volume applications, and spinosad (Success, Spintor, Entrust). Residual products kill both by contact and by residual action, probably through injestion as the adult cleans itself or feeds on the treated tree surface. Products in this group include imidacloprid (Provado), carbaryl (Sevin), diazinon, azinphos-methyl (Guthion), and dimethoate. 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 20 fluid ounces per acre diluted in about 1.5 to 2 gallons of water per acre. (application rig) Dimethoate is the only registered product that can control the larvae inside the fruit, and is most effective when applied within a week after harvest. Non-residual," knock-down" products are usually applied by air or ground sprayer about every week to prevent egg laying. Products that kill flies both by contact and residue are generally applied by air-blast sprayer every 10-14 days. Careful planning is necessary as harvest nears, as pre-harvest intervals, export restrictions, and control leading up to and during extended harvest must be taken into consideration. Most control failures occur in the last 10 days prior to, and during harvest. There is no evidence of resistance to any insecticide in the Pacific Northwest cherry fruit fly population.

As biological controls have been ineffective to date, reducing the potential impact of cherry fruit fly control has recently centered on the assessment and registration of alternative spray control products. While formerly "organic" products such as rotenone and piperonyl butoxide have recently been taken off of the market, very effective, but less toxic, products such as spinosad (Success, Entrust) and nicotinal class insecticides (imidacloprid, Provado) are being registered. Horticultural summer weight mineral oils at 1 percent solution, sprayed weekly, controlled cherry fruit fly on heavily infested trees, but dulled the skin of the fruit, rendering it commercially unacceptable. Most newly registered products have pre-harvest spray intervals of seven or more days, leaving the grower with few choices near and during harvest. A worthy exception to this is the GF-120NF bait, which has a 0 day re-entry and pre-harvest interval.

Concluding Remarks

This pest causes very limited direct damage to cherry fruit. It is, perhaps, the most successfully controlled tree fruit pest in the Pacific Northwest USA. However, due to the large populations that survive on neglected trees, cessation of control efforts would lead to rapid infestation of commercial orchards, and complete loss of this major crop. Research continues to find better monitoring tools and less toxic control options. Due to a zero tolerance for infestation due to quarantines, intensive spray programs are likely to continue as the major management approach.

References

Beers, E.H., Brunner, J.F., Willett, M.J., and Warner, G.M., 1993. Orchard pest management, a resource book for the Pacific Northwest. Good Fruit Grower, 276 pp.

Frick, K.E., Simkover, H.G., and Telford, H.S., 1954. Bionomics of the cherry fruit flies in Eastern Washington. Washington State University Tech. Bul. 13, 66 pp.

Jones V.P., Alston D.G., Davis D.W, Brunner J.F., Shelton M.E., 1991. Phenology of western cherry fruit fly (Diptera: Tephritidae) in Utah & Washington. Ann. Ent. Soc. Am. 84: 488-492.

Mayer, D.F., Long, L.E., Smith, T.J., Olsen, J., Riedl, H., Heath, R., Leskey, T., and Prokopy, R.J., 2000. Attraction of adult Rhagoletis indifferens (Diptera: Tephritidae) to unbaited and odor-baited red spheres and yellow rectangles. J. Econ. Entomol. 93(2): 347-351.