CHERRY FRUIT FLY: THE "PHANTOM" PEST  Timothy J. Smith WSU Extension North Central Washington

Cherry fruit fly damages very little fruit in Washington. Less than a dozen infested fruit are found in packing houses each year. Few growers have ever seen one, especially in their orchards.

So why do we pay so much attention to them? Markets, that's why. California says they don't have cherry fruit fly, and since they also produce cherries, don't want them. Under the present quarantine agreement, they will accept unfumigated cherries from us as long as we can assure them that the pest is not in packed fruit. We should do whatever is necessary to continue to produce high-quality, pest-free cherries to keep this agreement in force. Anything that slows the orderly marketing of cherries may depress prices, especially during seasons of higher than normal production.

KNOW THE PEST

The cherry fruit fly is a very weak flyer, so it spreads slowly. Two backyard unsprayed trees may be within a hundred feet of one another, one infested heavily, and the other free of the pest. The adult may fly into the orchard full of fertile eggs, ready to infest fruit, but this is probably fairly uncommon. It is much more likely that the adult emerges from the soil under the host tree and stays on or near the host for several days prior to laying its eggs. If you understand the ways of this pest, you can better understand its control. Below is a description of the insect's lifestyle and some general control suggestions:

As with all flies, this insect has four life stages: egg, maggot, pupa, and adult. ( Picture )

PUPA: The cherry fruit fly spends about 11 months of its' 12 month life in the pupa life stage. The pupa remains 1 to 5 inches deep in the soil under the tree where the larva crawled the previous summer. The development rate of the pupa is affected by variable soil temperatures, so, as a population, they do not all develop into adults and emerge at the same time.

There is some evidence that a few pupae remain dormant and emerge after two winters, assuring continued infestation even when no fruit is produced on the tree for a full season.

ADULT: There is only one generation per season., but the adults emerge from the soil over an extended period. Each adult can live for 16 to 40 days. First emergence timing depends on the Winter and Spring temperatures, and can be predicted very accurately with a model. Model information is posted each season elsewhere on this web site. Generally, adult emergence begins about four weeks prior to bing cherry harvest, and continues until about 4 weeks after harvest. ( Chart)

After hatching, the adults move up into the trees and feed on honeydew and other edible substances ( such as bird droppings) on the leaf surface. As the fruit begins to ripen, the females puncture the fruit with their ovipositors (egg-laying structures). These punctures do minor damage to the fruit, but the juice oozing from the fruit provides additional food for the flies.

The adults mate soon after they emerge, and may begin laying eggs 5 to 10 days after emergence. Rate of adult maturation depends on daily temperature, the warmer the temperature, the shorter the period between emergence and the beginning of egg laying. Eggs may be oviposited under the skin of green fruit, but the flies seem to prefer fruit in "straw color" or riper.

Each female can lay as many as 300 eggs during the month after she mates, but 50 to 150 is a more likely number under natural conditions. During the first few days after she starts ovipositing, the female can lay as many as 15-20 eggs per day. Due to this high activity per female, fruit infestation of unsprayed trees can rise from just a few percent one season to almost 100 percent the next.

When the female deposits an egg, she marks the fruit with a pheromone to signal other females that the fruit is already "in use". Generally, other females will not lay an egg in that fruit, but they will if the majority of fruit on the tree is already infested.

The adult is a very weak, or lazy, flier. A few cherry fruit flies will fly long distances, dispersed down-wind, especially if their host has no fruit, or is removed during the dormant season. However, most CFF will not travel much farther than the first cherry tree they find, which is usually within a tree or two of where they emerged. To reduce the spread of CFF from highly infested neglected trees, it is best to control the fruit flies on those trees prior to their removal, if practical.

Traps: Adult fruit flies may be trapped, using a yellow sticky board, (or much better yet, a 3 1/2 inch sticky red ball) hung low in the tree and baited with ammonium carbonate. However, unlike many moth pests, Cherry Fruit Fly are not strongly attracted by the colors or odor, so traps are not practical for control or monitoring for low numbers of flies in the commercial orchard. ( Trap picture )

EGG: The egg is deposited in a small slit, under the skin. They hatch in 5 to 8 days, depending on temperatures. As they are under the fruit skin, they are generally safe from predators and insecticides.

LARVA: After the egg hatches, the 1- to 2-millimeter-long, headless, legless larva (maggot) burrows directly to the pit. It feeds there for 2 to 5 days, then molts into the 2- to 4.5 mm-long second instar.

Feeding continues near the pit for about 4 days. After the maggot molts into its third stage, it makes its most rapid growth, developing to about 8 mm long (3/8 inch) during the next 8 days.

The total time it takes the larva to develop in the fruit can vary from 10 to 21 days, warmer temperatures leading to most rapid development.

About three days before it emerges from the fruit, the larva bores a ventilation hole to the surface. It then emerges from the fruit, drops to the ground, and burrows into the soil, where it pupates. This completes the life cycle of the cherry fruit fly.

MANAGEMENT OF CHERRY FRUIT FLY:

1. REDUCING CHERRY FRUIT FLY SOURCES

Identification and eradication of fruit fly sources in the cherry production region has been handled very effectively by the Chelan-Douglas Tree Fruit Pest and Disease Control Board. ( Marlane Gunnard 667-6677) and the

Okanogan County Tree Fruit Pest and Disease Control Board ( Dan McCarthy - 422-710 )

Reduction of fruit fly numbers through the control of unsprayed populations is a key to long-term CFF control within production regions. Those areas with many abandoned or neglected trees have much more difficulty assuring others that their fruit is free of infestation.

2. PROPER SPRAY TIMING

There has always been strong emphasis on proper timing of the first cherry fruit fly cover spray. Were we starting too late? Maybe the fly was getting a head start? Most growers use to apply their first cover spray a week or more prior to the time we caught the first fly in a highly infested "monitor" trees, and two weeks or more ahead of possible fruit fly egg-laying, so it was not too likely that CFF was "sprayed too late". With the advent of the highly accurate Cherry Fruit Fly Model, most of the industry is applying their first CFF cover with proper timing and confidence.

Most "timing" difficulties are actually within the spray season. The interval between sprays is now the critical question to be addressed. When you properly spray your orchard, the adults that are present are almost always killed by spray contact, if you are using an effective product and spraying so that the entire tree is contacted by the spray. Some old and new products do very little other than kill by contact. A few products leave a residual that kills the fruit fly as it feeds on the tree surface during the 5-7 days after application. These residual products are becoming fewer by the year.

The days of effective control you can expect from a residual spray depends on the product, and the amount of rain that occurs after spraying. In general, the more conservative (and less fly-affected) growers never let more than 10 days pass between applications of residual materials, and contact sprays are applied at least every 7 days. The between-spray interval listed on the product label should not be used as a guideline as to how long the product will effectively control the pest. Products must often be alternated to allow 10 day or less spray intervals. Cherry Fruit Fly seem to slip in between sprays when the interval is more than 7 days between "contact" materials and more than 10 days between "residuals."

Most of the larvae found in fruit at harvest are quite small, indicating infestation took place in the two weeks prior to picking. In many instances, infested lots were picked near the end of that grower's harvest, often a week or more after they began picking and two to four weeks since their last ground-applied spray. Remember that harvest of mixed varieties in the same block may take 7-10 days to complete. Sprays must be applied on a 7 day schedule (at least), even when harvest is under way. Apply a product with short re-entry and pre-harvest interval in the few days that may occur between the picking of varieties.

Most of the season's flies are emerging the final two weeks before harvest, and during harvest, so frequently applied "contact" materials with short re-entry and pre-harvest intervals are doing most of the pre-harvest control in orchards.

3. POST-HARVEST MANAGEMENT:

The final 25 to 40 percent of the cherry fruit fly population emerges over a 3-4 week period after harvest into an (usually) unprotected orchard with 10,000 to 25,000 fruit per acre (yes, that many!) left behind in a well picked orchard. These cherries are under leaves, behind scaffold limbs, or on limbs above the reach of even the tallest pickers. If there are any unpicked trees, the sky's the limit for potential infestation.

These unsprayed fruit fly have plenty of time to infest the missed cherries and set up a population of flies to emerge the next season. These flies are likely the vast majority of what will be in the orchard each year.

Letting the pest have free access to your orchard after harvest for several years may allow them to build up to levels that will be difficult to control. Since picking absolutely all of the fruit is nearly impossible in a large-scale operation, spray to control the flies for at least 3 weeks after harvest. The easiest, and most effective way to do this is with one high-volume (dilute) application of dimethoate applied within the week after harvest is completed. This is the only product that will kill larvae INSIDE the fruit. High volumes of water greatly reduce the minor effect this product may have on the trees. The rumor that this spray application increases fruit doubling is not true. Hotter than usual (95-105F) temperatures during July-early August causes fruit doubling and suturing.

Controlling the post-harvest half of the fruit fly population will greatly reduce pressure the next season, and is another key to the control of this pest.

4. IMPROVING CONTROL OPTIONS

 Most of the effective products used to control this pest in commercial orchards are in some danger of being lost for this use during the re-registration process brought about by the "Food Quality Protection Act".

There are at least two very safe, but effective products that are or will be registered for Cherry Fruit Fly control in Sweet Cherries, perhaps in time to partially offset the loss of the older, safe, but politically unpopular products now in use.

Tests are under way- watch for results.

There is no evidence that the products we presently use are losing their effect due to resistance in the cherry fruit fly population. Many of the cherry fruit fly that survive from one season to the next are not exposed to pesticides at all, which maintains the population in a highly susceptible state. Spray application, rather than lack of spray product choices is likely to remain the greatest spray problem.

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