Managing Pepper Maggots with OMRI-Approved GF-120 Fruit Fly Bait

The pepper maggot (PM), Zonosemata electa, is a Tephritid fruit fly native to horsenettle in North America, which first attacked peppers and eggplant in 1921. It has a patchy distribution throughout its range, which includes the eastern United States and Ontario, but also extends west to Kansas and south to Texas and Florida. In New England, it is quite common in the CT River Valley and along the shore in CT, MA, and RI, and has been found as far north as Epping, NH.

This pest has a single generation each year. The adult flies emerge from the soil in early to mid-July over a 10- to 14-day period. Males emerge up to 7 days before females. Each female can lay up to 50 eggs, beginning 6 to 7 days after mating. Eggs take 10 to 12 days to hatch. Mating is thought to occur exclusively on the host plant and fruit, where males will fight for territory to win a receptive female. However, the flies spend much of their time feeding in and inhabiting nearby trees, returning to the crop fields during daylight hours for mating and egg laying. Over 94% of egg laying occurs in the first 4 weeks after emergence. The flies may live through mid- to late August if not controlled.

Fully grown maggots emerge from the fruit in late August or early September, and enter the soil to pass the winter as pupae. Adults have been known to migrate at least ½ miles to infest new pepper plantings.  Up to 75% of the fruit have been lost in the second year of production on some farms, while other farms go years without experiencing pepper maggot problems. Fruit may be rendered unmarketable by the presence of maggots in the flesh or seed head (placenta). In late August and early September, fruit may rot when soft rot bacteria enters exit holes left by emerging maggots. The insect survives the winter as a pupa about two inches below the soil surface.

Females lay eggs by inserting their hollow, sword-like ovipositor though the fruit flesh and depositing an egg so that it sticks out into the void on the interior of the pepper pod. In doing so, the female leaves a small, white, round scar on the surface of the fruit, which can be used to monitor for the pest and to time initial insecticide applications. Only 5 to 50% of the egg-laying attempts (or scars) result in an egg being deposited in the fruit. These scars are particularly easy to detect on the glossy surface of cherry pepper pods: a favorite host. The insect will also attack other thick-fleshed, blocky fruit such as bell, cheese, and apple pepper varieties and, less frequently, eggplant. Hot cherry peppers (i.e. ‘Cherry Bomb’) can be used as indicator plants to time insecticide applications, if planted every 50-75 feet in the outer row of peppers along the tree line or in the entire perimeter row of the crop.

Another way to monitor for this pest is to bait a yellow, sticky AM Trap with a vial of 28% ammonium hydroxide solution (Fisher Scientific, Fair Lawn, NJ). We use a common 20 dram drug store vial, stuffed with cotton, with a ¼ -inch hole drilled in the top to dispense the ammonia. Using a paper clip, attach the vial to a hole made in the center of the AM trap. The trap works best 20 feet high in a maple tree beside the pepper field.  Lower elevations and other types of trees have proved less reliable. Recent experience has demonstrated that you can eventually trap all the flies out of one tree if that tree is used year after year for monitoring. Simply switching to a nearby tree will allow you to catch more flies.

On IPM farms, pest control involves insecticide sprays with dimethoate or acephate (Orthene) applied within a week of catching the first PM fly or detecting the first stings on fruit. Two or three applications may be needed at 8- to 10-day intervals to cover the entire adult emergence and egg-laying period. Orthene will control both PM and European corn borer, but has a 7 day-to-harvest (dh) restriction. Dimethoate generally has a 0 or 1 dh restriction, but will not control borers.

Pepper maggot can also be controlled using perimeter trap cropping. For example, when planting bell peppers, simply replace the outer perimeter row with a row or two of cherry pepper trap crop plants. You will need to plant a couple of cherry pepper plants at each end of the bell rows too, so that the trap crop completely encircles the cash crop. Within a week of detecting flies or stings, spray just the trap crop with an effective insecticide.  For light populations, insecticide applications may not be necessary the first year to stop the flies from reaching the bell peppers. However, maggots that develop in the cherry pepper fruit the first year will produce so many flies the second year that the perimeter will be breached unless the trap crop is sprayed. Perimeter trap cropping has been successfully used to protect eggplant by planting two rows of hot cherry peppers in the perimeter and using a shielded sprayer to apply insecticide applications only to the trap crop.

Until recently, the only effective management strategy for an organic farm was to cover host crops with a floating row cover throughout the entire flight period (5 to 8 weeks) or to market fruit with maggots inside. The use of row covers often proved impractical, especially during harvest. However, spinosad, which is ineffective when applied as a foliar spray (i.e., as Radiant or Entrust), has recently been reformulated as a fruit fly bait: GF-120. The new bait formulation allows the fly to ingest more of the active ingredient to provide a higher kill. In research trials, this formulation has been shown to provide 67 to 98% efficacy for related pests, such as apple and blueberry maggots.

The fact that GF-120 is not readily available makes using this product a challenge.  Because it is not commonly stocked by local suppliers, GF-120 will have to be ordered prior to the planting season to insure that it arrives on time. For our trial, a representative from Dow AgriSciences provided the product for us to try.

A second challenge is that label directions call for GF-120 to be applied with large orifice nozzles (droplet size 4-6 mm), but as an ultra low volume spray: approximately 1 gallon per acre of finished spray solution. It is also recommended that the material be applied to the underside of leaves “to reduce exposure to sun and rain.” Since large orifices are usually associated with high volumes of finished solution, this essentially meant that we had to travel at approximately 12 mph in an ATV, over rough terrain, while directing the spray up to the underside of the leaf canopy. We accomplished this on an organic farm in CT, both by using perimeter trap cropping to reduce the area requiring spray and by rolling the drive rows early in the season prior to spraying in order to allow high-speed travel around a 1-acre block. A high rate of GF-120 was used: 20 fl oz/a in 140 fl oz of finished solution. Originally, based on recommendations from Dow, we used a SJ3-02 TeeJet fertilizer nozzle with a 50 mesh screen at 20 psi to produce the correct flow rate and droplet size. However, the grower switched back to the nozzles, which came with the sprayer for greater spray distance after the first application. The perimeter rows of cherry peppers were sprayed so that applications were directed to the top and underside of the plant canopy in two separate passes. An additional application was directed to the underside of the surrounding tree canopy per label directions. This process was repeated the first three weeks after flies were first detected in traps (starting July 12). The electric sprayer purchased for the ATV application then failed and required repair, which delayed the fourth application for 2.5 weeks or until Mid-August. Additional flies were captured during early August and new stings were detected on the fruit during that time period, which necessitated the final application. The unanticipated delay and heavy August rains may have reduced the efficacy of that final application.

In 2010, pepper maggots were not controlled on this farm and approximately 75% of the fruit were infested and unmarketable. In 2011, using the interrupted GF-120 spray schedule (no spray in early Aug), 60 to 70% of the crop remained maggot-free at harvest.  Providing an unsprayed control patch of peppers at the same site to measure the untreated damage level was considered impractical, due to the mobility of the adult fly. The grower was pleased with the results. This grower hedged his bet on the new fruit fly bait by covering a second block of peppers with floating row covers from the first week of July through August. The covered block remained maggot-free through harvest.

T. Jude Boucher, University of Connecticut
24 Hyde Ave., Vernon, CT 06066, 860-875-3331

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.