Investigating New Management Approaches for Picture-Winged Flies in Sweet Corn

View the project final report

• 2016 annual report

Commodities

• Vegetables: sweet corn

Practices

• Pest Management: chemical control

Sweet corn IPM trials were conducted investigating insecticide efficacy for picture-winged flies and crop destruction methods to prevent the completion of fly development in the soil after harvest. Pyrethroid treated plots reduced silk fly populations and resulted in low levels of silk fly ear damage, but did not protect against fall armyworm. Spinetoram mixed with a feeding stimulant provided fall armyworm protection and intermediate silk fly protection in the fall and equivalent protection in the spring. Crop destruction techniques did not affect adult fly emergence from soil in the fall, but residue incorporation reduced fly emergence in the spring.

Introduction

The purpose of this project is to investigate more environmentally and economically sustainable approaches for silk fly management in fresh market sweet corn fields. Florida is the second largest producer of fresh market sweet corn (USDA/NASS 2016), and much of that is grown in southern Florida where silk flies (Diptera: Ulidiidae) are severe pests. Sweet corn is treated mostly with pyrethroids to prevent unacceptable damage to harvestable ears by silk flies. Females oviposit in fresh silks, and the larvae consume the developing ear. There are few non-pyrethroid options due to label and efficacy limitations. Adult flies quickly re-infest treated fields (Goyal 2010) and insecticide residues rapidly degrade. Fields are sometimes treated more than three times per week to prevent females from ovipositing in the silks (Anonymous 2009). Due to heavy pyrethroid reliance, silk flies are under heavy selection pressure for developing resistance. Growers have commented that flies are increasingly more difficult to control. Some of the flies present soon after treatment may have simply recovered from initial intoxication. Laboratory bioassays published in 2004 raised concerns over the potential for resistance development (Nuessly and Hentz 2004). This project will perform much needed resistance monitoring of pyrethroid treated flies in the field.

Poor canopy coverage and penetration when using contact insecticides can also lead to control failure. Silk flies congregate near the top of the canopy in the evening hours (Seal et al. 1995), but can be found throughout the crop canopy during the day. Most sweet corn acreage is treated by air, but ground equipment is used in areas where this is not possible. This project will examine if canopy penetration can be improved by modifying application volume or pressure or by using adjuvants.

There is need for additional chemical and cultural control tactics. Several newer reduced risk insecticides that have fly efficacy are available, but must be ingested by the flies to be effective. Some of these are currently incorporated into bait formulations for tephritid fruit fly control (Mangan et al. 2006). These products have not been extensively tested as a control option for silk flies. Mixing these materials with the feeding stimulant sucrose has been adopted by small fruit growers for control of Drosophila suzukii (Diptera: Drosophilidae) in the Northeast (Cowles et al. 2015). This project will examine if reduced-risk insecticides mixed with feeding stimulants can provide adequate silk fly control.

Finally, fields are often left standing for prolonged periods after harvest, allowing fly larvae to complete development. There have been no studies to determine the most efficient or timely means of removing crop residue to remove silk fly larvae that would contribute to the next generation of silk flies. Growers that do remove crop residue in a timely manner will often disc or mow fields. This project will document the effect of delayed crop destruction on silk fly production and which method of crop removal is the most efficient for preventing silk flies from completing their life cycle.

1. Determine how insecticide application methods can be improved for increasing canopy penetration
2. Determine if silk flies demonstrate reduced pyrethroid susceptibility in field settings
3. Determine if the reduced risk insecticide spinetoram can be applied in a feeding stimulant to provide adequate fly control
4. Determine most efficient and timely means of crop residue removal to reduce silk fly production