By Julien Beuzelin and Anna Mészáros
Maggots feeding on silks, kernels and sometimes the cob of sweet corn ears harvested in Florida represent the greatest insect pest threat to the state’s fresh market sweet corn industry. These maggots are the larvae of three fly species known as corn silk flies.
What to Look For
Although the maggots of the three species look alike, the adults are different. Chaetopsis massyla adults have three dark bands on the wings. These adults can be numerous in sweet corn fields but generally do not result in severe maggot infestations. Euxesta eluta adults have four dark bands on the wings with a small clear dot near the front margin of each wing and can be associated with severe maggot infestations if not controlled. Euxesta stigmatias adults have four dark bands without a clear dot and can cause severe infestations even when intensively controlled.
Silk fly adults can live several weeks, are mobile and are found from the fall to the spring in sweet corn fields and surrounding habitats such as sugarcane fields and non-crop weedy or woody areas. In a statewide survey conducted 15 years ago, C. massyla and E. eluta were observed throughout Florida. E. stigmatias did not occur much further north than Lake Okeechobee in the southern region of the state.
Insecticide applications targeting adults before they lay eggs on husks and silks are the main tactic to control silk flies. Formal thresholds for the number of adults observed in sweet corn fields justifying the application of insecticides have not been developed because of the extremely low market tolerance for damaged ears and quick changes in adult numbers and species composition.
Thus, the presence of silk flies, especially E. eluta or E. stigmatias adults, generally justifies insecticide applications from silk emergence until about four days before harvest. However, scout and grower experience, field history, regional pest pressure and market prices also play a role in determining the need for insecticides. Three insecticide applications per week are common. The frequency of application should be increased to prevent further damage if small maggots are found infesting young ears.
E. stigmatias adults are less susceptible to insecticides than E. eluta adults. Thus, species identification while scouting is important because management should be more intensive when E. stigmatias is prevalent. However, species identification is challenging because flies are relatively small and fast moving. Research showed that Multilure traps (transparent dome with yellow inverted funnel) baited with ammonium acetate and a floral volatile [1,4 dimethoxybenzene (DMB)] are attractive to adults of the three species and can assist with scouting. Ammonium acetate lures are commercially available. DMB lures have been prepared by entomologists at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Everglades Research and Education Center (EREC).
In experimental sweet corn fields, trap captures are correlated with visual observations. In addition, trapping at nearly 50 sites on commercial farms between 2020 and 2022 confirmed silk fly population trends observed by crop consultants and increased confidence in management decisions.
Chemical Controls
Control of silk flies with insecticides relies almost exclusively on the application of pyrethroids. A premix of zeta-cypermethrin and bifenthrin (Hero) has caused the greatest adult mortality, followed by zeta-cypermethrin (Mustang Maxx) and lambda-cyhalothrin (e.g., Warrior II). Bifenthrin (e.g., Brigade), beta-cyfluthrin (Baythroid XL) and esfenvalerate (Asana XL) have been slightly less effective.
To increase the efficacy of pyrethroids under high pest pressure, the addition of piperonyl butoxide at rates listed on the label is recommended. There is some evidence that early evening pyrethroid application might be more effective than morning or mid-day applications.
In addition to pyrethroids, the carbamate methomyl (e.g., Lannate) and the spinosyn spinetoram (Radiant), which are used for fall armyworm control, have activity against silk fly adults. Methomyl is effective but has practically no residual activity. Spinetoram is costly and relatively less effective. Additionally, repeated foliar applications of abamectin have only provided suppression of maggot infestations.
Field evaluations have not shown evidence that diatomaceous earth (e.g., Celite 610), kaolin clay (Surround) or biological insecticides such as Beauveria bassiana (BoteGHA), Chromobacterium subtsugae (Grandevo) and azadirachtin (Aza-Direct) protected sweet corn ears. Thus, there are no alternatives to pyrethroids under current production practices.
Plant Resistance and Natural Enemies
Natural enemies, including generalist predators such as minute pirate bugs and assassin bugs, prey on silk flies. However, these beneficial insects do not play a role in silk fly management because intensive insecticide use in sweet corn fields prevents predator buildup. In addition, sweet corn hybrids currently grown in Florida are considered highly susceptible to silk flies. Thus,natural enemies and host plant resistance are not integrated into the current pest management strategy. Nevertheless, UF/IFAS scientists have been working toward the development of sweet corn hybrids more resistant to silk flies, and recently developed breeding lines that may exhibit reduced silk fly injury.
Other Factors
Fall armyworms should be well controlled even during sweet corn vegetative stages because silk fly adults are attracted to fall armyworm excrement and injured plants.
Weeds in sweet corn fields and surrounding habitats should be controlled, when feasible, because they provide shelter to silk fly adults.
In addition, sweet corn plants should be destroyed as soon as possible after harvest to reduce the number of maggots completing their life cycle and producing adults. However, the effectiveness of post-harvest residue destruction may be inconsistent.
New Breakthroughs
Silk fly management is challenging because of the behavior of the insects and the limited number of effective control tactics. Advances in management with insecticides are expected during the next five years because research has shown that the ingestion of selected insecticides is highly toxic to silk fly adults.
In addition, foliar applications of a pre-commercial insecticide with a novel mode of action provided control levels comparable to those provided by a pyrethroid in a field evaluation conducted in 2023 at the UF/IFAS EREC.
Breeding efforts might also result in more resistant sweet corn hybrids.
Julien Beuzelin is an assistant professor at the UF/IFAS EREC in Belle Glade. Anna Mészáros is a UF/IFAS commercial vegetable Extension agent based in Palm Beach County.