Variable Pyrethroid Susceptibility Among the Sweet Corn-Infesting Ulidiidae (Diptera) in Florida and New Baseline Susceptibilities

Substrate and Moisture Affect the Pupation Depth of the Corn Silk Flies Chaetopsis massyla and Euxesta eluta (Diptera: Ulidiidae)

Corn silk flies, or picture-winged flies (Diptera: Ulidiidae), are important pests of fresh market sweet corn in commercial production areas in southern Florida. Issues with pest management related to insecticide resistance, problems in insecticide application, and alternate crop population sources constitute a significant challenge for the protection of developing corn ears. Developed larvae leave cobs and pupate in the soil; however, relatively little is known about these behaviors. In this study, two soil types collected from fields were compared with sand under six different moisture levels in the laboratory to determine the pupation depths of the larvae. Comparisons were carried out concerning the pupation depth of Chaetopsis massyla and Euxesta eluta, which are major pest species in Florida. Both soil type (muck, loamy sand, and sand) and moisture levels (0, 10, 25, 50, 75, 100% field holding capacity) significantly affected pupation depth, with shallow pupation observed under dry or saturated wet conditions. The addition of structures such as pipe cleaners simulating corn roots resulted in deeper pupation under most conditions.

Laboratory Evaluation of Pupal Parasitoids for Control of the Cornsilk Fly Species, Chaetopsis massyla and Euxesta eluta

Cornsilk flies are serious pests of sweet corn through damage to cobs and secondary fungal establishment. As pupae are generally outside the infested cob on the ground, there can be potential for use of pupal parasitoids for control. Two species of gregarious parasitoids, Muscidifurax raptorellus and Nasonia vitripennis, and three species of solitary parasitoids, Spalangia endius, Spalangia cameroni and Muscidifurax raptor, were evaluated against pupae of the two cornsilk fly species, Euxesta eluta and Chaetopsis massyla. House fly pupae, the most common host for most of the parasitoids, were included for comparison. All of the parasitoids killed and successfully parasitized pupae of the two cornsilk fly species at rates that were similar to house fly pupae. Adult parasitoids that emerged from cornsilk fly hosts were somewhat smaller than parasitoids reared from house flies and had proportionally fewer females. These parasitoids, which are widely and commercially available for filth fly control, warrant further consideration for their potential against cornsilk flies in the field.

Pyrethroid Susceptibility in Field Populations of Picture-Winged Flies (Diptera: Ulidiidae) Infesting Fresh Market Sweet Corn in Florida

Picture-winged flies (Diptera: Ulidiidae) are the most damaging insect pests of sweet corn (Zea mays L.) produced in Florida for the fresh market. Management of these pests, referred to as corn silk flies, relies on frequent pyrethroid applications targeting adults. In response to the need for an insecticide resistance management (IRM) program for corn silk flies in this highly intensive crop system, glass vial bioassays were conducted to determine the susceptibility of 12 corn silk fly populations to the pyrethroid beta-cyfluthrin. Two Euxesta eluta Loew and nine Euxesta stigmatias Loew populations were obtained by collecting infested ears in commercial and experimental fields in 2020 and 2021. One E. eluta laboratory colony was used as a susceptible reference population. The E. eluta reference colony was the most susceptible population, with an LC50 value of 0.01 µg/vial. The E. stigmatias field populations were generally less susceptible to beta-cyfluthrin than the E. eluta field populations, with the highest LC50 values attaining 3.51 µg/vial and 0.19 µg/vial, respectively. In addition, the five E. stigmatias populations from commercial sweet corn fields were as much as 17.6 times less susceptible than the four E. stigmatias populations from nontreated fields. Results suggest that E. stigmatias is less susceptible to pyrethroids than E. eluta. Results also suggest that corn silk flies in commercial sweet corn fields are selected for reduced pyrethroid susceptibility throughout the growing season. This study successfully used the glass vial bioassay method for corn silk flies, providing a new tool to initiate an IRM program.

Sweet Corn Research around the World 2015–2020

Modern sweet corn is distinguished from other vegetable corns by the presence of one or more recessive alleles within the maize endosperm starch synthesis pathway. This results in reduced starch content and increased sugar concentration when consumed fresh. Fresh sweet corn originated in the USA and has since been introduced in countries around the World with increasing popularity as a favored vegetable choice. Several reviews have been published recently on endosperm genetics, breeding, and physiology that focus on the basic biology and uses in the US. However, new questions concerning sustainability, environmental care, and climate change, along with the introduction of sweet corn in other countries have produced a variety of new uses and research activities. This review is a summary of the sweet corn research published during the five years preceding 2021.

Management of Euxesta spp. in Sweet Corn with McPhail Traps

Pests attacking the ear of sweet corn, such as Helicoverpa and Euxesta species, cause economic losses for the producer and the processing industry. Feeding on the style-stigmata preventing fertilization and on the developing grain and the association with pathogens are the main causes of product depreciation. The traditional control such as spraying with chemicals is not effective, even with several applications directed to the corn ear. Bacillus thuringiensis (Bt) corn also does not reach the fly. McPhail traps that have been used to monitor the pest can be a control strategy. This work evaluated the efficiency of food attractants placed inside McPhail traps to remove adult insects, in order to reduce ear damage. Twelve McPhail-type traps were installed in a randomized complete block design containing Bio Anastrepha® alone or combined with different doses of insecticide. Every 10 days, all the captured insects were counted and separated by species and sex. Only Euxesta eluta and Euxesta mazorca were found. The occurrence of insects was greater in the period between silk emergence and grain filling. The number of females was higher, probably due to the need to feed before oviposition. The number of E. mazorca females caught in the treatment containing only Bio Anastrepha® was higher compared with that of others. The mean ear damage was very low, and there was no interaction between the production parameters and the distance between the trap and the harvested plant. In short, the use of McPhail trap containing food attractants may be a viable alternative to control corn silk flies.

Developmental Studies of Maize-Infesting Picture-Winged Flies (Diptera: Ulidiidae)

Eleven species of picture-winged flies (Diptera: Ulidiidae) attack maize (Zea mays L.) in the Americas. Field and laboratory studies were used to determine developmental times on sweet corn ears for the three most common species attacking the crop in the United States, Chaetopsis massyla (Walker), Euxesta eluta Loew, and Euxesta stigmatias Loew. Egg plus larval stage developmental times were evaluated in early Spring and late Fall 2009, and late Spring 2010, by placing newly deposited eggs in protected ears in the field. Newly formed puparia were removed daily from cages and held in the laboratory to determine pupal developmental times. Developmental times were compared with flies reared on artificial diet in the laboratory. Ear- and diet-reared adults were held until their death to determine adult longevity. Developmental times, including for pupae from ear-reared larvae, were significantly affected by species and season. All three species required nearly twice as long to complete development in the late Fall compared to late Spring studies. Flies required 3-13 d longer to complete development on artificial diet than on ears. Euxesta eluta adults lived two to three times longer than the other species, and females of all species lived 10-15% longer than males. Species and seasonal developmental times are discussed in relation to ear developmental times and control strategies. It is estimated that 16-19 generations per year of all three fly species can develop in the field in the sweet corn production area of southern Florida.