The picture-winged fly Euxesta stigmatias Loew (Diptera: Ulidiidae) is a serious pest of sweet corn (Mossier 1999; Nuessly & Hentz 2004) causing up to 100% damage to untreated crops (Van Zwaluvenberg 1917). It was first described as a sweet corn pest in Florida in 1938 (Barber 1939). Fly larvae emerge from eggs deposited primarily on corn silks (styles) and feed on silks, kernels, and cobs. Economic losses can occur even in fields where insecticides are frequently applied (Seal & Jansson 1994; Seal 1996; Nuessly & Hentz 2004). While additional Euxesta spp. have been reported damaging corn ears in Central and South America (Painter 1955; Frias-L 1978; Diaz 1982), no other ulidiid has been reported attacking corn in Florida. However, during the course of a sweet corn variety trial in 2007 conducted at Belle Glade, FL, we sweep netted and reared from infested ears what appeared to be a second species. Gary J. Steck (Division of Plant Industry, Florida Department of Agricultural and Consumer Services, Gainesville, FL) identified adults collected and reared from the field as E. stigmatias Loew and Chaetopsis massyla (Walker) (Diptera: Ulidiidae).

Previously referred to as Otitidae, Ulidiidae is the family name currently accepted by dipterists and used in the BioSystematic Database of World Diptera (Thompson 2006). This family is reported to have 671 species worldwide (Anonymous 2008a), including 285 in the Americas south of the United States (Steyskal 1968). Ulidiidae is divided into 2 subfamilies, Otitinae and Ulidiinae, based on aedeagus differences (Hennig 1939). Both Euxesta and Chaetopsis belong to the subfamily Ulidiinae. The genus Chaetopsis is represented by 7 species in North America (Steyskal 1965) and 10 species in the Americas south of the United States with 4 species common to both (Steyskal 1968). Most species of this family have saprophagous feeding habits, although some are primarily phytophagous (Allen & Foote 1992). Genera that are considered to be phytophagous include Chaetopsis, Eumetopiella, Euxesta, Tetanops, and Tritoxa.

The literature indicates that C. massyla has been reared from several other monocots, but these finds have been associated with prior or concurrent insect or fungal infestations. Chaetopsis massyla has been reared from onions, Allium cepa L. (Liliales: Liliaceae) (Merrill 1951), decaying Narcissus bulbs (Liliales: Liliaceae) (Blanton 1938), and cattail, Typha latifolia L. (Typhales: Typhaceae) (Keiper et al. 2000). Allen & Foote (1992) collected C. massyla larvae from decomposing cattail stems previously damaged by Noctuidae (Lepidoptera) larvae and from Carex lacustris Willd. (Cyperales: Cyperaceae) stems previously damaged by Epichlorops exilis (Coquillett) (Diptera: Chloropidae) larvae. The objective of this study was to determine the pest nature of C. massyla on corn.

MATERIALS AND METHODS

No-choice trials on sweet corn (Seminis ‘Obsession’) were conducted in Dec 2007 in a greenhouse and again in May 2008 in a field at the Everglades Research and Education Center (EREC), Belle Glade, FL. Pre-silking ears were protected from infestation by other ulidiids or Lepidoptera by 50mesh cloth bags secured with rubber bands. Seven days after the start of silking, C. massyla male-female pairs were added to the cages and provided with 50% honey solution on cotton balls. Each ear was caged with five, 8–14 d-old pairs in the 2007 trials and one 5–15 d-old pair in the 2008 trials. As part of a larger experiment designed to evaluate fly development on uninfested ears, 8 ears and 9 ears were caged solely with C. massyla flies in 2007 and 2008, respectively. All ears were collected 14 d later and placed individually in 0.93 L plastic bags with paper towels in a room maintained at 26.0 ± 1°C and photoperiod of L14:D10 h. Pupae were collected from the bags and held for adult emergence on moist filter paper within parafilm-sealed Petri plates. Emerged adults were preserved in 70% ethyl alcohol.

To evaluate C. massyla infestation of ears in full choice tests, ears naturally infested by ulidiids were collected from 2 additional field trials conducted in 2008 at the EREC. Ears were selected randomly from the fields at harvest (21-dold ears). Seventy-five ears were collected in May from a Bt-enhanced sweet corn field (Syngenta ‘GSS 0966’), and 360 ears were collected in December from a standard sweet corn field (‘Obsession’) and examined for fly larvae within silk channels. To remove the possibility of C. massyla attacking only ears damaged previously by other insects, ears with fly larvae and the presence of or previous damage by Lepidoptera larvae, either Helicoverpa zea (Boddie) or Spodoptera frugiperda L. (J. E. Smith) (Lepidoptera: Noctuidae), were discarded from the samples. Ears infested only with fly larvae were held for pupal development and adult emergence as described above.

RESULTS

No choice trials resulted in 100% infestation by Chaetopsis massyla with no other insects emerging from caged ears. A mean (±SEM) of 13 ± 4 (range 6 to 40) adults emerged from each ear in the Dec 2007 greenhouse trial, while 13 ± 6 (range 4 to 20) per ear emerged in the May 2008 field trail. These tests indicated that C. massyla could successfully colonize and develop in sweet corn ears not previously or concurrently infested with other insects.

Chaetopsis massyla emerged from ears collected in both choice field trials. In the Bt-enhanced sweet corn trial, C. massyla were found in 19 of the 30 ears infested only with fly larvae. Three of these ears were infested solely by C. massyla while 16 also held E. stigmatias. Mean C. massyla adult emergence per ear was 25 ± 6 (range 7 to 61) when infested only by C. massyla and 16 ± 3 (range 2 to 43) when infested by both fly species. Of the 50 standard sweet corn ears found infested solely by fly larvae, 42 contained C. massyla. Half of these were infested by only C. massyla, whereas the others were also infested by E. stigmatias. Mean C. massyla emergence per standard sweet corn ear was 7 ± 2 (range 2 to 21) when infested only by C. massyla and 6 ± 2 (range 1 to 13) when infested by both fly species. Chaetopsis massyla was subsequently reared from fly-infested corn ears collected from commercial sweet corn fields throughout Palm Beach County. This county is the major sweet corn producing county in Florida (Anonymous 2008b; A. Kirstein, Palm Beach Co. Coop. Ext. Economist, personal communication).

Fig. 1.

Adult females of Chaetopsis massyla (a) and Euxesta stigmatias (b); heads of C. massyla (c) and E. stigmatias (d); ovipositors of C. massyla (e) and E. stigmatias (f); flgm1, first flagellomere; fr vit, frontal vitta.

DISCUSSION

In choice and no-choice trials, C. massyla successfully infested and developed in ears without prior infestation by other insect species. This is the first known report of C. massyla as a primary pest of corn ears. However, in Nov 2009, examinations of Ulidiidae specimens at the California Department of Food and Agriculture collection by S. D. Gaimari (CDFA Dipterist, personal communication) and at the Smithsonian National Museum of Natural History by G. S. Nuessly (in collaboration with Dipterist A. Norbaum) revealed C. massyla adults were reared from sweet corn tassels in Orange County, CA in 1942 and from corn ears in Riverside County, CA in 1996, respectively. The later specimens were collected from ears also infested with E. stigmatias. Labels on C. massyla specimens at the Smithsonian indicate they have been collected from across the continental United States.

Two other Chaetopsis species are reported to feed on corn and other plants as larvae. Chaetopsis aenea (Wiedemann) were reared from damaged corn stems in Ohio (Gossard 1919) and decayed and smut-infected onions in Michigan (Severin & Severin 1915). Larvae of Chaetopsis fulvifrons (Macquart) were found in tunnels of Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) within corn stalks in Texas (Knutson 1987), and from barnyard grass, Echinochloa crus-galli (L.) P. Beauv (Cyperales: Poaceae) damaged by Eumetopiella rufipes (Macquart) (Diptera: Ulidiidae) (Valley et al. 1969).

Chaetopsis massyla can be distinguished from other pestiferous ulidiids occurring in Florida corn by several characters. Wings of C. massyla have 3 dark bands (Fig. 1a), while Euxesta spp. attacking corn have a fourth band near the wing base (Fig 1b). The legs of C. massyla are yellow, whereas the legs of other species attacking corn are brown to black in color. The upper apex of the first antennal flagellomere is angulate or pointed in Chaetopsis (Fig. 1c), but is rounded in Euxesta spp. (Fig. 1d) (Steyskal 1987). The frontal vitta of C. massyla usually is bare (Fig. 1c), while it has several scattered setae or cruciate bristles in Euxesta spp. (Fig. 1d) (Steyskal 1987). The ovipositor is broad, depressed, thin and laminar apically in Chaetopsis (Fig. 1e) compared with Euxesta spp. where the ovipositor is narrow, soft and not laminar apically (Fig. 1f) (Steyskal 1987).

In conclusion, the results from field surveys and artificial infestation studies indicate that C. massyla can attack and develop within corn ears with or without prior infestation by other insect species. Larval feeding by C. massyla renders the ears unmarketable. Therefore, we report C. massyla as a pest of corn and also confirm its primary nature of attack on corn ears in contrast to reports by Allen & Foote (1992) that suggested the species was limited to scavenging or secondary invasion of plant tissues. Cooperative studies are in progress to determine the geographical distribution of corn-infesting populations of C. massyla throughout the southeastern United States.