Environmental Conditions

During the 11-mo study period, temperature, humidity, and rainfall were recorded 60 cm above ground by the Florida Automated Weather Network (FAWN), Homestead. Mean monthly temperatures for Feb through Dec 2010 ranged from 27.9 °C in Jun to 14.2 °C in Dec with maximums of 35 °C in Jun, Jul and Aug, and a minimum of -1 °C in Dec (FAWN 2010). Relative humidity ranged from a mean monthly maximum of 85% in Aug and Sep to 76% in Dec, and rainfall ranged from 28 cm (11.0″) in Aug to 2 cm (0.7″) in Dec (FAWN 2010).

Seasonal Abundance and Diversity of Arthropods

Field Preparation. There were 3 field tests in the spring, summer, and fall of 2010 with one test per season. At each location (Kendall and Homestead), one field was used for all field data collection. Field tests were conducted in Krome gravelly loam soil (loamy-skeletal, carbonatic, hypothermic, lithic, udorthents) that was well drained, had a pH of 7.4–8.4, was 34–76% limestone pebbles (< 2 mm diam), and had low organic matter (< 2%) (Nobel et al. 1996; Li 2001). Each test field (0.4 ha) was divided into 40 equal 0.01-ha plots that were each 2 rows wide by 27.5 m long. At Homestead, ‘Obsession’ sweet corn (Seminis Vegetable Seeds, Oxnard, California) was planted in rows with a garden seeder (Model 1001-B, Earthway® Products, Bristol, Indiana) on 15 Feb, 2 Jun, and 1 Oct for the spring, summer, and fall tests, respectively. Field corn instead of sweet corn was planted at Kendall. Planting sites were spaced 0.3 m apart in rows separated by 0.9 m. Three to 5 seeds were planted per planting site to assure at least 1 seed germinated, but seedlings were thinned to 1 plant per site upon emergence. Granular fertilizer (N-P-K: 8–16–16, Diamond R fertilizers, Fort Pierce, Florida) was applied at 1,347 kg/ha at planting in parallel bands spaced 0.1 m from the seed rows. In addition, liquid fertilizer (N-P-K: 4–0–8, Diamond R fertilizers, Fort Pierce, Florida) was applied twice as a foliar spray 21 and 35 d after planting (DAP) to provide 2.8 kg of N/ha/d. To control weeds, the pre-emergence herbicide, trifluralin (Treflan™ 4EC, Dow AgroSciences, Indianapolis, Indiana), was applied at 0.09 kg/ha at planting, and plants were irrigated as needed by drip irrigation.

Data Collection. The sweet corn fields were sampled once during each of the 3 corn development stages described by Bean (2010). The first sample was during the silking (R1) stage and 4 days after first silk; the second sample was 5 days after the beginning of the blister (R2) stage, and the final sample was during the first week of the milk (R3) stage. For the spring and fall, the first, second, and third samples were 49, 63, and 70 DAP for the R1, R2, and R3 stages, respectively, and for the summer they were 56, 70, and 77 DAP, respectively. Two corn ears, 1 per plant, were randomly collected from each plot per sample date resulting in 80 ears per sample date. Each ear was separately placed in a 17 ± 22 cm self-sealing plastic bag, and samples that could not be immediately processed were stored at 26 ± 5 °C until processing. Each corn ear was removed from storage, cut in half at mid-point, and placed in a 100mL beaker filled with 75% ethanol for 5 min to collect arthropods, then carefully removed with forceps and discarded. The corn husk, silk, and pieces of corn cob were also rinsed in alcohol to dislodge arthropods. Alcohol rinses were filtered through a 25-µm mesh (USA Standard Testing Sieve, W. S. Tyler Co., Mentor, Ohio) to collect arthropods, which were then saved in 70% ethanol for identification. In the commercial field corn site at Kendall, only the top 5 cm of ear sheath (excluding the cob but including leaf tips and silk) were collected and handled as described for sweet corn. Arthropods were counted with a 10X microscope and identified to species where possible. Unidentified arthropods were sent for identification and voucher specimens submitted to the Florida Department of Agriculture and Consumer Services, Division of Plant Industry, Gainesville, Florida.

Laboratory Evaluation of Arthropod Species for Predatory Status

Ulidiid Colonies. Colonies of E. stigmatias, E. eluta, and E. annonae each originated with 100 adults collected from corn fields in Summer 2010. Each Euxesta sp. was reared in a separate cage (31 × 31 × 31 cm) that was maintained at 30 ± 5 °C and 75 ± 10% relative humidity; rearing methods were the same for each fly species. Colonies were fed a beet armyworm artificial diet (BAW, Southland Co., Lake Village, Arkansas) using methods described by Seal & Jansson (1989), and adult flies were supplied with fresh water and 1% honey solution. Diet mixture was attached to the bottoms inside plastic 28-g vials (BioServe™, Beltsville, Maryland, USA) that were affixed upside-down to cage ceilings with the tops facing down to encourage adult oviposition. Eggs were collected from oviposition vials every 24 h, then transferred to fresh BAW diet for larval emergence in the same environmental conditions as adults. Freshly eclosed first instars were removed from the vials every 24 h, transferred into plastic 28-g vials with BAW diet, and allowed to complete development into pupae. Vials with diet and larvae were checked every 4 h to remove pupae, which were rinsed gently with tap water to remove dietary residue and to reduce fungal infection. Pupae were air-dried, then placed into Petri dishes each with a moist paper disk (5 cmdiam) to prevent desiccation. Petri dishes containing pupae were placed in cages (31 × 31 × 31 cm) to allow adult emergence and checked every 2 h to collect newly emerged adults.

Collection of Arthropods for the Predatory Status Test. The study was conducted in the summer and fall using 150 ears per season. The source was mainly sweet corn from the foregoing Homestead field, but field corn from the Kendall field was also used. Nymphs and adults of O. insidiosus and third instar larvae of A. insignitus and Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae) were each evaluated for predation on different ulidiid life stages. The species and developmental stage of each arthropod was determined by its size and color. To preserve corn ears (including arthropods) and to prevent the escape of arthropods until their use in tests, 10 ears were placed per self-sealing plastic bag (17 × 22 cm) with bags temporarily stored at 26 ± 5 °C and 30 ± 5 °C for the summer and fall tests, respectively. After 1 h, corn ear tips were excised 10 cm from the top by a knife and inspected using a 10X microscope. For each species, individuals of a similar developmental stage were collected using a fine paint brush to transfer immatures and adults from corn ears to Petri dishes (10.5 cm diam). One predator of a given taxon and stage was initially placed in a Petri dish along with water and honey as food, but it was deprived of water and food for 24 h before being offered ulidiid prey in a no-choice test. There were 3 sets of petri dishes for each predator of a given taxon and stage. In one set, the predator was provided with 20 eggs (1-day old) of one of the 3 species (E. stigmatias, E. eluta, and E. annonae) from the foregoing laboratory colonies. In a second set of Petri dishes, the predator was provided with 20 one-day-old, first, second, and third instar larvae of an Euxesta sp., and in a third set, the predator was provided with twenty 1-day-old adults of an Euxesta sp. Each Petri dish thus contained one predator of a given taxon and age and 1 Euxesta sp. and was 1 of 4 replications. Petri dishes were checked at 24-h intervals to record numbers of ulidiid eggs and larvae. Dead (evidenced by deflation) or missing eggs and larvae were assumed to have been consumed by predators, which were often seen deflating eggs as they fed in an environment too moist to allow desiccation. The study was conducted in a growth chamber at 28 ± 0.5 °C and 75% RH.

Statistical Analyses.

Data for counts of ulidiid eggs and larvae and abundances of other arthropod species were transformed using square-root transformations for statistical analyses, however, non-transformed means and standard errors of means are reported in the figures. Treatments were then compared using one-way ANOVAs or non-paired T-tests as appropriate (SAS Institute 2003). Within each corn stage, abundances of ulidiid eggs and larvae were compared by non-paired T-tests. However, comparisons of numbers of arthropods other than ulidiids found in corn ears and cumulative seasonal abundances of Euxesta spp. eggs, larvae, and O. insidiosus were analyzed by one-way ANOVAs (PROC GLM, SAS Institute 2003). Differences among means were evaluated for statistical difference by Tukey-Kramer HSD tests (P < 0.05).

Seasonal Abundance and Diversity of Arthropods

In corn ears, 5 species of arthropods other than ulidiids were found in the spring, 11 species in the summer, and 8 species in the fall of 2010 (Table 1). Nymphs and adults of O. insidiosus occurred throughout the survey season as did larvae and adults of Lobiopa insularis Castelnau and Carpophilus lugubris Murray (Coleoptera: Nitudilidae) and an unidentified mite species (Acari) (Table 1; Figs. 1a and b, 2). Larvae of Anotylus insignitus (Gravenhorst) (Coleoptera: Staphylinidae) were found only during the spring and summer blister (R2) and summer milk (R3) stages (Table 1, Figs. 1c and 2). Other multiple-season finds included an unidentified Thrips sp. (Thysanoptera: Thripidae) and an unidentified muscoid fly (Diptera: Muscidae) each in the summer and fall (Table 1, Fig. 2). Single-season finds in the summer included larvae of C. carnea, and in field corn, an unidentified species of an ant (Hymenoptera: Formicidae), an earwig (Dermaptera: Forficulidae), and a booklouse (Psocoptera) (Table 1, Fig. 1d). Single-season finds in the fall included an unidentified roach species (Blattaria) and Zelus longipes (L.) (Hemiptera: Reduviidae) (Table 1). When stages of corn were combined for each season, mean numbers of ulidiid eggs and larvae and of O. insidiosus (nymphs and adults combined) were each significantly greater in the summer than in the fall or spring, when they were least abundant (Figs. 3 and 4).

Spring 2010. The only arthropods found in the spring R1 stage that may have preyed upon ulidiids were O. insidiosus (Figs. 1a and b; 2). In R2, the arthropods included O. insidiosus, Thrips sp., larvae and adults of L. insularis and C. lugubris, and A. insignitus larvae (Figs. 1a, b, c and 2). There were significant differences among arthropod taxa in numbers found in the R2 stage with the mean number of larval L. insularis and C. lugubris the highest (F = 10.19; df = 5, 39; P < 0.0005). During R3, arthropods included O. insidiosus, unidentified mites, and adults of L. insularis and C. lugubris (Figs. 1a, b and 2) with the latter group significantly more abundant than the other taxa (F = 17.49; df = 5, 39; P < 0.0001). The mean number of ulidiid eggs was significantly greater than larvae during R1 (t = 10.89; df = 1,39; P < 0.002), but larvae significantly outnumbered eggs during R2 (t = 61.69; df= 1, 39; P < 0.001) and R3 (t = 48.2;df= 1, 39; P < 0.0001) (Fig. 3a).

Table 1.

Arthropods other than ulidiids found on corn ears (including husk and silk) in southern Florida in 2010.

Summer 2010. Significant differences occurred in numbers of arthropods among taxa found in corn ears during the summer R1 (F = 21.67; df = 5, 39; P < 0.0001) (Fig. 2). The mean number of O. insidiosus was significantly greater than the other groups, which included Thrips sp., unidentified mites, and adults of L. insularis and C. lugubris. All four of these groups were also found during R2 and R3 stages along with larvae of L. insularis, C. lugubris, and A. insignitus (Figs. 1a, b, c and 2). A muscoid fly species and larvae of C. carnea were also found at the study sites (Table 1, Fig. 1d). At the Kendall site in field corn, unidentified species of an ant, an earwig, and a booklouse were also found (Table 1). There were significant differences in numbers of arthropods found in summer R2 (F = 61.13; df = 5, 39; P < 0.0001) with mean numbers of larval L. insularis, C. lugubris, and A. insignitus significantly greater than Thrips sp. or mites (Figs. 1c and 2). Significant differences also occurred in numbers of arthropods in summer R3 (F = 69.11; df = 5, 39; P < 0.0001) with mean numbers of larval and adult L. insularis and C. lugubris and larvae of A. insignitus significantly greater than O. insidiosus, Thrips sp., or mites (Figs. 1a, b, c, and 2). Anotylus insignitus larvae were found only during the R2 stages of spring and summer and the summer R3 stage; C. carnea larvae were found only during the summer (Table 1; Figs. 1c and d, 2). The other summer arthropods included unidentified species of a muscoid fly, an ant, an earwig, and a booklouse (Table 1). As in the spring, mean numbers of ulidiid eggs were significantly greater than numbers of larvae in the R1 stage (t = 53.17; df = 1, 39; P < 0.0015), but mean numbers of larvae were significantly greater than numbers of eggs in the R2 and R3 stages (t = 69.66; df = 1, 39; P < 0.0001, and t = 72.9, df = 1, P < 0.002, respectively; Fig. 3b).

Fall 2010. In the R1 stage, there were significant differences in numbers of arthropod taxa found on corn ears (F = 20.29; df = 5, 39; P < 0.0001). The mean number of O. insidiosus was significantly greater than Thrips sp., the only other arthropod taxon found other than ulidiids (Figs. 1a, b and 2). In the R2 stage, there were also significant differences among arthropod taxa (F = 23.37; df = 5, 39; P < 0.003). Although mean numbers of O. insidiosus were numerically the greatest, they were not statistically higher than other arthropods present. The other taxa found in R2 were L. insularis and C. lugubris adults, Thrips sp., and mites (Fig. 2). In R3, there were also significant differences among arthropod taxa (F = 48.2; df = 5, 39; P < 0.005) with mean numbers of adult L. insularis and C. lugubris significantly greater than O. insidiosus or Thrips sp., the only other non-ulidiids found (Figs. 1a, b and 2). In R1, the mean number of ulidiid eggs was significantly higher than larvae (t = 10.98; df = 1, 39; P < 0.0015), while in R3, ulidiid larvae significantly outnumbered eggs (t = 53.11; df = 1, 39; P < 0.0002). However in R2, there were no significant differences between ulidiid eggs and larvae (Fig. 3c). Arthropods collected in the fall also included unidentified species of a roach and a muscoid fly and Z. longipes (Table 1).

Fig. 1.

Predators of Euxesta spp. (A) First instar nymph of Orius insidiosus feeding on a Euxesta sp. egg. (B) Adult of O. insidiosus feeding on a third instar larva of E. stigmatias. (C) Anotylus insignitus larva feeding on Euxesta sp. eggs. (D) Larva of Chrysoperla carnea feeding on an adult Euxesta sp. fly.

Laboratory Evaluation of Arthropod Species for Predatory Status

In the laboratory, nymphs and adults of O. insidiosus fed on eggs and larvae of E. stigmatias, E. eluta, and E. annonae (Table 1, Figs. 1a and b). Anotylus insignitus larvae also consumed eggs and larvae of Euxesta spp. (Table 1, Fig. 1c), and larvae of C. carnea fed on larvae and adults of Euxesta spp. (Table 1, Fig. 1d). Hence, at least 3 predatory species in the present study consumed ulidiid flies (Table 1, Fig. 1).