D. A. Dean

Insectivorous activities of spiders in United States field crops

A survey of average spider numbers in U.S. field crops (considering a geographic range from the east to the west coast) gave an overall mean density of ≅ 1 plant‐dwelling spider/m2(± 0.18 SEM). This value is more than 100 times lower than Turnbulls famous ‘overall mean value’ (= 130.8/m2) computed from 37 published censuses of spider numbers in a wide variety of environments from all over the world. Crop fields are disturbed systems whose spider numbers are drastically reduced by agricultural practices such as pesticide use, cultivation, harvest, etc. Small sized spider individuals (including large percentages of immatures) numerically dominate the faunas of U.S. field crops, and these feed primarily on tiny prey organisms (< 4 mm in length). Small web‐building spiders are almost strictly insectivore (insects constituting > 99% of total prey). In contrast, the ‘hunters’ (nonweb‐building spiders) that actively search the plant surface for prey, exhibit a mixed strategy of araneophagic and insectivorous foraging patterns (insects constituting ≅ 90% of total prey). The aggressive ‘active searchers’ are highly polyphagous (compared to the small web‐weavers), but can narrow their feeding niche significantly when a suitable prey species reaches high numbers relative to other prey groups. The numerically dominant spider predators in U.S. field crops feed heavily on small plant‐sucking insects such as plant bugs (Miridae), fleahoppers (Miridae), leafhoppers (Cicadellidae), treehoppers (Membracidae), planthoppers (Delphacidae), and aphids (Aphididae), which is of interest from a biocontrol point of view. A typical agroecosystem spider (i.e., striped lynx spider) may capture an average of ≅ 1 prey per rainfree day in the field (by multiplying this value with the overall mean spider density of 1/m2, it follows that the average prey kill may be in the order of magnitude of ≅ 1 prey/m2/day). At prey densities of 100–300/m2 (as recorded in literature), spiders kill perhaps ≅ 0.3‐1 % of the potential prey per day. Extensive field and laboratory observations, experiments, and computer models conducted by research groups in different parts of the U.S. indicate that the collective predation impact of spiders may contribute significantly to lower pest levels in some crop fields.

Spiders as predators of arthropod eggs

Examples of spiders preying upon the eggs of Araneae and Insecta are given from descriptions found in the literature from different regions of the world. In many cases, spiderlings or female spiders were feeding on the eggs of conspecifics (cannibalism). Evidence of conspecific predation on eggs has been reported for the families Dipluridae, Uloboridae, Scytodidae, Loxoscelidae, Pholcidae, Theridiidae, Agelenidae, Lycosidae, Gnaphosidae, Clubionidae, Thomisidae and Salticidae. Spiders feeding on the eggs of other species (interspecific predation by Salticidae upon eggs of other Salticidae and Uloboridae; Theridiidae upon the eggs of orb‐weaving spiders) has also been noted.

The southern black widow spider, Latrodectus mactans (Araneae, Theridiidae), as a predator of the red imported fire ant, Solenopsis invicta (Hymenoptera, Formicidae), in Texas cotton fields

In cotton fields of east Texas, the southern black widow spider, Latrodectus mactans (F.), was observed to capture the red imported fire ant, Solenopsis invicta Buren, which constituted 75% of the prey of L. mactans. Remains of workers, queens, and males of S. invicta were found in the spider webs. L. mactans were observed to feed on ants captured in the webs indicating that S. invicta were used as a food source. Juveniles as young as third instar were observed capturing S. invicta workers. In the same area, 16 other species of spiders were observed to be predators of S. invicta.

Genetic Variation of Anastrepha suspensa (Diptera: Tephritidae) in Florida and the Caribbean Using Microsatellite DNA Markers

ABSTRACT Anastrepha suspensa (Loew) (Diptera: Tephritidae), the Caribbean fruit fly, is indigenous to Florida and the Greater Antilles where it causes economic losses in fruit crops, including citrus. Because of the geographic separation of many of its native locations and anecdotal descriptions of regional differences in host preferences, there have been questions about the population structure of A. suspensa. Seven DNA microsatellite markers were used to characterize the population genetic structure of A. suspensa, in Florida and the Caribbean from a variety of hosts, including citrus. We genotyped 729 A. suspensa individuals from Florida, Puerto Rico, Cayman Island, Dominican Republic, and Jamaica. The investigated seven loci displayed from 5 to 19 alleles, with expected heterozygosities ranging from 0.05 to 0.83. There were five unique alleles in Florida and three unique alleles in the Caribbean samples; however, no microsatellite alleles were specific to a single host plant. Genetic diversity was analyzed using FST and analysis of molecular variance and revealed low genetic diversity between Florida and Caribbean samples and also between citrus and noncitrus samples. Analyses using migrate revealed there is continuous gene flow between sampling sites in Florida and the Caribbean and among different hosts. These results support previous comparisons based on the mitochondrial cytochrome oxidase I locus indicating there is no genetic differentiation among locations in Florida and the Caribbean and that there is no separation into host races.

Predation on blackmargined aphids (Homoptera: Aphididae) by spiders on pecan

Spiders were collected in a pecan orchard and examined in the laboratory and in the field as potential predators of the blackmargined aphid, Monellia caryella (Fitch). Representatives from 11 families, 23 genera, and over 25 species were observed eating from 2 to 36 aphids per day. This is the first quantified report of aphid predation by spiders in pecan. Previous studies indicate a density of one spider to every 10–100 pecan leaves, and <1–10 blackmargined aphids per leaf typically occur on pecan. Spider predation in pecan appears to be important in biological control of the blackmargined pecan aphid.

SPIDERS COLLECTED FROM PEANUTS AND NON-AGRICULTURAL HABITATS IN THE TEXAS WEST CROSS-TIMBERS

The spider fauna of peanut fields in Erath and Comanche counties in the Texas West Cross-Timbers region was sampled during 1981-83. A total of 134 species comprising 79 gen- era and 18 families was collected. The Salticidae had the greatest diversity with 22 species. The families Araneidae, Clubionidae, Dictynidae, Gnaphosidae, Linyphiidae, Lycosidae, Philodromi- dae, Theridiidae and Thomisidae were also species-rich. Spider collections from habitats other than peanut fields added another 138 species, making a total of 272 species and 129 genera belong- ing to 28 families. Twenty-four species and one family (Zoridae) are new records for Texas. Recent studies by Whitcomb et al. (1963) and Leigh and Hunter (1969) in cotton; Bailey and Chada (1968) in sorghum; Negm et al. (1969) in sugar- cane; Howell and Pienkowski (1971), Wheeler (1973), and Yeargan and Dondale (1974) in alfalfa; and LeSar and Unzicker (1978) in soybeans dem- onstrated the high diversity of spiders in agroecosytems. There were no pre- vious studies of spiders in peanut fields, but the spider fauna of Texas has received attention from several workers. Vogel (1970) listed 574 species known to occur in Texas. With some reductions due to synonomy and recent additions in the literature due to revisions and checklists, the number known from the state prior to this study was about 782. The large number of species is due to the diverse flora, geology and climatic conditions in Texas. Published lists of Texas spiders include Jones (1936) for species from Dallas, Reddell (1965) for cave spiders, Carpenter (1972) for the salticids of Wichita Co., Brown (1974) for species from Nacogdoches, and Cokendolpher et al. (1979) for north central Texas crab spiders. A few workers made collections in field crops. Woods and Harrel (1976)