Winfield L. Sterling

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.

Dislodgement of Heliothis zea (Lepidoptera: Noctuidae) eggs from cotton due to rain and wind: a predictive model

Abstract This study provides mechanisms and means for estimating Heliothis zea egg mortality to rain and wind under field conditions. Building on laboratory studies with simulated weather factors, equations are presented to describe the dislodgement of Heliothis zea eggs from cotton plants by the action of rain and wind alone and in combination. These estimates can be used as correction factors in studies of Heliothis spp. egg predation and as an aid in making management decisions for this pest.

Inaction levels for the red imported fire ant, Solenopsis invicta (hym.: formicidae): A predator of the boll weevil, Anthonomus grandis (col.: curculionidae)

Abstract The density of red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae), necessary to limit boll weevil abundance, Anthonomus grandis Boheman (Coleoptera: Curculionidae), below the action level was determined. An inaction level of 0.4 fire ants per shake bucket sample (i.e., four ants per 10 plant terminals) will provide boll weevil control approximately 90% of the time. A dynamic inaction level is also provided. Total mortality of boll weevil developmental stages including eggs through teneral adults averaged 97.9%. Of 100 boll weevil eggs deposited in cotton squares (flower buds), an average of two adults survive of which about half are female. Thus, the net replacement rate per generation ( R o is 1.0 which, in this case, indicates a stable boll weevil population.

Natural control of the cotton fleahopper, Pseudatomoscelis seriatus (Reuter) (Hemiptera, Miridae), in Texas

The population dynamics of the cotton fleahopper, Pseudatomoscelis seriatus (Reuter), on cotton was predicted using the Texas Cotton Insect Model (TEXCIM) and compared with observed field data from east Texas. The presence of natural enemies and their estimated rate and efficacy of fleahopper consumption were used to calibrate the model to east Texas conditions. A correcting equation was incorporated into the model to account for fleahopper mortality due to rain. Current economic thresholds for cotton fleahoppers were used to determine the frequency of correct decisions made by the model. TEXCIM was free of type I and type II errors in 93% of 71 sampling periods compared with a previous rate of 86% in 57 sampling dates in an earlier version of TEXCIM. Cotton producers may now use either traditional chemical control or monitoring and conserving natural enemies to maximize profits.

Stress Ethylene Production in a Plant-Insect-Microbe System

Cotton fleahopper (CFH), Pseudatomoscelis seriatus (Reuter), which causes extensive shedding of small floral buds of cotton, Gossypium hirsutum (L.), also induces ethylene synthesis and microbial contamination of excised shoot tips. Salivary glands from lab reared insects were contaminated with microbes while those from insects from several native plant hosts were largely microbe-free. CFH were shown to transmit Xanthomonas campestris pv. malvacearum (Smith) Dye to cotton during feeding. Salivary gland extracts from insects from hosts yielding microbe-free CFH, when injected into cotton shoot tips, duplicated the magnitude and pattern of ethylene production induced by insects. Microbe additions amplified the effect of salivary extracts. Filter sterilized salivary fluid was shown to contain a heat- and aging-labile, ethylene inducing activity. Commercial pectinases also induced ethylene production, and they, as well as CFH and CFH salivary fluids, caused darkening and softening of tissue. Characterization of the ethylene inducing enzyme in salivary fluid is in progress.