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Composition, abundance and pest control potential of spider communities in agroecosystems: a comparison of European and US studies

Preface. Acknowledgments. Contributors. Part 1: Perspectives. 1. Critical Issues Related to Nontarget Effects in Classical Biological Control of Insetcs L.E. Ehler. 2. Nontarget Effects of Biological Control: What Are We Trying to Miss? J.A. Lockwood. 3. The Frequency and Strength of Nontarget Effects of Invertebrate Biological Control Agents of Plant Pests and Weeds P. Stiling, D. Simberloff. 4. The Impact of Nontarget Concerns on the Practice of Biological Control R.H. Messing. Part 2: Parasitoids and Predators. 5. Predicting the Risk from Biological Control Agent Introductions: A New Zealand Approach B. Barratt, et al. 6. Parasitoid Drift in Hawaiian Pentatomoids P.A. Follett, et al. 7. Evaluating Nontarget Effects of Classical Biological Control: Fruit Fly Parasitoids in Hawaii as a Case Study J.J. Duan, R.H. Messing. 8. Trichogramma Nontarget Impacts: A Method for Biological Control Risk Assessment D.B. Orr, et al. 9. Coccinellid Introductions: Potential for and Evaluation of Nontarget Effects J.J. Obrycki, et al. 10. Food Webs as a Tool for Studying Nontarget Effects in Biological Control J. Memmott. Part 3: Weeds. 11. Why Things Bite Back: Unintended Consequences of Biological Weed Control P.B. McEvoy, E.M. Coombs. 12. Importation Protocols and Risk Assessment of Weed Biological Control Agents in Australia: The Example of Carmenta nr ithacae T. Withers, et al. 13. Negative Ecological Effects of the Musk Thistle Biological Control Agent, Rhinocyllus conicus S.M. Louda. 14. Biological Control of Musk Thistle: A Reassessment J.R. Nechols. Part 4: Pathogens. 15. Predicting the Host Range of Entomopathogenic Fungi A.E. Hajek, L. Butler. 16. Monitoring the Effects of Bacillus thuringiensis kurstaki on Nontarget Lepidoptera in Woodlands and Forests of Western Oregon J.C. Miller. 17. Environmental Impacts of Entomopathogenic Nematodes Used for Biological Control in Soil M. Barbercheck, L.C. Millar. Index.

Nontarget effects of biological control.

Seven native coccinellid species inhabited alfalfa, corn, and small grain fields in eastern South Dakota prior to invasion and establishment of Coccinella septempunctata L. Six species occurred in all crops, however, Adalia bipunctata (L.) occurred only in corn. The structure of native coccinellid communities differed significantly for years prior to compared with years after establishment of C. septempunctata in fields of the three agricultural crops. Differences in community structure were accounted for mainly by reduced abundance of two species, C. transversoguttata richardsoni Brown and Adalia bipunctata (L.). Annual abundance of C. transversoguttata richardsoni averaged 20-32 times lower during post-invasion years than in years prior to invasion, depending on crop; while annual abundance of A. bipunctata averaged 20 times lower in corn after invasion. Addition of C. septempunctata to the community did not result in a significant increase in total abundance of coccinellids in the crops. Coccinellid abundance in agricultural crops may be limited by the total abundance of prey or by the availability of other requisites in the landscape as a whole. Therefore, introduction of a new species, while resulting in reductions in native species populations, may not increase total coccinellid abundance, and may therefore have no net effect on biological control of aphid pests.

Effects of an invading coccinellid on native coccinellids in an agricultural landscape.

We describe polymerase chain reaction (PCR) primers for gut analysis of aphid predators. The primers amplify aphid mitochondrial COII fragments ranging in size from 77 to 386 bp. Using these primers, we were able to distinguish six species of US Great Plains cereal aphids, including two congeners, Rhopalosiphum maidis (Fitch) and R. padi (L.), and to detect them in extracts of coccinellid and chrysopid predators. We devised a protocol for deriving half-lives of detectability for the DNA of a single aphid consumed by predators maintained under simulated field dietary and temperature conditions. Using this protocol and primers that amplify a 198-bp fragment, we determined statistically different half-lives of detectability for a single R. maidis of 3.95 h in Chrysoperla plorabunda (Fitch) and 8. 78 h in Hippodamia convergens Guerin. The detectability half-life for a 339-bp R. maidis fragment was statistically longer in C. plorabunda but not in H. convergens. The sensitivity of the assay for the 198-bp fragment is 10-7 aphid equivalents. For species-specific predator gut analysis, PCR is superior to monoclonal antibody technology, giving comparable detectability half-lives with lower expense, much shorter development times, and greater certainty of a successful outcome.

Identifying key cereal aphid predators by molecular gut analysis

An analysis of 29 faunal surveys of spiders found in nine field crops in the United States indicates the presence of 614 species in 192 genera and 26 families . These species represent 19% of the ca . 3311 species occurring in North America . Five families included 61% of the species reported in field crops : Salticidae (89 spp .), Linyphiidae (78), Araneidae (77), Theridiidae (64),and Lycosidae(62) .

Spiders in united states field crops and their potential effect on crop pests

Principal components analysis was used to assess niche partitioning between four aphid predators on the basis of oviposition strategies. The study was conducted by sampling abundance and position of the eggs of these predators in corn monocultures in two locations of southern Quebec. The results indicated that the chrysopid Chrysopa occulata Say laid its eggs on corn leaves usually without aphid colonies, and late in the season. All other predators reacted to aphid concentrations. The syrphid Sphaerophoria philanthus (Mg.) oviposited close to the ground, early in the season, and among colonies of the aphid Rhopalosiphum padi (L.). The two coccinellids laid their eggs during July and August with a maximum during tasseling for Coleomegilla maculata lengi (Timberlake) and after pollination for Hippodamia tredecimpunctata tibialis Say. Of these two species, the latter one selected sites at higher levels on the plant, and was more abundant at the edges of the field. It is suggested that the observed niche partitioning cannot be the result of interspecific competition in variable environments such as maize monocultures.

Oviposition and niche partitioning in aphidophagous insects on maize

Laboratory experiments were performed to determine the functional responses and switching behavior of the two dominant spider species of southern Quebec (Canada) corn crops, Tetragnatha laboriosa Hentz and Clubiona pikei Gertsh, and for the two dominant aphid species, Rhopalosiphum maidis (Fitch) and Rhopalosiphum padi (L.). Each spider species was tested with one, and then with the two aphid species at increasing prey density. Switching tests were done by offering the spiders two different ratios of aphid species (4:1 and 1:4). Both spider species showed sigmoid, complex, domed-shaped and, sometimes, linear functional responses in both the monospecific and bispecific tests. Complex functional responses were observed only with C. pikei . Hence, the capture rates were often density-dependent, although inverse density-dependent capture rates were obtained at high prey densities. Both spider species consumed disproportionally more R. maidis than R. padi , whatever the ratio. A greater exploration rate of the experimental spaces by R. maidis could explain the predators' preference. These results suggest that these spiders have some potential for being effective biological control agents.

Functional Responses and Switching of Tetragnatha laboriosa Hentz (Araneae: Tetragnathidae) and Clubiona pikei Gertsh (Araneae: Clubionidae) for the Aphids Rhopalosiphum maidis (Fitch) and Rhopalosiphum padi (L.) (Homoptera: Aphididae)

Louis Provencher

Papers

Oviposition and niche partitioning in aphidophagous insects on maize

Functional Responses and Switching of Tetragnatha laboriosa Hentz (Araneae: Tetragnathidae) and Clubiona pikei Gertsh (Araneae: Clubionidae) for the Aphids Rhopalosiphum maidis (Fitch) and Rhopalosiphum padi (L.) (Homoptera: Aphididae)