Potential for biotic interference of a classical biological control agent of the soybean aphid

The degree to which resident biota can inhibit the ability of an introduced biological control agent to establish and be effective is termed biotic interference. Studying biotic interference prior to a release using the actual agent is logistically difficult, however, due to quarantine restrictions. An alternative solution is to study biotic interference against a surrogate species in the intended range of introduction, with the expectation that biotic interference against the actual agent will be similar. This study assessed how biotic interference, mostly by generalist predators, may affect establishment of classical biological control agents of the soybean aphid, Aphis glycines Matsumura, in North America. The parasitoid Aphidius colemani Viereck was used as a surrogate for Asian aphidiine braconids such as Binodoxys communis (Gahan). We conducted a factorial field experiment that measured the effect of releasing A. colemani and of excluding resident natural enemies using field cages on soybean aphid populations. We also conducted molecular gut-contents analyses on predators collected in release plots to determine which species fed upon A. colemani. Releasing A. colemani in open field plots increased soybean aphid control beyond that observed in open field plots alone, despite indications that intraguild predation of A. colemani occurred. Thus, biotic interference was not sufficient to eliminate the contribution of A. colemani on soybean aphid suppression during the course of our experiment. Molecular gut-contents analysis revealed that at least two predators, Harmonia axyridis (Pallas) and Chrysoperla carnea Stephens, engaged in intraguild predation against A. colemani. The prolonged effect of intraguild predation on parasitoid establishment remains to be determined.     

Problems of detection and interpretation of vertical migration

Simple counts of organisms as functions of time and depth, orsonic scattering records, can yield only minimal estimates ofrange or velocity of vertical migrants. It is incorrect thoughcommon to equate mean population movements so determined torates of motion of the individual animals. If migratory movementsare not synchronized, conventional studies will generally showthe population to have a bimodal depth distribution, but maynot indicate any migration. A simple model based on hunger as a mediator and light as asynchronizer can explain many instances of apparently anomalousdid migrations, and relate did to longer term migratory behavior. There are a number of alternative or supplementary investigativetechniques which can and should be used to increase understandingof vertical migration. The uses and limitations of some of theseaie discussed.     

Population dynamics and food of bleak (Alburnus alburnus L.) in the littoral zone of Lake Balaton,Hungary

Intra- and interspecific effects together with changing environmental conditions regulate the dynamics of the fragile bleak populations in Lake Balaton. Size- and age-structures, length-weight relationships and mortality were studied on 1493 individuals and growth history were determined using scales from 703 specimens. The population growth rate was slow and the basic population parameters showed area variations along the longitudinal axis of the lake. Gut contents were analyzed in 224 individuals. The bleak stocks are trophically strongly related to the littoral zooplankton and benthos; and they play a significant energy-mediating role in the food-webs between the periphytic communities and fish assemblages in the littoral zone of Lake Balaton.