The Inadequate Environment

Hot-Blooded Insects

The food and feeding habits of riparian ground beetles were studied in four alpine floodplains (Bavaria, Germany): a 5th-order stream (the Isar) and three 3rd-order streams. The riparian fauna along the streams mainly consists of predaceous species. Riparian ground beetle densities were much higher along the Isar than along the small streams. Aquatic invertebrates composed 89% of the potential prey for carnivorous terrestrial insects along the Isar. Besides aquatic organisms washed ashore, stoneflies emerging on land are of considerable importance as potential prey for terrestrial predators. In contrast, only 34% of the potential prey organisms collected along the small streams were of aquatic origin. Food abundance was 9 times higher in the shore region of the Isar compared to the small streams. Surface drift in the Isar, a potentially important food source for riparian organisms, was about 106 organisms and exuviae per meter stream width in 24 h. The drift density in the Isar was 59 times higher than that in a small stream. Terrestrial organisms provided only 3% of the drifting particles in the Isar, but 50% in the small stream. Gut content analysis reveals, that riparian ground beetles in the Isar floodplain mainly feed on aquatic organisms washed ashore or emerging on land. While small Bembidion species prefer chironomids (larvae and adults) the larger species Nebria picicornis feeds on emerging stoneflies, terrestrial riparian organisms and aquatic organisms accumulating along the shoreline. The prey of riparian ground beetles in the floodplain of the three small streams mainly consists of terrestrial species some of which may have been washed ashore.

Riparian ground beetles (Coeloptera, Carabidae) preying on aquatic invertebrates: a feeding strategy in alpine floodplains.

During their lifetimes, mobile species are likely to experience a mosaic of habitat types offering a variety of food resources and shelter. The lifetime fitness of such species may depend on the quantity and quality of resources encountered in the different habitat types as the organisms move across the landscape. Because alternative farming practices can lead to contrasting degrees of resource availability among habitats, agroecosystems provide opportunities to address such questions experimentally. Here, I examine how landscape structure and pesticide use influence feeding rate, measured with the Energy Reserve Index (ERI) based on body mass and length, and fecundity of a generalist arthropod predator Pterostichus cupreus L. (Coleoptera, Carabidae) in an agroecosystem setting. Landscape structure was measured at the scale of the lifetime range of P. cupreus. Landscape was characterized by the number of fields within range of P. cupreus, the size distribution and perimeter-to-area ratios of arable fields...

Reproduction and energy reserves of a predatory carabid beetle relative to agroecosystem complexity

Previous work has suggested that the condition of polyphagous predatory carabid beetles can be influenced by landscape structure and farming practice. However, earlier studies aimed at testing the effect of farming practice on insect condition and reproduction have not been designed to test this accurately because farms representing different cultivation systems have not had similar landscape complexity. In this study we measured the condition of the polyphagous predatory carabid beetles Harpalus rufipes, Pterostichus cupreus, P. melanarius, and P. niger from five pairs of conventional and organic farms in central east Sweden. The farms within each pair were near to each other and had similar size, land use, and landscape structure. The condition of the beetles was measured as the residuals from the regression between ln(elytra length) and ln(body mass). We show that the residuals for P. melanarius were positively correlated with their fat reserves. For P. melanarius, residuals were higher on farms where the fields had high perimeter-to-area ratio. The pooled residuals of H. rufipes, P. cupreus, and P. niger were positively correlated with the perimeter-to-area ratio and crop diversity. Pterostichus me- lanarius from organic farms had better condition than conspecifics at conventional farms, and for H. rufipes, P. cupreus, and P. niger combined there was a tendency toward better condition on organic farms than at conventional farms. Harpalus rufipes, P. cupreus, P. melanarius, and P. niger, which are all polyphagous predatory carabid beetles, are natural enemies of pest aphids in the agroecosystem. We conclude that, by manipulating landscape structure and composition, improvement in the condition of polyphagous predatory carabid beetles could be achieved, thereby potentially increasing populations of these beneficial insects.

Landscape complexity and farming practice influence the condition of polyphagous carabid beetles

Using an assemblage of tiger beetles in SE Arizona, USA, as test organisms, the potential for food as a limiting resource in each of the life cycle stages was determined. Laboratory experiments established at what levels food quantity affected adults and larvae. Adult females at low feeding levels produced significantly fewer eggs and larvae than females at high feeding levels. Larvae at low feeding levels took significantly longer to pass through all three larval stages, and their pupae and emergent adults were significantly smaller than those individuals raised at higher feeding levels. Large adults survived longer than small adults when deprived of food. Small adult females produced fewer eggs and larvae at low feeding levels than larger conspecific females at the same feeding levels. Field observations established that the feeding levels of wild larvae and adults in several habitats over five years was lower than the level which produced maximum offspring and survival in the laboratory, except on permanent pond edges where food was abundant. Year to year fluctuations in food availability indicated that food in some habitats was more likely limiting in some years than others.

C. B. Knisley

Papers

Evidence for food as a limiting resource in the life cycle of tiger beetles ( Coleoptera: Cicindelidae).