Effect of Soil Amendments on Pest Insects in Potatoes

I tested effects of amending soil with compost containing lignocellulosic substrates and biocontrol agents known to suppress soil-borne diseases (a parasitic fungus Trichoderma virens, a bacterium Bacillus subtilis, and a hypovirulent strain of the fungus Rhizoctonia solani) on the Colorado potato beetle and potato-colonizing aphids. Experiments were conducted on one conventional and one organic farm in northern Maine during 2007-09 growing seasons. At weekly intervals, thirty plants were selected at random and visually examined for pest insects. The numbers of Colorado potato beetle adults, small larvae (first and second instars), large larvae (third and fourth instars) and egg masses, as well as the number of potato aphids, green peach aphids, and buckthorn aphids were recorded. There were two trends over the length of the study. First, compost increased the numbers of adult Colorado potato beetles while negatively affecting the number of egg masses and larvae. For adult beetles, an increase on plants amended with compost due to redistribution is unlikely in a commercial setting when the entire field is amended with compost. Although the main goal of applying compost amendments is soil improvement, this demonstrates an additional benefit of its application. Aphids did not appear to be affected by compost applications. In contrast, T. virens increased populations of Colorado potato beetles and aphids. In greenhouse experiments we tested the survival and development of the Colorado potato beetle and potato-colonizing aphids on amended or non-amended plants. B. subtilis increased development time for Colorado potato beetle larvae when added to soilless potting media alone or with hypovirulent R. solani. Hypovirulent R. solani applied alone also increased development time and decreased survivorship in affected larvae, but only when applied at an intermediate rate of eight grams per pot. Our results support the presence of plant-mediated effects of soil amendments on the development of the Colorado potato beetle larvae. This should be taken into account when developing management plans for soil-borne disease