ELEVATED ATMOSPHERIC CARBON-DIOXIDE CONCENTRATION AFFECTS INTERACTIONS BETWEEN SPODOPTERA-EXIGUA (LEPIDOPTERA, NOCTUIDAE) LARVAE AND 2 HOST-PLANT SPECIES OUTDOORS

Beet armyworm, Spodoptera exigua (Hubner), larvae were placed on sugar beet (Beta vulgaris L.) and pigweed (Amaranthus hybridus L.) plants in outdoor chambers in which the plants were growing at either the ambie nt (almost-equal-to 350 mul liter-1) or ambient plus 350 mul liter-1 ( almost-equal-to 700 mul liter-1) carbon dioxide concentration. A serie s of experiments was performed to determine if larvae reduced plant gr owth differently at the two carbon dioxide concentrations in either sp ecies and if the insect growth or survival differed with carbon dioxid e concentration. Leaf nitrogen, water, starch, and soluble carbohydrat e contents were measured to assess carbon dioxide concentration effect s on leaf quality. Insect feeding significantly reduced plant growth i n sugarbeet plants at 350 mul liter-1 but not at 700 mul liter-1 nor i n pigweed at either carbon dioxide concentration. Larval survival was greater on sugarbeet plants at the elevated carbon dioxide concentrati on. Increased survival occurred only if the insects were at the elevat ed carbon dioxide concentration and consumed leaf material grown at th e elevated concentration. Leaf quality was only marginally affected by growth at elevated carbon dioxide concentration in these experiments. The results indicate that in designing experiments to predict effects of elevated atmospheric carbon dioxide concentrations on plant-insect interactions, both plants and insects should be exposed to the experi mental carbon dioxide concentrations, as well as to as realistic envir onmental conditions as possible.