AFFECTS OF LARVAL INJURY BY WESTERN CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) ON GAS-EXCHANGE PARAMETERS OF FIELD CORN

The effect of larval-induce root injury by western corn rootworm, Diabrotica virgifera virgifera LeConte, on plant physiological parameters of field com (Zea mays L.) was examined in a 2-yr field study. Controlled infestations of 200, 500, and 1,000 eggs per 30.5 row-cm were established in an irrigated, silty clay loam soil in 1989 and in silty clay loam, loam, and sandy loam soil textures with irrigated and dryland treatments in 1990. Infestations were made 16 d after planting and at planting in 1989 and 1990, respectively. Gas exchange parameters were examined before egg hatch, during the injury period, and almost-equal-to 2 wk following the cessation of the injury. During both years, com photosynthetic rates at full sunlight were reduced by an average of 7.9% coinciding with the initial period of larval injury; i.e., feeding by primarily first and second instars. During the periods of maximum injury and postinjury, western com rootworm injury to com roots resulted in either no significant effect on photosynthetic rate (1990) or in an apparent stimulatory effect (by up to 11.2%) on photosynthetic rate (1989). The photosynthesis effects were not consistently related to changes in stomatal conductance or intercellular CO2 concentration. Plant response to root injury was similar in all three soil textures and in differing soil moisture levels (occurring during postinjury period only) in 1990; however, soil texture and soil moisture did have significant direct effects on plant physiology. Plant developmental stage at the time of injury may be an important factor in determining the plant response to injury. Peak injury occurred in the V12-V15 and V9-V11 stages of development in 1989 and 1990, respectively. The increased amount of photosynthetically active biomass (i.e., leaf tissue) may have enhanced the plants' compensatory response in 1989.