EFFICACY OF THE STEM-GALLING MOTH EPIBLEMA-STRENUANA WALK. (LEPIDOPTERA, TORTRICIDAE) AS A BIOLOGICAL-CONTROL AGENT FOR RAGWEED PARTHENIUM (PARTHENIUM-HYSTEROPHORUS L.)

The efficacy of Epiblema strenuana Walk., a stem-galling moth, as a bi ological control agent for Parthenium hysterophorus L. (ragweed parthe nium) was examined in a glasshouse pot experiment. The importance of t he timing of insect attack and the presence of competition from Cenchr us ciliaris L. (buffelgrass) to the level of control obtained was test ed. This was achieved by applying E. strenuana eggs at two stages of t he weed's development: prior to stem elongation (35 days after emergen ce) and after stem elongation (53 days after emergence), in the presen ce or absence of moderate competition from C. ciliaris seedlings. Appl ication of a moderate number of E. strenuana larvae, at 53 days, reduc ed the number of immature capitula (36%), mature capitula (41%), and v iable seeds (39%) produced by P. hysterophorus plants. Timing of the a pplication of E. strenuana was important, with earlier application (at 35 days) causing a significant reduction in plant height (34%) and a more significant reduction in the number of mature capitula (74%) and viable seed (74%) produced. Competition from C. ciliaris had a signifi cant, and usually greater, effect on all of these characters, as well as significantly reducing the aboveground biomass of the weed. Plant h eight was the only measured character which was less affected by compe tition from C. ciliaris than by E. strenuana attack. Reduction in weed seed production was greatest when E. strenuana was applied prior to s tem elongation and when the weed was also experiencing competition fro m C. ciliaris. In fact, a synergistic interaction was detected between plant competition and insect attack, and seed production was reduced by more than expected when these factors were combined. With both fact ors simultaneously present, seed production was reduced to 2% of that of the control plants. E. strenuana attack did not affect the quality of P. hysterophorus seeds produced. These levels of control do not oft en occur in the field and a number of possible explanations for this a re discussed. (C) 1998 Academic Press.