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 biological control agent for Parthenium hysterophorus L. (ragweed parthenium) was examined in a glasshouse pot experiment. The importance of the timing of insect attack and the presence of competition from Cenchrus ciliaris L. (buffelgrass) to the level of control obtained was tested. This was achieved by applying E. strenuana eggs at two stages of the weed's development: prior to stem elongation (35 days after emergence) and after stem elongation (53 days after emergence), in the presence or absence of moderate competition from C. ciliaris seedlings. Application of a moderate number of E. strenuana larvae, at 53 days, reduced the number of immature capitula (36%), mature capitula (41%), and viable seeds (39%) produced by P. hysterophorus plants. Timing of the application 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 significant, and usually greater, effect on all of these characters, as well as significantly reducing the aboveground biomass of the weed. Plant height was the only measured character which was less affected by competition from C. ciliaris than by E. strenuana attack. Reduction in weed seed production was greatest when E. strenuana was applied prior to stem elongation and when the weed was also experiencing competition from 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 factors 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 often occur in the field and a number of possible explanations for this are discussed.