ABSTRACT
Summary Our objectives were to examine (1) whether increases in yield and persistence of Italian ryegrass varieties following frequent pesticide application are related to a reduction in frit-fly larval populations, and if so (2) whether the use of a nonpersistent pesticide would be a feasible method of reducing such damage. In two field experiments, the yield and persistence of normally short-lived Italian ryegrasses (Lolium multiflorum L.) were greatly increased by application of large and frequent doses of the potent insecticide phorate. Much of the insect damage was attributed to stem-boring dipterans of the frit-fly complex (e.g., Oscinella frit).
582In one experiment, small plots of 23 varieties of Italian ryegrass were either treated with phorate every 6 weeks or left untreated. Varietal differences in larval numbers and yield in response to pesticide use were compared. In a second experiment, chlorpyrifos was applied each year on three occasions to coincide approximately with the oviposition periods of frit-fly. In the first experiment, larval numbers varied greatly among varieties, but there was a positive correlation between larval numbers and tiller density. Larval numbers were related to yield increase following pesticide application in the first year only, but there were large differences among varieties in their response to pesticide treatment in all 4 years. At the end of the third year of the experiment, many plants on untreated plots had died, but plants on treated plots in general remained vigorous. In the second experiment, chlorpyrifos treatment, although not as effective as phorate, improved yields of the three varieties tested over a 3-year period by an average 25%, or 2.7 t/ha/yr. It is concluded that there are large differences among varieties of Italian ryegrass in susceptibility to frit-fly that appear to be linked to yield response to pesticide treatment and sward longevity. This finding suggests that there are possibilities of breeding specifically for frit-fly resistance in order to increase yield and improve persistence. In the interim it may be feasible to use infrequent doses of nonpersistent pesticides to enhance yield and varietal persistence.
