NATURAL PARASITISM BY TRICHOGRAMMA MINUTUM OF THE EGGS OF THE SUGAR-CANE MOTH BORER, DIATRAEA SACCHARALIS, IN THE CANE FIELDS OF PUERTO RICO

The initiation in the formation of waves of abundance of egg-clusters of Diatraea saccharalis F. each spring in the northwestern corner of Puerto Rico is seasonal, but their height, duration and sudden drop is late summer are due to the near perfection of natural control by Trichogramma minutum Riley. In the remaining four-fifths of Puerto Rico, the factors responsible for the initiation, height and usually much shorter duration of waves of abundance, of egg-clusters lare not seasonal at all, but apparently depend on temporary and partial failure of biologic control in previous generations of the host. Natural control in the egg stage, even with often almost as many egg-clusters eaten by ants as attacked by parasites, rarely occurs, because of the shortness of the period of the wave. Irrigation so modifies humidity that rainfall, varying from less than 30 inches to nearly 90 inches per year, can not be proved to be 3 factor, and the variations in temperature are within too narrow limits to produce an effect. Height of cane and- variety of cane has no effect on egg-clusters, but ratoon cane averages greater abundance of host eggs, and higher parasitism, than plant cane or ratoon cane of which the trash has been burned. Parasitism invariably averages higher when or where host eggs are most numerous, but great variation in abundance of eggs in individual fields is paralleled by comparable variation in parasitism not depending on host abundance. Fields with more than five fresh eggclusters per man-hour and less than 33% parasitism are one out of every ten or eleven, and generalized predictions as to the occurrence of such conditions can be made. The release of laboratory-reared parasites can be commercially justified only in fields meeting these conditions, which occur most often in plant cane on the south coast during the winter.