EFFICACY OF BACILLUS THURINGIENSIS BERLINER AND ITS COMPATIBILITY WITH THE PREDATOR, GEOCORIS PUNCTIPES (SAY) FOR CONTROL OF HELIOTHIS VIRESCENS (F.), ON COTTON IN ARIZONA

Abstract

Efficacy of Bacillus thuringiensis Berliner against the first three larval instars of Heliothis virescens (F.) was studied under greenhouse and field conditions. The primary objective of these studies was to determine the susceptibility of the tobacco budworm to Dipel, a commercial preparation of Bacillus thuringiensis. Both greenhouse and field studies showed that first-instar larvae were more susceptible to all dosage levels of Dipel tested (0.125, 0.25, 0.5, and 1.0 1b/A). The higher rates of 0.5 and 1.0 1b/A gave better control and remained effective for longer periods against the first-, second- and third-larval instars than did the lower rates of 0.125 and 0.25 1b/A. The efficacy of all Dipel rates decreased significantly with each subsequent post-application-date bioassay, and only the higher dosages of 0.5 and 1.0 1b/A remained effective at the three-day residue period in the field studies. The addition of a feeding stimulant, Coax, at 0.25 1b/A, to all Dipel treatments resulted in a significant increase in larval mortality of all instars tested. Dipel plus Coax exhibited longer residual activity than did Dipel alone, indicating that Coax may protect the spores and toxin from degradation by the direct sunlight. When second-instar tobacco budworm larvae were allowed to feed on Dipel-treated terminals for 6, 18 or 30 hours and then transferred to untreated diet, they showed a great capacity to recover from the effect of the toxin. However, as the Dipel rate or exposure time was increased, the ability of larvae to recover was decreased. Dipel did have an effect on the biological activity of treatment-surviving larvae. Larval and pupal development was delayed, pupal weight was decreased, and female fecundity was reduced. In all cases the changes were proportional with the Dipel rate and with the length of exposure. No apparent effect was shown relative to adult longevity or egg viability for those surviving Dipel treatments as larvae. When Dipel was applied directly on tobacco budworm eggs, it had no effect on egg hatch but did significantly affect survival of newly-emerged larvae, indicating the larvae consumed a lethal dose of the toxin during eclosion. The compatibility of Dipel in combination with Geocoris punctipes (Say) against first-instar larvae of Heliothis virescens was also studied under greenhouse and field conditions. Results of these studies indicated the possibility of utilizing the two biological agents to suppress tobacco budworm populations in the field. A minimum density of one Geocoris nymph per 4 row feet is required, plus an effective field rate of Dipel. When adult tobacco budworms were fed on Dipel at 32000 IU/ml, their longevity and fecundity were significantly reduced. An adult feeding stimulant is needed to lure the adults to a Dipel source.