No abstract

Persistence of recombinant and wild-type nucleopolyhedroviruses (NPV) was compared in field and laboratory microcosm experiments. Horizontal and vertical distribution of the viruses also was monitored in the field agricultural soil. Mixed populations of the bollworm, Helicoverpa zea, and tobacco budworm, Heliothis virescens, in cotton were sprayed five times during a growing season with wild-type H. zea NPV (HzSNPV.WT) or with a genetically modified H. zea NPV expressing an insect-specific neurotoxin (HzSNPV.LqhIT2). HzNPV.WT accumulated 2.3 times as many occlusion bodies (OB) as HzSNPV.LqhIT2 in soil by the end of the growing season in October 1997. Both NPVs were detected at all soil depths down to 26-35 cm. Both NPVs were randomly distributed among 0-2 cm soil samples throughout the plots according to analysis with Taylor's power law. By 4 August 1998, soil concentration of HzSNPV.WT was only 11-13 OB/g at depths from 0 to 14 cm, and the wild-type virus was not detected below 14 cm. HzSNPV.LqhIT2 was detected only in trace amounts at 0-2 cm at this time. Neither NPV was detected in bioassays of cotton leaves nor in insects sampled from the plots in 1998. Viral persistence also was monitored in laboratory soil microcosms. Three viruses-wild-type Autographa californica NPV (AcNPV.WT), A. californica NPV expressing a scorpion toxin (AcNPV.AaIT), and A. californica NPV expressing juvenile hormone esterase (AcNPV.JHE-S201G)-were introduced into soil microcosms by each of two methods, in water suspension or in host cadavers, for a total of six treatments plus controls. After 17 months, the number of viable OB remaining did not differ among the treatments. The results indicate that the only differences in soil populations of wild-type versus recombinant NPVs are due to the greater amounts of the wild-type viruses that accumulate, probably because they have a greater capacity to replicate in the host insect population.

The effect of the two pathogens, the bactospeine and the nuclear polyhedrosis virus was tested on larvae of Heliothis armigera separately and in a combination. A retardation in larval development was observed after feeding the larvae on a diet containing different concentrations of the bacterium. Larvae of the 5th instar could escape virus treatment by a rapid transformation into the prepupal stage. It was also observed that the susceptibility of the larvae to each of the two pathogens when applied separately decreases with larval age. By using a combination of both pathogens an antagonistic effect was produced in case of first instar larvae. As the larvae increase in age, the two pathogens interact synergetically and an additive effect was observed specially when the mixture contained the LC50 of bactospeine. Zusammenfassung Zur Wirkung von Bacillus thuringiensis Berl. und eines Kernpolyedervirus auf Raupen der Amerikanischen Baumwolleule, Heliothis armigera Hbn. (Lepid., Noctuidae) Es wurden die Wirkungen zweier Pathogene: des Bacillus‐Präparates Bactospeine und eines Kernpolyedervirus bei getrennter und kombinierter Applikation auf die Raupen von H. armigera untersucht. Eine Verzögerung der Entwicklung der Raupen wurde nach Beimischung von Bactuspeine zur Nahrung festgestellt. Raupen des 5. Stadiums überstanden eine Virusbeigabe zur Nahrung durch vorzeitige Umwandlung zur Puppe. Die Empfindlichkeit der Raupen gegenüber den 2 Pathogenen nahm mit dem Raupenalter ab. Bei Verwendung einer Kombination aus beiden Pathogenen entstand bei den Raupen ein antagonistischer Effekt. Mit Zunahme des Raupenalters wandelte er sich zu einem Synergismus und, sofern die Mischung die LC50 von Bactuspeine enthielt, zur additiven Wirkung.

Temperature had various effects on the predacious efficacy of immature and mature stages of the coccinellid predator, Stethorus punctillum on the two-spotted spider mite, Tetranychus urticae. In the case of immature stages, food consumption at the lowest tested temperature (158C) was significantly higher than that at higher temperatures (25 and 358C). On the contrary, positive correlation between food consumption and temperature was evaluated in the case of adult predator. Regarding predator responses to different prey density, a high positive correlation between food consumption and prey density was evaluated among 4 th instar larvae of the predator, followed by adult predator, while younger instars did not show reasonable increases with increasing prey densities. These results confirm that larval and adult stages of S. punctillum exhibit ''Type II'' functional response. In conclusion, the 4 th instar larvae and adult predator are the most preferable stages in winter and summer crops to control T. urticae, respectively.

The retarded effect of continuous larval treatment with Bacillus thuringlensis var. thuringiensis BERLInEa on the biological potency of adult Anagasta kuehniella(Zell) throughout successive generations was studied under controlled laboratory conditions. The results revealed a marked decrease in adult emergence and prolongation in the generation period that are directly proportional to concentration, but a negligible effect on the sex ratio. A significant reduction in the fecundity of the Fl-moths was only observed with the highest concentration used.

No abstract

A standard laboratory strain of the Egyptian cotton leafworm, Spodopteva littorulis (Boisduval) was treated with Bacillus thuvingiensis var. Kuvstaki-HD1 (Dipel 2x) for eight successive generations. Selection with B. thuringiensis did not establish true resistance in the population. On the contrary, a latent toxicity was demonstrated. Slight delay in response to the toxic effect of the pathogen was developed after continuous selection for four successive generations. This may be attributed to the increase in vigor tolerance due to intrinsic or extrinsic factors rather than development of true resistance.

No abstract