MORTALITY, FEEDING INHIBITION, AND RECOVERY OF SPRUCE BUDWORM (LEPIDOPTERA: TORTRICIDAE) LARVAE FOLLOWING AERIAL APPLICATION OF A HIGH-POTENCY FORMULATION OF BACILLUS THURINGIENSIS SUBSP. KURSTAKI

Abstract

A larval population of spruce budworm, Choristoneura fumiferana (Clemens), was monitored for 5 d following aerial application of a commercial formulation of Bacillus thuringiensis Berliner subsp. kurstaki to investigate dose acquisition and expression (larval mortality, recovery, feeding, and growth) in relation to spray deposition and persistence of spray deposits. The main objective was to test if previous laboratory observations on how B. thuringiensis affects feeding and dose ingestion by spruce budworm larvae hold true under field conditions. About 40% of the treated population ingested a lethal dose within 1 d after spray application. Lethally dosed larvae died without further feeding upon transfer from treated foliage to (untreated) artificial diet. Resumption of feeding by larvae that survived the treatment was delayed relative to larvae from the control population during 3 d following spray application; during that time, normal feeding activity and larval weight gain were suppressed. Inhibited feeding by survivors appeared to prevent further dose uptake because the proportion of lethally dosed larvae in daily collections did not increase despite significant residual spray deposits in budworm feeding sites. Restoration of "normal" recovery times by the fourth day coincided with a 65–85% reduction in persistence of the pathogen on the foliage and did not result in further lethal dose acquisition, as treatment-induced mortality dropped to about 20% on the 4th and 5th days. The observations are consistent with previous laboratory observations of how B. thuringiensis affects larval feeding and with the hypothesis that feeding inhibition may be a limiting factor in the acquisition of a lethal dose.

Response of Lymantria dispar L. (Lepidoptera: Lymantriidae) to Bacillus thuringiensis subsp. kurstaki at different ingested doses and temperatures

“…To the best of our knowledge, no information is available on feeding inhibition that occurs in gypsy moth larvae following ingestion of sublethal doses (Maczuga and Mierzejewski, 1995), a process that is known to affect dose acquisition (van Frankenhuyzen and Payne, 1993). Because both mortality and feeding inhibition depend on dose, instar and temperature (van Frankenhuyzen and Nystrom, 1987;Maczuga and Mierzejewski, 1995) and are critical determinants of spray efficacy (van Frankenhuyzen et al, 2000;Régnière and Cooke, 1998), we conducted extensive bioassays involving $21,000 larvae to investigate and quantify those relationships in a systematic fashion. We developed novel statistical approaches to analyze the complex mortality data sets.…”

Carry over effects of the entomopathogen Bacillus thuringiensis ssp. Kurstaki on Choristoneura fumiferana (Lepidoptera: Tortricidae) progeny under various stressful environmental conditions

“…During the last 15 years, formulations of Btk products have been improved (van Frankenhuyzen, 1995) and van Frankenhuyzen et al . (1998) demonstrated that approximately 30–35% of the toxin, resulting from an application of Foray 48B at 30 BIU/ha, persisted on foliage after 4 days, and still had insecticide proprieties (van Frankenhuyzen et al ., 2000). Thus, in the field, spruce budworm larvae might have been reinfected and recovered from several feeding inhibition periods, which could have had detrimental effects on larval growth and pupal weight.…”

Lethal and Sublethal Effects of Single and Double Applications of <I>Bacillus thuringiensis</I> Variety <I>kurstaki</I> on Spruce Budworm (Lepidoptera: Tortricidae) Larvae

“…Differences between the results reported by Ré gniè re and Cooke (1998) and our results can be explained by the fact that larvae in the Þeld encounter Btk as discrete droplets and not as a product uniformly spread over an area as experienced on feeding disks. In the Þeld, the probability of encountering a droplet containing a lethal dose is lower at Þfth than at fourth instar because the LD 50 increases with larval age (van Frankenhuyzen et al 2000). Because feeding inhibition resulting from ingestion of a sublethal dose can hinder the acquisition of a lethal dose (van Frankenhuyzen et al 2000), it is possible that Btk droplets in the Þeld contain enough active elements to induce feeding inhibition but not to kill a Þfth-instar larva.…”

“…In the Þeld, the probability of encountering a droplet containing a lethal dose is lower at Þfth than at fourth instar because the LD 50 increases with larval age (van Frankenhuyzen et al 2000). Because feeding inhibition resulting from ingestion of a sublethal dose can hinder the acquisition of a lethal dose (van Frankenhuyzen et al 2000), it is possible that Btk droplets in the Þeld contain enough active elements to induce feeding inhibition but not to kill a Þfth-instar larva. As a possible remedy, aircraft applicator nozzles could be adjusted to spray larger droplets that would contain more active elements during the second application without increasing the application rate.…”