Transmission of a Gammabaculovirus within Cohorts of Balsam Fir Sawfly (Neodiprion abietis) Larvae.

Roger Graves,Dan T Quiring,Christopher J Lucarotti

doi:10.3390/insects3040989

Roger Graves, Dan T Quiring + Show 1 more

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https://doi.org/10.3390/insects3040989

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Abstract

Nucleopolyhedroviruses (NPV: Gammabaculovirus: Baculoviridae) of diprionid sawflies (Diprionidae: Hymenoptera) are highly host specific and only infect the midgut epithelium. While still alive, infected sawfly larvae excrete NPV-laden diarrhea that contaminates food sources. The diarrhea can then be consumed by conspecific larvae, resulting in rapid horizontal transmission of the virus. To better understand the efficacy of Gammabaculovirus-based biological control products, the horizontal spread of such a virus (NeabNPV) within cohorts of balsam fir sawfly (Neodiprion abietis) larvae was studied by introducing NeabNPV-treated larvae into single-cohort groups at densities similar to those observed during the increasing (field study) and peak (laboratory study) phases of an outbreak. In field studies (~200 N. abietis larvae/m2 of balsam fir (Abies balsamea) foliage), NeabNPV-induced mortality increased positively in a density-dependent manner, from 23% (in control groups) to 51% with the addition of one first-instar NeabNPV-treated larva, to 84% with 10 first–instar-treated larvae. Mortality was 60% and 63% when one or 10 NeabNPV-treated third-instar larva(e), respectively, were introduced into groups. Slightly higher levels of NeabNPV-induced mortality occurring when NeabNPV-treated larvae were introduced into first- rather than third-instar cohorts suggests that early instars are more susceptible to the virus. In the laboratory (~1330 N. abietis larvae/ m2 of foliage), NeabNPV-caused mortality increased from 20% in control groups to over 80% with the introduction of one, five or 10 NeabNPV-treated larvae into treatment groups of first-instar larvae.

Highlights

In North America, many diprionid sawflies (Diprionidae: Hymenoptera) are defoliators of coniferous trees and represent some of the most economically important pests of forest trees [1,2].Typically, diprionid population outbreaks are of short duration, lasting only 3±5 yr, and are brought back to low density levels by natural enemies, especially nucleopolyhedroviruses (NPV)(Gammabaculovirus, Baculoviridae; [3]) [1,2,4].Since the early 1990s, western Newfoundland (Newfoundland and Labrador (NL), Canada) has experienced the largest outbreak of balsam fir sawfly (Neodiprion abietis Harris) populations on record
To better understand the efficacy of Gammabaculovirus-based biological control products, the horizontal spread of such a virus (NeabNPV) within cohorts of N. abietis larvae was studied by introducing NeabNPV-treated larvae into single-cohort groups at densities similar to those observed during the increasing and peak phases of an outbreak
Larval mortality was significantly influenced by the number (Wald chi-square = 683.0, p < 0.001) and the instar (Wald chi-square = 46.6, p < 0.001) of NeabNPV-treated larvae introduced into cohorts

Summary

Introduction

In North America, many diprionid sawflies (Diprionidae: Hymenoptera) are defoliators of coniferous trees and represent some of the most economically important pests of forest trees [1,2].Typically, diprionid population outbreaks are of short duration, lasting only 3±5 yr, and are brought back to low density levels by natural enemies, especially nucleopolyhedroviruses (NPV)(Gammabaculovirus, Baculoviridae; [3]) [1,2,4].Since the early 1990s, western Newfoundland (Newfoundland and Labrador (NL), Canada) has experienced the largest outbreak of balsam fir sawfly (Neodiprion abietis Harris) populations on record. In North America, many diprionid sawflies (Diprionidae: Hymenoptera) are defoliators of coniferous trees and represent some of the most economically important pests of forest trees [1,2]. Diprionid population outbreaks are of short duration, lasting only 3±5 yr, and are brought back to low density levels by natural enemies, especially nucleopolyhedroviruses (NPV). (Gammabaculovirus, Baculoviridae; [3]) [1,2,4]. Since the early 1990s, western Newfoundland (Newfoundland and Labrador (NL), Canada) has experienced the largest outbreak of balsam fir sawfly (Neodiprion abietis Harris) populations on record. Tens of thousands of hectares (ha) of forests dominated by balsam fir (Abies balsamea (L.). Natural declines in balsam fir sawfly populations in western Newfoundland have been associated with epizootics of a species-specific NPV, NeabNPV [5,7±9]. NeabNPV has been demonstrated to be an effective control agent [10±12] as NeabNPV infection most often results in the death of infected balsam fir sawfly larvae [13]

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