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Prescribed Fire Effects on Biological Control of Leafy Spurge

Introduction


Leafy spurge (Euphorbia esula L.) is a major noxious weed on wildlife refuges, parks, waterfowl management areas, and other grasslands dedicated to biological conservation throughout the northern Great Plains (Wallace et al. 1992). Although studies are just beginning to document ecological impacts of leafy spurge (Belcher and Wilson 1989, Trammell and Butler 1995), the species' ability to form nearly monotypic stands (Watson 1985) clearly threatens native biodiversity.

Conventional chemical and physical control of leafy spurge is seldom practical on conservation lands because of cost (Messersmith and Lym 1983), risks to the native flora, or conflicts with the needs of wildlife. Biological control may thus be a preferred approach to control of leafy spurge on conservation lands, provided the control agent(s) is compatible with practices used to manage grassland habitats. Of primary concern is prescribed burning, which is widely used to manipulate prairie vegetation for the benefit of native communities (Higgins et al. 1989).

Flea beetles of the genus Aphthona (Coleoptera: Chrysomelidae) appear to be the most promising of the insects currently approved for biocontrol of leafy spurge in the U.S. (Rees and Spencer 1991). All but 1 approved Aphthona species are univoltine. Adults of univoltine species begin to emerge in mid- to late June and lay eggs in the soil near leafy spurge crowns until early September. Although adults feed on leafy spurge foliage, control is exerted by the larvae, which feed on leafy spurge roots. Larvae overwinter in the soil and pupate in late spring to early summer.

The univoltine A. nigriscutis Foudras appears generally adapted to upland soil types and moisture conditions on many wildlife management areas in the northern Great Plains (Rees and Spencer 1991). However, possibly because the litter layer interferes with reproduction, the species is difficult to establish in the dense, mixed stands of leafy spurge and grass that often prevail on these lands (N.R. Spencer, pers. comm).

The first objective of this study was to determine whether burning to remove the litter layer would facilitate establishment of A. nigriscutis. We tested both fall and spring burns to assess influence of burn season on establishment. This phase of the study also provided opportunity to assess initial combined effects of fire and beetle populations on leafy spurge stem density. Our second objective was to determine the ability of established colonies of A. nigriscutis to survive prescribed burns conducted during the early fall and late spring.


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