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Biological Control of Perennial Weeds
Eight insects for biological control of leafy spurge have been released in North Dakota.
Flea beetles (Aphthona spp.) have been the most effective for leafy spurge control because the larvae feed on the root system, the populations can increase rapidly after introduction, and the insect is easily captured for transport to additional locations. Flea beetles are distributed through the North Dakota Biological Control Program. Contact your county weed officer or a member of your local weed board for information. To best establish, the flea beetles should be released on a well-drained south-facing slope and a moderate density of leafy spurge (60 to 90 plants/square yard) with minimal grass cover and thatch. Flea beetle populations take 3 to 5 years to become well established and the insects should not be collected or moved during this time. While the insects are establishing, do not spray insecticides within a quarter-mile of the site, cultivate or burn the release site. During this time, landowners should prevent expansion of the leafy spurge infestation by monitoring the insect population to determine the boundaries and treating the perimeters with herbicides.
Begin scouting for flea beetle establishment when the total accumulated growing degree days (AGDD) for sunflower reaches 1100 to 1200. Flea beetle density prior to 1200 AGDD is low which makes it difficult to monitor for establishment. Likewise waiting until 1600 AGDD, the flea beetle population is beginning to decline. The best time to collect flea beetles for redistribution and/or monitoring for establishment is between 1200 to 1500 AGDD. Estimating beetle density should be done using a standard insect sweep net. Sweep 5 times covering an area of approximately 1 m2 and count the number of flea beetles in the net. If flea beetles are too numerous to count individually, remove excess trash and non-flea beetle insects and pour net contents into a graduated container. Every 10 ml of flea beetles is approximately 1000 individuals.
Leafy spurge gall midge (Spurgia esulae) prevents galled stems from flowering, thereby decreasing seed production. The gall midge generally infests only part of a leafy spurge population so seed production is reduced, but not eliminated. A second control method is needed to reduce the original infestation and prevent spread by roots and seeds of plants not galled. Research at NDSU has shown that the leafy spurge gall midge is compatible with herbicide treatment in an integrated leafy spurge management program. Herbicides such as Tordon or 2,4-D should be applied at the optimum growth stage for leafy spurge control. Some of the area (perhaps 15 to 25%) must be left untreated to sustain the insect population. This integrated program may be most useful near wooded areas or rough terrain. Consult NDSU Ext. Service Circulars W-866 Integrated Management of Leafy Spurge and W-1088 Leafy Spurge Biology, Ecology, and Management for further details.
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