How to use leafy spurge gall midge for biocontrol
Biocontrol category: Classical - released once and persists
When to use: When moving gall midges to a new area, collect the first galls formed in a season (prior to mid-summer) and move to the new site within 24 hrs.
Maximizing effectiveness: Combine the use of leafy spurge gall midge with another control method to reduce the original infestation and prevent spread by roots and seeds of plants not galled. Redistribute only galls collected early in the season (before mid-summer).
The gall midge effectiveness is also influenced by leafy spurge genotype. In a research study at North Dakota State University, genotypes from South Dakota and Nebraska were most susceptible to gall formation and had the highest larvae survival, while the genotypes from Montana and Manitoba were most resistant.
Morphological characteristics of the leafy spurge stem tips, such as stem diameter, leaf length, width, and area did not correlate with gall formation or larvae survival. The two genotypes most resistant to galling by S. esulae, Manitoba and Montana, had the same chloroplast genotype, but also were closely related to the two most susceptible genotypes. Because eggs were laid on all genotypes, it appears that adult females were not preferentially selecting appropriate host genotypes, but that egg and larvae survival was strongly influenced by genotype.
Pest stage: Female leafy spurge gall midge adult lays eggs on the main growing tip, which is often killed by the feeding larvae, and secondary shoots are then produced. The secondary shoots are attacked by subsequent generations of the gall midge.
Mode of action: Weed-feeder
The gall midge prevents galled stems from flowering, thereby decreasing seed production.
Conservation: The first galls of the season are the best to collect these insects for redistribution. Simply cut off the stems below the galls and place the “bouquet” in water for transport. The galls do best when moved to the new site in 24 hr or less. Galls formed in mid-summer or later can be attacked by Zaptopis nigroaenus, a native parasite which feeds on the larvae of the gall midge and greatly reduces the effectiveness of the gall midge on leafy spurge.
Compatibility: Herbicides can be applied at the optimum growth stage for leafy spurge control. Some of the area (perhaps 15 to 25 percent) must be left untreated to sustain the leafy spurge gall midge population. The integrated program may be most useful near wooded areas or rough terrain. Herbicide treatment would prevent leafy spurge spread outside the tree area or inaccessible site and the gall midge would reduce seed production within the areas where herbicides generally cannot be used. (Any time you use a pesticide, you must read and follow the label directions and comply with all applicable laws and regulations related to pesticide use. Also be sure that any pesticide used is approved for use in your country and state/province.)
Commercially available: No
