Neither herbicides nor Aphthona spp. biological control agents provide satisfactory leafy spurge control in all environments where the weed is found. Open-release and caged field studies were conducted to determine the effect of herbicides used in combination with Aphthona spp. flea beetles on both the leafy spurge control and the biological control agent populations. Leafy spurge density was reduced more rapidly when A. nigriscutis or a mixture of A. czwalinae and A. lacertosa was combined with a fall-applied treatment of picloram plus 2,4-D than when either method was used alone. The combination treatment reduced leafy spurge density 3 to 5 yr earlier than did either the herbicides or the biocontrol agents alone and once reduced, the Aphthona flea beetles maintained acceptable control for at least 7 yr. Spring-applied herbicides eliminated the adult food source and negatively affected the flea beetle establishment, but fall-applied treatments did not reduce the flea beetle establishment or its reproduction. The combination treatment was most successful when used with established Aphthona spp. populations that were not increasing in size or reducing leafy spurge density. Land managers could realize a three- to fivefold cost savings with the combination treatment compared with herbicides alone.

The combination of Aphthona spp. flea beetles and herbicides can increase leafy spurge control when compared with either method used alone, but the effect of herbicides on the biocontrol agent and conversely the effect of Aphthona on herbicides within the plant are unknown. Understanding this interaction will allow for maximum weed control with little or no negative effect on the establishment and reproduction of the biological agent. Fall application of picloram plus 2,4-D had a minimal negative effect on the A. nigriscutis population and no effect on overwintering fitness. The number of A. nigriscutis adults collected from soil cores taken from the field was similar regardless of the herbicide application date, although emergence was variable. Leafy spurge soluble and insoluble carbohydrate and soluble protein concentrations, which are indirect measurements of the herbicide's effect on nutrient availability to the insect, were similar in treated and untreated plants. Aphthona nigriscutis larval feeding did not affect the absorption or translocation of 14C–2,4-D or 14C-picloram alone or in combination in leafy spurge. The observed increase in leafy spurge control due to the combined treatment was likely an additive effect of herbicide toxicity to root tissue and A. nigriscutis larval feeding on root buds.