control Biological of viburnum leaf beetle Principal Investigator Paul A. Weston, Senior Research Associate Department of Entomology, Cornell University, Ithaca, NY Cooperators Brian Eshenaur, Horticulture Extension Educator, Monroe Co. Coop. Ext., Rochester, NY Jan S. Lamboy, IPM Extension Educator, IPM Program, NYSAES, Geneva, NY Background and Objectives Viburnum leaf beetle, Pyrrhalta viburni (Paykull), is a relatively new landscape pest in New York State. As its common name suggests, this pest has a host range that is restricted to plants in the genus Viburnum. This insect is very destructive to its host plant because the larvae feed extensively on new foliage in the spring, and adults resulting from this first generation consume considerable portions of the second flush of foliage produced by the plant following spring defoliation. The first record of the insect in New York State was from a planting of native viburnums along the shore of Lake Ontario in 1996 (Rick Hoebeke, personal communication), and the species has spread quickly through counties bordering the lake. As of this past summer, the insect had been detected in 27 counties of New York State, and was found for the first time in in states bordering New York (1 county each in Pennsylvania and Vermont). Given the rapid spread of the insect and the extent of damage observed to date, it seems likely that viburnum leaf beetle will soon pose a serious threat to viburnums throughout the Northeast and beyond. We have been evaluating pesticides for controlling viburnum leaf beetle, but there are several incentives for developing non-pesticidal methods. First, pesticide use invariably poses risks for non-target organisms, be they humans that apply the products or come in contact with treated plants, or beneficial insects that can be very effective control measures in their own right. Second, repeated pesticide use generally results in development of pesticide resistance, decreasing the efficacy of pesticides and requiring larger doses of the products to achieve the same level of control. Thirdly, pesticide use is being severely restricted or banned outright in many municipalities in New York State, which means that we must have alternative control methods if we are to keep this pest species in check. One alternative method for management of insect pests is biological control. A variety of biological control agents can be used for insect control; these include pest-specific parasitoids, generalist predators, and pathogens (including viruses, bacteria, fungi, or nematodes). For this project, we sought to evaluate generalist predators and pathogenic nematodes for control of viburnum leaf beetle in the laboratory. Materials and Methods As candidate biocontrol agents, we chose only organisms that are commercially available in the U.S. Four predators were evaluated: 1) Coleomagilla maculata, a ladybird beetle (adults and larvae), 2) Harmonia axyridis, another ladybird beetle (adults only; larvae are not commercially available), 3) larvae of Chrysoperla carnea (green lacewing), and 4) Orius insidiosis, minute pirate bug (adults only; nymphs are too small). In addition, we evaluated a 1
species of parasitic nematode, Heterorhabditis bacteriophora, because it attacks insects in the soil; we reasoned that immature stages of viburnum leaf beetle might be susceptible to such an agent, either as larvae as they crawl into the soil to pupate, or as pupae in the soil. This nematode is often used against other chrysomelids. Predatory efficiency was evaluated by placing a viburnum leaf containing larvae of Pyrrhalta viburni in petri dishes (9 cm diam) at two densities (1 or 4 larvae per dish) along with an individual predator. Larval P. viburni in all trials were 3rd instar except for the second trial with lacewing larvae; in this trial, we used 1st instar prey larvae because the small predators had a difficult time handling the larger (3rd instar) prey. Prey mortality and signs of predator attack were assessed at the following times following introduction: 4, 24, 48, and 96 h. The various combinations of predator and prey life stages can be seen in Table 1 (we were unable to try every predator with both larval stages of prey because of lack of prey larvae). Five replicates were conducted for each predator:prey combination and each prey density. For those predators that appeared most effective, we video-recorded interactions between predator and prey and quantified a number of parameters related to aggressiveness and attack efficiency of the predators. Because the emphasis of this portion of the project was on absolute as opposed to comparative predatory efficiency, results were not statistically analyzed (mortality of larvae not confined with predators was nil). Predators killing less than half of prey items were considered to be poor candidates for further study. Incidence of predation in the field was observed in Highland Park Arboretum in Rochester, N.Y. where population levels of P. viburni have been very high. Efficacy of H. bacteriophora was assessed by placing viburnum a leaf with five3rd instar P. viburni larvae in a container (9 cm diam x 11 cm tall) containing soil medium dosed with nematodes. The target number of nematodes was 666 per container, which is equivalent to 1 million nematodes per square foot, the recommended application rate. Ten such containers were placed in a growth chamber at 22C under 15:9 L:D, and checked daily for adult emergence. Ten control containers, identical to the treatment containers except lacking nematodes, were placed beside the treatment containers and similarly monitored. Percent emergence was analyzed with ANOVA. Results Adult Harmonia axyridis paired with 3rd instar and larval Chrysoperla carnea paired with 1st instar P. viburni larvae exhibited efficacy at each of two prey densities, individually consuming 100% of single prey and 45-65% of four prey within 24 hours (Table 1). In direct observations, first instar C. carnea, despite a 65% greater duration of attacks than H. axyridis, seemed unable to kill 3rd instar P. viburni larvae. Post-attack mortality from inflicted injuries might occur, however. H. axyridis successfully killed and consumed large prey. Larval and adult Coleomagilla maculata and adult Orius insidiosus were ineffective predators of 3rd instar larvae; the only mortality with these two predators was 20% mortality observed with adult C. maculata paired with individual P. viburni larvae. The nematode Heterorhabditis bacteriophora was highly efficacious against pupating P. viburni, reducing adult eclosion from 74% to 12% (F1,18 = 21.4; P = 0.0002). We suspect that efficacy might even be higher because mortality induced by the nematode was 100% in 8 of 10 experimental units and 80% in the ninth. The only incidence of predation in the field was by H. axyridis. When present, it was 2
found at a density of approximately 1 H. axyridis (adult or larvae) per 100 P. viburni larvae. In all cases, H. axyridis was found on plants with populations of aphids, and were more often found in the vicinity of aphids than P. viburni larvae. Discussion These studies have revealed that two generalist predators and a pathogenic nematode offer potential as biocontrol agents for P. viburni. Although both predators (H. axyridis and C. carnea) are widely distributed in the northeastern U.S., it is not clear whether populations occurring in the wild would be sufficiently high to suppress P. viburni populations, especially when young P. viburni larvae are present (early to mid May). Thus, augmentation might be needed to elevate predator numbers to levels high enough to effect control. H. axyridis seems to prefer aphids over P. viburni larvae, so it is unclear whether predation on P. viburni will be substantial if large aphid populations are present. Augmentation with C. carnea larvae might also be effective in reducing numbers of P. viburni, but we have not seen lacewing larvae feeding on P. viburni in the field. This may be due to later emergence of lacewings; augmentation with commercially reared C. carnea in early May could possibly be...