Alexei A. Sharov

BIOECONOMICS OF MANAGING THE SPREAD OFEXOTIC PEST SPECIES WITH BARRIER ZONES

Exotic pests are serious threats to North American ecosystems; thus, economic analysis of decisions about eradication, stopping, or slowing their spread may be critical to ecosystem management. We present a model to analyze costs and benefits of altering the spread rates of invading organisms. The target rate of population expansion (which may be positive or negative) is considered as a control function, and the present value of net benefits from managing population spread is the criterion that is maximized. Two local maxima of the present value of net benefits are possible: one for eradication and another for slowing the spread. If both maxima are present, their heights are compared, and the strategy that corresponds to a higher value is selected. The optimal strategy changes from eradication to slowing the spread to finally doing nothing, as the area occupied by the species increases, the negative impact of the pest per unit area decreases, or the discount rate increases. The model shows that slowing population spread is a viable strategy of pest control even when a relatively small area remains uninfested. Stopping population spread is not an optimal strategy unless natural barriers to population spread exist. The model is applied to managing the spread of gypsy moth (Lymantria dispar) populations in the United States.

MODEL OF SLOWING THE SPREAD OF GYPSY MOTH (LEPIDOPTERA: LYMANTRIIDAE) WITH A BARRIER ZONE

When attempts to eradicate an introduced pest species fail and it becomes established, barrier zones are often used to stop or to slow the spread of the population into uninfested areas. The U.S. Forest Service is currently conducting a Slow-the-Spread (STS) pilot project to evaluate the feasibility of slowing the spread of the gypsy moth (Lymantria dispar L.) in several areas along the population front. To predict the effect of barrier zones on the rate of gypsy moth spread we developed a model that assumes establishment of isolated colonies beyond the expanding population front. These colonies grow, coalesce, and thereby contribute to the movement of the population front. The model estimates the rate of spread from two functions: (1) colonization rate as a function of the distance from the population front and (2) population numbers in a colony as a function of colony age. Eradication of isolated colonies in a barrier zone was simulated by truncating the colonization rate function beyond a specific dist...

What affects the rate of gypsy moth (Lepidoptera: Lymantriidae) spread: winter temperature or forest susceptibility?

1u2003The effect of winter temperature and forest susceptibility on the rate of gypsy moth Lymantria dispar (L.) range expansion in the lower peninsula of Michigan was analysed using historical data on moth counts in a grid of pheromone‐baited traps collected from 1985 to 1994 by the Michigan Department of Agriculture. The rate of spread was measured by the distance between population boundaries in consecutive years. Boundaries were estimated for population thresholds of 1, 3, 10, 30, and 100 moths per trap using a polar coordinate system.

Evaluation of Preventive Treatments in Low-Density Gypsy Moth Populations Using Pheromone Traps

Abstract Pheromone traps can be used for evaluating the success of treatments that are applied to either eradicate or delay the growth of isolated low-density populations of the gypsy moth, Lymantria dispar (L.). We developed an index of treatment success, T, that measures the reduction in moth counts in the block treated adjusted by the change in moth counts in the reference area around it. This index was used to analyze the effectiveness of treatments that were conducted as part of the USDA Forest Service Slow-the-Spread of the gypsy moth project from 1993 to 2001. Out of 556 treatments that were applied during this period, 266 (188,064 ha) were selected for the analysis based on several criteria. They included 173 blocks treated with Bacillus thuringiensis (Berliner) variety kurstaki and 93 blocks treated with racemic disparlure. Analysis using general linear models indicated that disparlure treatments were significantly more effective than B. thuringiensis treatments in reducing moth captures. The frequency of repeated treatments in the same area was higher after B. thuringiensis than after disparlure applications. Treatments were more successful if the pretreatment moth counts outside of the block treated were low compared with moth counts inside the block.

Optimization of pheromone dosage for gypsy moth mating disruption

The effect of aerial applications of the pheromone disparlure at varying dosages on mating disruption in low‐density gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), populations was determined in field plots in Virginia, USA during 2000 and 2002. Six dosages [0.15, 0.75, 3, 15, 37.5, and 75 g active ingredient (AI)/ha] of disparlure were tested during the 2‐year study. A strongly positive dose–response relationship was observed between pheromone dosages and mating disruption, as measured by the reduction in male moth capture in pheromone‐baited traps and mating successes of females. Dosages of pheromone 15 g AI/ha (15, 37.5, and 75 g AI/ha) reduced the mating success of females by >99% and significantly reduced male moth catches in pheromone‐baited traps compared to untreated plots. Pheromone dosages <15 g AI/ha also reduced trap catch, but to a lesser extent than dosages 15 g AI/ha. Furthermore, the effectiveness of the lower dosage treatments (0.15, 0.75, and 3 g AI/ha) declined over time, so that by the end of the study, male moth catches in traps were significantly lower in plots treated with pheromone dosages 15 g AI/ha. The dosage of 75 g AI/ha was initially replaced by a dosage of 37.5 g AI/ha in the USDA Forest Service Slow‐the‐Spread (STS) of the Gypsy Moth management program, but the program is currently making the transition to a dosage of 15 g AI/ha. These changes in applied dosages have resulted in a reduction in the cost of gypsy moth mating disruption treatments.

Evaluation of Aestival Diapause in Hemlock Woolly Adelgid (Homoptera: Adelgidae)

Abstract Two hemlock woolly adelgid, Adelges tsugae Annand, generations complete their development on hemlocks (Tsuga spp.) that are native to eastern North America. Progrediens are present in the spring and sistens are present from early summer until the following spring. Following the settling of sistens crawlers at the base of hemlock needles, first-instar sistens go into aestival diapause for ≈4 mo. We conducted studies to determine if we could prevent the induction of diapause and determine the environmental conditions required to do so. Diapause was determined to be maternally regulated. We were able to prevent the induction of diapause by preconditioning parents at 12 and 14.5°C, but not at 17°C, indicating that temperature is a critical preconditioning cue. Preventing the induction of diapause was also most successful under a photoperiod of 12:12 (L:D) h and was therefore chosen as a standard for rearing hemlock woolly adelgid. Egg stage through second-instar progrediens were found to be the maternal lifestages sensitive to diapause-inducing cues.

A model for testing hypotheses of gypsy moth, Lymantria dispar L., population dynamics

A model for simulating long-term gypsy moth (Lymantria dispar L.) population dynamics in North America has been developed. Simulated ecological processes include larval dispersal, foliage consumption on different host trees, reproduction, and mortality due to natural enemies (virus, 4 guilds of parasitoids, and 2 guilds of predators). Population dynamics in several forest stands can be simulated taking into account the migration of gypsy moth and its natural enemies. The model fits well to the following observations and data: (1) quasi-periodic outbreaks at approximately 10-year intervals, (2) life-tables in different outbreak phases, (3) increased parasitism in artificially augmented gypsy moth populations, and (4) phase plots of population dynamics. Simulated survival curves are intermediate between those reported by J.S. Elkinton et al. and by J.R. Gould et al. It is shown that low-density gypsy moth populations can be stabilized by immigration from high-density areas without any density-dependent local processes. The model supports the following hypotheses about gypsy moth population dynamics: (1) density fluctuations of small mammal predators is the most probable synchronization factor in gypsy moth populations; (2) the outbreak frequency depends on the proportion of susceptible tree species and the density of small mammal predators; (3) bacterial insecticide can be applied less frequently for gypsy moth control than can a chemical insecticide of the same killing power because the bacterial insecticide spares invertebrate natural enemies; (4) the success of gypsy moth eradication programs depends on the initial gypsy moth population density and on the type of functional response of predators. An alternative form of population bimodality hypothesis is suggested, according to which there are two types of gypsy moth population dynamics: eruptive in susceptible stands and stable in resistant stands.

Influence of Temperature on Development of Hemlock Woolly Adelgid (Homoptera: Adelgidae) Progrediens

There are three generations of hemlock woolly adelgid, Adelges tsugae Annand, that develop on the secondary host Tsuga spp. Two of these generations, the progrediens and sexupara, are present concu...

Effect of Synthetic Pheromone on Gypsy Moth (Lepidoptera: Lymantriidae) Trap Catch and Mating Success Beyond Treated Areas

Abstract Racemic disparlure sprayed at doses of 37 to 75 g/ha (AI) for mating disruption of gypsy moths, Lymantria dispar (L.), interfered with male moth search behavior outside of treated plots. Counts of feral male moths in pheromone-baited traps and the number of recaptured laboratory-reared moths gradually increased with increasing distance from treated areas. In most cases this effect was observed up to 250 m from treated plots. However, in one location it extended to 600 m along a narrow valley. The proportion of tethered females that mated during 1-d exposure increased gradually with increasing distance from treated plots. The relationship between male moth capture rates in pheromone traps and mating success of tethered females near treated plots was the same as the one observed in previous studies in pheromone-free areas.

Modelling the epizootiology of gypsy moth nuclear polyhedrosis virus

Qualitative understanding of the dynamics of epizootics of the nuclear polyhedrosis virus of gypsy moth has become complete enough to justify attempts to quantitatively predict the timing and intensity of epizootics within a season. In earlier work (Dwyer and Elkinton, 1993), we compared the predictions of a simple differential equation model derived from Anderson and May (1981) to time series of virus mortality in each of eight gypsy moth populations (Woods and Elkinton, 1987). The models predictions were very accurate for high density populations, but seriously under-estimated virus mortality in low density populations. Here we compare the predictions of the simple model to those of the gypsy moth life system model (GMLSM, Sheehan, 1985; Colbert and Racin, 1991), a highly complex computer simulation of gypsy moth population dynamics and forest stand growth that incorporates much of the existing knowledge of the many factors influencing gypsy moth populations. In particular we looked at two different versions of the GMLSM that incorporate two different models of virus transmission. One was identical to that of the simple model (Anderson and May, 1981; Dwyer and Elkinton, 1993), in which the rate of transmission was a constant (the transmission coefficient) times the product of the densities of healthy larvae and the densities infectious virus particles on foliage. The other approach, developed by Valentine and Podgwaite (1982) was a detailed model of production and consumption of infective particles on foliage. The model took account of age-related changes in the amount of foliage consumed and the variation in susceptibility of larvae to virus (LD50). The Anderson and May version of the GMLSM performed about as well as the simple differential equation model, but only for values of the transmission coefficient about 250 times higher that those we had determined experimentally (Dwyer and Elkinton, 1993). The Valentine and Podgwaite (1982) version of the GMLSM gave a much better fit, but only for values of LD50 that were 100 times higher than those determined experimentally. Future research will focus on efforts to refine our understanding of virus transmission in order to explain and reduce the discrepancies between model predictions and observed mortality from virus in fieldpopulations.