The ability to estimate and predict spread in exotic invaders is a keystone process in the development of management guidelines and policy. On detection of a new invader, key questions that often arise are which areas are susceptible to infestation; how long before populations become established in these susceptible areas; and what will be the economic and ecological impacts. Moreover, a quarantine zone is often placed around the area of a new invasion, which also serves as a zone of biological interest in which detection surveys can be made to determine the extent of the invasion. In this case, the expected rate of spread can be a critical ingredient in determining the appropriate perimeter around an infestation. In established invaders that have not yet fully invaded all susceptible habitats, estimates of spread rates can be useful predictors in determining future boundaries of infestation. In the case of forest insect pests, for example, silvicultural interventions can often minimize the inimical effect of exotics if such tactics can be implemented prior to invasion (Gottschalk, 1993).One obstacle in developing sound approaches to managing biological invasions is the lack of reliable methods for estimating and predicting the spread of an introduced species. One critical need is the availability of sensitive survey tools for characterization of the spatial distribution of the invading 1 ABSTRACT Aim To compare different quantitative approaches for estimating rates of spread in the exotic species gypsy moth, Lymantria dispar L., using county-level presence/absence data and spatially extensive trapping grids.Location USA Methods We used county-level presence/absence records of the gypsy moth's distribution in the USA, which are available beginning in 1900, and extensive grids of pheromone-baited traps, which are available in selected areas beginning in 1981. We compared a regression approach and a boundary displacement approach for estimating gypsy moth spread based on these sources of data. ResultsWe observed relative congruence between methods and data sources in estimating overall rates of gypsy moth spread through time, and among regions. Main conclusionsThe ability to estimate spread in exotic invasive species is a primary concern in management programmes and one for which there is a lack of information on the reliability of methods. Also, in most invading species, there is generally a lack of data to explore methods of estimating spread. Extensive data available on gypsy moth in the USA allowed for such a comparison. We show that, even with spatially crude records of presence/absence, overall rates of spread do not differ substantially from estimates obtained from the more costly deployment of extensive trapping grids. Moreover, these methods can also be applied to the general study of species distributional changes, such as range expansion or retraction, in response to climate change or other environmental effects.

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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.