Lymantria dispar (gypsy moth)

Hatching larvae usually start feeding on flushing buds and later on newly-expanded leaves. High populations often result in total tree defoliation, often across a large spatial area.

Cultural Control

Silvicultural manipulation has been used as a long-term management strategy to limit the ability of gypsy moth populations to increase to outbreak densities. Such strategies are based on thinning strategies. Thinning to reduce host species preferred by the gypsy moth would theoretically reduce stand susceptibility, but is not very satisfactory because the most susceptible tree species, such as oak species, are also usually considered the most valuable timber species. Gottschalk (1993) also suggested presalvage thinning to remove low-vigour trees to lower stand vulnerability. Effects of silvicultural manipulations on gypsy moth populations and tree mortality are discussed by Muzika et al. (1998) and Liebhold et al. (1998).

Biological Control

Following the introduction of gypsy moth into North America in 1869, it was the target of several early and extensive biological control programmes (Howard and Fiske, 1911; Burgess and Crossman, 1929). About 80 species of natural enemies, parasitoids, predators and pathogens were introduced from 1906 to the present but most have failed to establish, possibly due to the lack of alternate hosts (Hoy, 1976). Only 11 parasitoids, one predator and two pathogens established upon their release, some of which have become important mortality factors in North America. Of major interest is the fungal pathogen Entomophaga maimaiga, which was probably introduced accidentally from eastern Asia in the 1980s. Since then, this pathogen has become an important natural enemy of the gypsy moth (Hajek et al., 1993) and it has recently been observed to have replaced the gypsy moth nuclear polyhedrosis virus as the dominant pathogen in outbreaking populations in the USA (Hajek et al., 2015).

Classical biological control programmes have also been implemented in Morocco, where the gypsy moth lacks several of its major natural enemies. The egg parasitoid Ooencyrtus kuvanae and the nuclear polyhedrosis virus were introduced from Europe (Fraval and Villemant, 1995). Other biological control attempts against the gypsy moth include mass releases of O. kuvanae were made in Bulgaria (Chernov, 1976), which resulted in 60% higher egg parasitism. Maksimovic and Sivcev (1984) released gypsy moth eggs to sparse populations to maintain a low density of hosts and sustain parasitoids, which increased parasitism and prevented defoliation in subsequent years.

In the gypsy moth's native range in Eurasia, outbreaks sometimes occur, but they tend to be localized and of short duration. Severe defoliation results in reduced growth increment and crown dieback, but tree mortality is only occasionally observed. This is in contrast to North America, where major outbreaks tend to occur every 5-10 years, last 2-3 years each time, and occur over a spatially widespread area (Johnson et al., 2005, 2006; Haynes et al., 2009). Two to three years of complete defoliation often results in significant tree mortality, particularly during drought conditions or when trees are stressed by other factors, such as plant pathogens. The difference in outbreak frequency and intensity between gypsy moth in its native Eurasia and North America could be due to absence of certain natural enemies.

L. dispar is considered one of the most important non-native forest pests in the northeastern and Midwestern USA. From 1924-2013, over 37 million hectares were defoliated, including over 11 million hectares between 1980 and 1983; during this outbreak, in Pennsylvania in 1981 alone, timber loss was estimated to be more than US$ 72 million (Montgomery and Wallner, 1988). Other notable outbreaks in the USA occurred between 1989-1993 (>7.4 million hectares) and 2006-2010 (>2.3 million hectares). As the range of the gypsy moth continues to expand, these impacts are also likely to increase (Tobin et al., 2012). In addition to timber impacts, other impacts include costs and losses to the urban and suburban forest including hazard tree removal and replacement, residential impacts, and impacts to the recreational sector (Leuschner et al., 1996; Bigsby et al., 2014).