A Review of Sampling Procedures Available for Ipm Decision-Making of Forest and Shade Tree Insects in North America

RELATIVE ABUNDANCE OF FEEDING GROUPS AND SPECIES

The majority of sampling procedures developed for forest and shade tree insects were published on defoliating insects, which is the largest group represented in the sampling database (58%; Table 1). Forty percent of all species were classified as defoliating insects (Table 1). Forest and shade tree defoliators are found in five insect orders, including Lepidopetra, Hymenoptera, Coleoptera, Orthoptera (including Phasmida and Mantodea), and Diptera. Fettig et al. (2001) provided only summaries on publications representing three orders, Lepidoptera (65 publications), Hymenoptera (7 publications), and Coleoptera (1 publication). At least two papers are available on Orthopteran pests that did not meet the criterion for inclusion in their database (Readshaw 1965; Hodson 1972). To our knowledge, no sampling technique or sampling program has been published on a Dipteran defoliator of forest or shade trees. In general, these species are of limited importance. Among the best known is the native holly leafminer, Phytomyza iliccola Loew (Diptera: Agromyzidae), a common pest of American holly, Ilex apaca Ait., in the eastern United States (Coulson and Witter 1984).

The most commonly referenced defoliators were the gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae) (14 publications); Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough) (Lepidoptera: Lymantriidae) (11 publications); spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) (10 publications); and western spruce budworm, C. occidentalis Freeman (8 publications) (Table 2). These four species alone accounted for 35% of all publications in the database, and they are also the most common species represented among the five feeding groups. All are notable pests that have had a significant impact on our forested resources. Lymantria dispar was first introduced into Medford, Massachusetts, in 1869, and is now a major defoliator of hardwoods throughout eastern North America. Orgyia pseudotsugata is a major defoliator of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, and true firs, Abies spp., throughout western North America. Choristoneura fumiferana is the most destructive defoliator of balsam fir, A. balsamea (L.) Mill., and white spruce, Picea glauca (Moench) Voss, in eastern North America. Historically, control programs for C. fumiferana were among the largest in the world (Sterner and Davidson 1982). Choristoneura occidentalis is an important pest of Douglas-fir; true firs; Engelmann spruce, Picea engelmannii Parry ex Engelm.; and larch, Larix occidentalis Nutt., in the western United States and Canada. Infestations in mature stands cause growth loss, top kill, and occasional tree mortality (Coulson and Witter 1984).

The second largest group represented was classified as bud, shoot, and root insects (Table 1). This group included 16% of all publications and 18% of all species reported (Table 1). Recently, research efforts have focused on the development of sampling techniques for regeneration weevils, Hylobius spp., and tip moths, Rhyacionia spp., in forest and Christmas tree plantations. The pest status of these two genera has increased with the advent of intensive forest management (Fettig and Salom 1998; Fettig et al. 2001; Asaro et al. 2003; Coyle et al. 2003). For instance, the Nantucket pine tip moth, Rhyacionia frustrana (Comstock) (Lepidoptera: Tortricidae), caused a growth loss of US$366,590 (10,474 cords) in Florida in 1999. It is likely that losses in other states where the preferred host loblolly pine, P. taeda L., is grown more extensively would grossly exceed this figure (Fettig and Berisford 2002). Such figures are not available regarding the impact to the Christmas tree industry but are thought to be significant (Asaro et al. 2003).

Wood- and bark-boring insects accounted for 11% of all publications and included reviews on sampling techniques for the southern pine beetle, Dendroctonus frontalis Zimmerman. The southern pine beetle is the most important forest pest in the southern United States. In 2000, D. frontalis impacted 4.9 million ha (12.1 million ac) of pine forest, resulting in a loss of US$100.4 million (USDA-FS 2000). Much effort has gone into developing sampling methods to describe the population dynamics of this species (Coulson 1979, 1980), but few are useful for IPM purposes. This species was the fifth (6 publications) most frequently referenced species in the database (Table 2).

The piercing–sucking feeding group accounted for 10% of all publications, while seed and cone insects accounted for only 5% (Table 1). Although seed and cone insects are represented in seven orders, relatively few species are of importance in modern seed orchards. The most notable pests are routinely managed with insecticides on a calendar basis. Unfortunately, too many managers rely on calendar sprays and other nonjudicious uses of insecticides without knowledge of population trends, which often leads to secondary pest outbreaks (Cameron 1981).

RELATIVE ABUNDANCE OF ORDERS AND FAMILIES

The most common order represented was Lepidoptera, which accounted for 62% of all publications (Table 1). The vast majority of these publications were in the defoliating insect group. The order was absent from the piercing–sucking (by definition) and wood- and bark-boring feeding groups. There are notable Lepidopteran wood-boring insects, such as the carpenterworm, Prionoxystus robiniae (Peck) (Lepidoptera: Cossidae), but to our knowledge, sampling programs have not been developed for these species. Coleoptera was a distant second, accounting for 20% of all publications in the database (Table 1).

Tortricidae (Lepidoptera) was the most common family among species reported by Fettig et al. (2001). This family was unique in that representatives were present in three feeding groups (Table 2). Thirty percent of all publications included sampling methods or programs relevant to tortricid species. Lymantriidae (Lepidoptera) was a distant second and accounted for 20% of publications. Only two species, L. dispar and O. pseudotsugata, were represented. However, both are notable defoliators that receive near-continuous research attention in both urban and forested environments.

RELATIVE ABUNDANCE OF HOSTS

Eighty-one percent of summaries referenced a conifer as the primary host. Pinus (30%), Picea (19%), and Abies (15%) were the most commonly reported genera. Four of the most commonly referenced species are exclusively pests of conifers, including O. pseudotsugata, C. fumiferana, C. occidentalis, and D. frontalis (Table 2). Although L. dispar is known to feed on conifers, this phenomenon is rare and often occurs only in the absence of preferred hosts (Coulson and Witter 1984). Among hardwoods, the most common hosts were Quercus spp.

RELATIVE ABUNDANCE OF FOREST AND SHADE TREE PESTS

Ninety-two percent of the species reported were predominately forest pests (Table 2). The substantial lack of publications on shade tree insects is surprising. Shade trees, such as those located in residential, recreational, or administrative sites, are particularly susceptible to insect attack as a result of increased amounts of stress associated with drought, soil compaction, mechanical injury, or vandalism. Additionally, the effectiveness of biological controls is reduced in these systems (Coulson and Witter 1984). The value of these trees is also much higher than their forest counterparts (McPherson et al. 1999). For example, Nowak et al. (2001) estimated the maximum potential impact of the Asian longhorned beetle, Anoplophora glabripennis (Coleoptera: Cerambycidae), to urban forests at US$669 billion. Individual estimates ranged from US$72 million to US$2.3 billion per city. The urban forest is a large and currently expanding area that provides numerous benefits to its residents (Dwyer et al. 2003).

The need for sampling programs of shade tree insects is paramount. The system naturally lends itself to the development and implementation of such tools because of the “market values” of the crop, the increased susceptibility of the hosts, and their relatively isolated distribution within the landscape. Unfortunately, at present, such programs are lacking, and communities are most frequently forced to rely on crude judgments for control decision-making. A notable exception is the recent development and implementation of citywide monitoring programs for the elm leaf beetle, Xanthogaleruca luteola (Müller), in California (Lawson and Dahlsten 2003). This program resulted in a dramatic reduction in insecticide use at a considerable cost saving (Lawson and Dahlsten 2003) and is a prime example of the importance of developing such tools and the benefits that can be realized by their application to the urban forest.

RELATIVE ABUNDANCE OF SEQUENTIAL OR BINOMIAL SAMPLING PLANS AND ACTION THRESHOLDS

In our analysis, we tallied the total number of sequential sampling plans, action thresholds, and density–damage relationships (that did not explicitly specify an action threshold) in each feeding group because of their importance in control decision-making. In several publications, an action threshold (or economic impact level, EIP) was not specified. In these cases, we used the high-density estimates as an action threshold when available.

IPM IMPLEMENTATION

Integrated Pest Management (IPM) programs attempt to reduce insect-associated losses to acceptable levels using multiple techniques that are effective, economically viable, and ecologically compatible. Sampling to determine population density or classify pest status is the cornerstone of most IPM programs. Much effort has been devoted to the development, theory, and application of sampling methods (Fettig et al. 2001). However, our research indicates that only 38% of species listed as at least moderately important in the United States and Canada currently have useful sampling programs available. Even fewer of these programs have been incorporated into well-defined IPM programs. A notable exception is the recent development of an IPM program for R. frustrana (Asaro et al. 2003). The species is a common regeneration pest of pine plantations in the southeastern United States. Larval feeding can cause shoot mortality and tree deformity, height and volume reductions, compression wood increases, and occasional tree mortality (Asaro et al. 2003). The complex of pest, host, and system naturally lends itself to IPM practices.

The IPM program for R. frustrana includes six distinct components. Initially, host species (1) and plantation age (2) are used for basic hazard rating (3) of the infested stand (Hood et al. 1986). High hazard stands are recommended for further monitoring via pheromone-baited traps to estimate damage levels (4) (Asaro and Berisford 2001). If high damage is predicted, chemical control may be warranted to avoid economically significant growth losses, particularly on sites of medium to high quality (5). Insecticide spray timing models and schedules are reported that allow increased efficacy and reduced application frequency over conventional calendar sprays (Fettig and Berisford 1999, 2002; Fettig et al. 1998, 2000a, 2000b, 2003). Finally, post-treatment evaluations (6) are recommened because continual, nonjudicious use of insecticides is not economical or environmentally sound and can lead to secondary outbreaks of scales and aphids (Asaro et al. 2003). This IPM program incorporates knowledge accumulated during 40 years of both basic and applied research. Its development is a primary example of the need for developing sampling techniques and programs that later can be integrated with control methods (Asaro et al. 2003).

A significant opportunity exists for developing similar tools to aid in control decision-making for a large number of pests (Table 2). Significant efforts should be conducted to provide tools that are useful to the resource manager, urban forester, and arborist for the major forest and shade tree pests. Current trends suggest that efforts should concentrate on species important to the urban forest and intensively managed forest plantations.