Management of Crop Forest and Urban Pests

Management of crop, forest, and urban "pests" has been a major application of insect ecology. Insect roles in ecosystems may conflict with crop and livestock production and human health and habitation when conditions favor insect population growth. For example, densely planted monocultures of crop species, often bred to reduce bitter (defensive) flavors, provide ideal conditions for population growth of herbivorous species (see Chapter 6). Similarly, buildings provide protected habitats for ants, termites, cockroaches, and other species, especially when moisture and unsealed food create ideal conditions. Insects become viewed as pests when their activities conflict with human values.

Traditional views of herbivorous and detritivorous insects as destructive, or at least nuisances, and ecological communities as nonintegrated, random assemblages of species supported harsh control measures. Early approaches to insect control included arsenicals, although much classic research on population regulation by predators and parasites also occurred prior to World War II. With the advent of broad-spectrum, long-lived, chlorinated hydrocarbons and organophos-phates, developed as nerve toxins and used for control of disease vectors in combat zones during World War II, management of insects seemed assured. However, reliance on these insecticides exposed many target species to intense selection over successive generations and led to rapid development of resistant populations of many species (Soderlund and Bloomquist 1990). Concurrently, movement of the toxins through food webs resulted in adverse environmental consequences that became widely known in the 1960s through publication of Rachel Carson's Silent Spring (1962).

The last legal use of DDT (dichlorodiphenyltrichloroethane) in the United States, against the Douglas-fir tussock moth, Orgyiapseudotsugata, in 1974 during an outbreak in Oregon and Washington required emergency authorization by the U.S. Environmental Protection Agency, which had canceled use of DDT in the United States in 1972 (Brookes et al. 1978). This emergency authorization, based on apparent lack of practical alternatives, mandated intensified research on alternative methods of control. Although the importance of nuclear polyhe-drosis virus, Baculovirus spp., in terminating tussock moth outbreaks had been known since the 1960s, applications of DDT or other chemicals reduced larval densities to levels incapable of supporting epizootics (Brookes et al. 1978) and masked the importance of natural regulatory mechanisms. Subsequent research has demonstrated that enhancement of epizootics by application of technical-grade viral preparation to first instar larvae can cause population collapse within the same year; this currently is the preferred means of control. Accumulating evidence indicates that the Douglas-fir tussock moth may be an important regulator of forest conditions (see Chapter 15): compensatory timber production following outbreaks offsets economic losses (Alfaro and Shepherd 1991, Wickman 1980).

Much subsequent research has addressed the effects of pesticide residues on nontarget organisms and has led to cancellation of registration for chemicals with adverse environmental effects and to development and use of more specific chemicals, including insect growth regulators (IGRs) and chitin sythesis inhibitors (CSIs), with shorter half-lives in the environment. Research results also have led to greater use of microbial pathogens, including nuclear polyhedrosis viruses (NPV) and Bacillus thuringiensis (Bt). Effectiveness of these tools can be enhanced by attention to ecological factors. For example, invasive ants and termites, which often are inaccessible to broadcast application of toxins, can be controlled effectively by attracting foragers to a bait containing nonre-pellent, slow-acting toxin, IGR, or CSI that is shared with nestmates through trophyllaxis, accomplishing population reduction with minimal effect on nontarget species.

Much ecological research also has demonstrated the importance of using multiple tactics, including elimination of conducive conditions, enhanced plant defenses, insect growth regulators, pheromones, predators, and parasites, that constitute an integrated pest management (IPM) approach (e.g., Barbosa 1998, Huffaker and Messenger 1976, Kogan 1998, Lowrance et al. 1984, Rabb et al. 1984, Reay-Jones et al. 2003, Rickson and Rickson 1998, Risch 1980, 1981). An ecological approach emphasizes multiple tactics representing the combination of bottom-up, top-down, and lateral factors that regulate natural populations. For example, increased tree spacing can interrupt bark beetle and defoliator outbreaks in forests, reducing the likelihood of outbreaks and need for pesticides. Agroforestry and multiple-cropping systems that increase crop diversity also can interrupt spread of insect populations (Fig. 16.1). In addition, elicitors of induced defenses, such as jasmonic acid, could be used to elevate resistance to pests in crop plants and stimulate biological control at appropriate times (M. Stout et al. 2002). Because of the delay in expression of induced defenses, this approach would be most effective when infestations can be reliably anticipated and economic thresholds are high. Augmentation or introduction of predator and parasite populations for biological control requires retention of necessary habitat, such as native vegetation in hedgerows, or alternative resources, such as floral nectar sources (Hassell et al. 1992, Landis et al. 2000, Marino and Landis 1996, Thies and Tscharntke 1999). Implementation of control measures should be based on predictive models that indicate when the insect population is expected to exceed a calculated threshold, based on net cost-benefit of insect effect and control, above which intolerable loss of economic or environmental values would occur if the population is not controlled (Rabb et al. 1984).

Herbivorous insects also have been used to control invasive plant species. Introducing biological control agents from the pest's region of origin requires consideration of their ability to become established in the new community and their effects on nontarget species, as well as on the costs and benefits of invasive plant persistence and insect introduction.

Many crop species have been genetically engineered to express novel defenses, such as Bt toxins. However, reliance on such strategies threatens to undermine their long-term effectiveness, given insect ability to evolve resistance. Therefore, a high-dose-with-refuge strategy is recommended to prevent survival of pests on the Bt crop and maintain a large, nonadapted population in non-Bt refuges (Alstad and Andow 1995, Carriére et al. 2003). Management of resistance

Multiple Cropping
| Examples of multiple cropping to hinder spread of insect species over agricultural landscape in northeastern China. A: Embedded intercropping within rows. B: Multiple crop species arranged in strips.

development to transgenic crops could be undermined if pollen contamination of nontransgenic refuges or native vegetation leads to variable Bt concentrations and effects on nontarget species in the landscape (Chilcutt and Tabashnik 2004, Zangerl et al. 2001). This requires attention to the landscape structure of Bt and non-Bt crops (especially for insects with broad host ranges that might include multiple transgenic crops) and cooperation among scientists, growers, and government agencies (Carrière et al. 2001a). Another promising new tool includes use of chemicals, such as jasmonic acid, to elicit expression of targeted defenses by crop plants (e.g., M. Stout et al. 2002, Thaler 1999b, Thaler et al. 2001). However, expression of defenses by plants depends on adequate resources.

Advances in understanding of insect effects on a variety of plant and ecosystem attributes also has influenced evaluation of the need for insect management. Furthermore, management goals for natural ecosystems has become more complex in many regions, as societal needs have changed from a focus on extractive uses (e.g., fiber, timber, or livestock production) to include protection of water yield and quality, fisheries, recreational values, biodiversity, and ecosystem integrity. In many cases, insect outbreaks now are viewed as contributing to, rather than detracting from, management goals for natural or seminatural ecosystems. Recognition that low levels of herbivory stimulate primary production by many plants, including crop species (Pedigo et al. 1986, Trumble et al. 1993, S. Williamson et al. 1989), and may affect soil structure, infiltration, fertility, and climate requires evaluation of the integrated effects, or net cost-benefit, of changes in insect abundance or activity.

Many serious human diseases, such as malaria, yellow fever, bubonic plague, and equine encephalitis, are vectored by arthropods among humans and other animal species, especially rodents and livestock. Rodents are reservoirs for several important human diseases, but horses and cattle also are sources of inoculum. West Nile virus has a particularly broad reservoir of hosts, including birds, small mammals, and reptiles. The rapid spread of this disease across North America between 1999 and 2004 reflected a combination of insect transmission of the virus among multiple hosts and rapid bird movement across the continent (Marra et al. 2004). The importance of these diseases to human population dynamics, including the success of military campaigns, underscores the importance of understanding human roles in ecological interactions. Increasing human intrusion into previously unoccupied ecosystems has exposed humans to novel animal diseases that may involve insect vectors. Transmission frequency increases with density of human, reservoir, or vector populations. Management must involve a combination of approaches that augment natural controls and reduce exotic breeding habitat for vectors (e.g., tires, flower pots, roadside ditches) or reservoir hosts as well as inoculation of humans who may be exposed.

Termites, carpenter ants, and wood-boring beetles often threaten wooden structures. Considerable investment has been made in research to reduce damage, especially in historically important buildings. Again, management requires multiple approaches, including chemical barriers to make buildings less attractive to these insects; removal or treatment of infested building material, nearby wood waste, or infested trees; pheromone disruption of foraging behavior; nonrepellent termiticides that can be transferred in lethal doses to other colony members through trophyllaxis; and microbial toxins to inhibit gut flora and fauna (J. K. Grace and Su 2001, Shelton and Grace 2003). Other urban "pests" include nuisances and health hazards, such as exotic ants, biting or swarming flies, and even winter aggregations of ladybird beetles, that may be promoted by proximity of lawns, gardens, and ornamental pools. Frequent pesticide application or elimination of native vegetation in urban settings often reduces the abundance of desirable insects, such as butterflies, dragonflies, and biological control agents. Understanding the ecological factors that promote or suppress these insects in urban settings will enhance management strategies.

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