Conservation biological control is the implementation of practices that maintain and enhance the reproduction, survival, and efficacy of natural enemies (predators, parasitoids, and pathogens) of pests. Natural enemies are important in regulating populations of many agricultural and forest insect pests. Approaches to conservation of these natural enemies involve avoidance of practices harmful to them, as well as adoption of practices that benefit them. Like other animals, insect natural enemies require food, water, and shelter, and protection from adverse conditions. To achieve the goals of conservation biological control, fundamental knowledge of the biology and requirements of natural enemies is needed.
Practices Detrimental to Natural Enemies
Perhaps the most important rule of conservation biological control is the physician’s maxim, ‘first, do no harm.’ Many insecticides can have both direct and indirect effects on natural enemies. Direct effects include acute or chronic mortality as a result of direct contact with pesticides. Direct sublethal effects, such as decreased adult fecundity, reduced viability of offspring, and changes in feeding habits or other behaviors can also occur. Indirect effects can result from mortality in populations of alternate prey or hosts of the natural enemies. Use of broad-spectrum insecticides that have detrimental effects on natural enemies can lead to rapid resurgence of targeted pest populations. In addition, secondary pest outbreaks (rapid increase to pest status of populations of non-target arthropods) can result if naturally occurring biological control of these secondary arthropods is disrupted. Use of more selective ‘biorational’ insecticides, or insecticides with short residual activity, can be an effective strategy for conservation of biological control agents. Timing insecticide applications when natural enemies are absent or in life stages that are not susceptible to the insecticide can also aid in natural enemy conservation. Recent research involving development of pesticide-resistant natural enemies holds promise as well.
Conservation biological control also includes implementation of agricultural and silvicultural practices compatible with maintenance of natural enemy populations. Monoculture environments are highly advantageous to many herbivorous pests, but are usually very poor environments for natural enemies. Plowing, mowing, and harvesting operations, dust from these practices, burning of crop residues, and poorly timed irrigation practices can cause direct mortality of natural enemies. Of greater importance, however, is the habitat disruption associated with these practices. This disruption can create harsh conditions for natural enemies. Alternate hosts or prey, nectar and pollen sources, free water, and refugia are generally found in greater quantities in habitats with greater diversity of vegetation. Microclimatic conditions are generally more moderate as well. However, increasing reliance on intensive agriculture and forestry practices has tended to decrease habitat heterogeneity as well as genetic diversity of crops. Densities of parasitoids, as well as parasitism rates of pest species, have been found to be higher in mixed species habitats and in agricultural field edges near mixed species habitats. Avoidance or modification of cultural practices that disrupt natural enemy populations is an important strategy for maintaining natural enemy effectiveness.
Practices that Enhance Natural Enemy Effectiveness
Incorporating practices that are beneficial to natural enemies requires fundamental knowledge of natural enemy ecology and life history. These practices can be divided into two broad categories: (1) alternate foods or hosts of natural enemies, and (2) shelter and refugia.
Many predators and parasitoids require alternate food sources. For instance, certain ladybird beetles (Coccinellidae), which are important aphid predators, feed on plant pollen before switching to aphids. Many adult parasitoids require food in the form of pollen, nectar, or honeydew. Availability of such foods has been found to increase fecundity, longevity, survival, and effectiveness of many species. Availability of plant food sources may also increase the host searching efficiency of parasitoids, since degree of hunger can influence whether the parasitoids spend more time searching for hosts or for food. Starved parasitoids have demonstrated greater attraction to flower odors over host-associated odors. Maintenance of non-crop plants in or around agricultural fields or forest plantations can provide these foods, and also harbor alternate hosts or prey of natural enemies, helping to maintain natural enemy populations when pest populations are low. Provision of artificial supplementary food sources has also been shown to increase longevity of some agricultural and forest parasitoids in the laboratory and field.
Natural enemies also require shelter from the elements. Artificial shelters have been found to increase winter survival of various peach orchard predators, allowing them to provide improved control of early season peach pests. Windbreaks and shelterbelts may increase searching efficiency and oviposition of parasitoids and predators adversely affected by high winds. Within-field and border refugia in the form of mixed species habitats can also increase the natural enemy: pest ratio by providing overwintering and aestivation sites.
Conservation biological control is likely to increase in importance as agricultural and commercial forest systems become more intensively managed and restrictions on the use of conventional insecticides increase. It is an approach that requires integration of fundamental insect life history and natural enemy/pest interactions with knowledge of the ecology of the systems in which these interactions take place. This combination of factors makes conservation biological control research and implementation a challenging, but potentially rewarding, endeavor.