Adults of many predators and parasitoids may need or benefit from pollen, nectar, or honeydew (produced by aphids) during the summer. Many crop plants bloom evenly for a short time, so flowering plants may be needed along the edges of the field or within the field as supplementary sources of pollen and nectar. However, plant diversification in the field can also interfere with the efficiency of finding hosts, especially in the case of specialized parasitoids. Generalist predator populations can be stabilized thanks to the availability of pollen and alternative prey, but the effectiveness of predators still depends on whether they respond quickly enough, either by aggregation or multiplication, to outbreaks of the target pest. Therefore, plant diversification or other methods to supplement the nutrition of natural enemies must be carried out with knowledge of the behavior and biology of the natural enemy and the pest.
The seasonal inoculatory release of insect parasites and predators has been a highly successful strategy for biological control in greenhouses in Europe. Producers adopted this strategy because of the prevalence of insecticide resistance in many greenhouse pests and the increased costs of chemical control. The program was originally created around the use of the parasitoid Encarsia formosa against the whitefly in the greenhouse and the predatory mite Phytoseiulus persimilis against the two-spotted spider mite. Over the years, additional natural enemies have been added to control other pests, such as thrips, leafminers, aphids, caterpillars and other species of whiteflies, as needed. The costs of using biological control are now much lower in Europe than those of chemical control of insect pests.
Producers receive information on the details of the implementation of the program, new developments and new natural enemies through a network of extension advisors, specialized magazines and producer study groups. Beneficial insects can be broadly classified as predators or parasites. Insect predators, both adult and immature, actively seek out and consume multiple prey. They tend to be (but not always) larger than their prey. Predators include ladybugs, green lacewings, and damsel bugs.
Parasitic insects (called parasitoids) develop in or on a single host from eggs or larvae deposited by the adult parasitoid. Common parasitoids include tachinid flies and many types of wasps. Suppression is a common goal in many pest situations. The intention is to reduce the number of pests to a level where the damage they cause is acceptable. Once the presence of a pest is detected and a decision is made that it needs to be controlled, suppression and prevention are often joint objectives.
The right combination of control measures can often eliminate existing pests and prevent them from accumulating again to a level where they cause unacceptable damage. For example, reduced tillage favors benefits, but it also contributes to the infestation of pests such as the common stem borer and the European corn borer in corn. Continuous pests are almost always present and require regular control; sporadic pests are migratory, cyclical or other occasional pests that require control from time to time, but not on a regular basis; potential pests are organisms that are not pests under normal conditions, but that can become pests and require control under certain circumstances. Using resistant types, when available, helps to keep pest populations below harmful levels by making conditions less favorable to pests. Natural enemies are then carefully released, taking into account the right time in the life cycles of the enemy and the plague, in a place where the target pest is abundant and where the disruption of the newly released enemies is minimized. While importing new natural enemies is important for farmers, gardeners, and others who practice pest control, the scope of successful introduction projects (involving considerable experience, exploration abroad, quarantine, mass breeding and persistence despite many failures) is so great that only government agencies typically undertake such efforts. Thresholds are the levels of the pest population at which you must take pest control measures if you want to prevent pests in an area from causing unacceptable injury or damage.
Many of the most common predators in fruit production systems attack a wide range of pest species and help regulate pest population densities. Relying solely on pesticides for pest control can cause pests to develop resistance to pesticides, cause outbreaks of other pests, and damage surfaces or organisms that are not the target. The fact that the pesticide was unable to control the pest could be due to the resistance of the pest, the choice of the wrong pesticide, the misidentification of the pest, the application of the wrong amount, or the wrong application of the pesticide. The need for pesticides can be reduced by the use of resistant varieties, cultivation methods that reduce pest abundance or damage, methods for manipulating pest behavior to mate or find hosts and, in some cases, physical methods of control. The successful use of natural enemies in pest management requires a detailed understanding of insect biology and pest management techniques. Competitors are often overlooked in discussions about natural enemies, perhaps because many competitors to common crop pests are also pests in and of themselves.
Damsel bugs (Nabis species) are common in gardens and crops, where they feed on aphids and many other pests. As a pest comes under biological control, the population density of the pest and the biological control agent decreases because the host-specific natural enemies cannot hunt or reproduce in other species. Sometimes, applying a pesticide fails to control a pest because the pest was not correctly identified and the wrong pesticide was chosen. The more you know about the pest and the factors that influence its development and spread, the easier, more cost-effective and successful your pest control will be.