Aphids and Their Biocontrol
Abstract Aphids are small, soft bodied insects that are common pests of nearly all indoor and outdoor field crops, vegetables, fruit trees, and ornamental plants. There are different species of aphids, some of which attack only one host plant, while others attack numerous host plants. As well as causing direct damage by sucking sap and stunting growth and development, essentially reflecting yield parameters, they also act as potential vectors of plant viruses. The honeydew excreted by them occludes the stomatal openings of the leaves, hampering photosynthesis and respiration, and also favors the growth of black mold. Their prolific breeding, polyphagy, advanced degree of polymorphism, anholocyclic and/or holocyclic reproduction, parthenogenesis and telescopic generation, host alternation, and polyvoltinism make them a notorious pest. Concerns about the risks that chemical pesticides pose to the environment and human health, as well as their increased costs, have increased the need for more research into nonchemical methods of crop protection. In nature, there are several organisms that feed, parasitize, or infect aphids causing heavy mortality. Among those that regulate their populations are their parasitoids and predators that are commonly used in biocontrol programs in greenhouses and fields. The majority of aphid parasitoids belong to the subfamily Aphidiinae (Hymenoptera: Braconidae) and a few species to Diptera. Aphid parasitoids have enormous potential that can be used in regulating the aphid population both in glasshouses and open fields. At present, a number of parasitic species are utilized in biocontrol of aphids. A dozen species are commercially propagated and traded throughout the world, more intensely in European countries and the United States. The constraints of biocontrol of aphids are multifold. The parasitoids are killed by their natural enemies (hyperparasitism and intraguild predation) The biggest obstacle to the use of parasitoids in aphid control is their mass propagation at an affordable cost. Unless natural enemies are made as readily available, biocontrol is likely to be a subject of academic interest with no practical role whatsoever. The potential for mass rearing of parasitoids is bright because new technologies are being developed to produce both aphids and parasitoids using artificial media. However, there is a need to consider the trade off between producing and storing large quantities of parasitoids at low costs and the overall quality of the individuals that are released in crops. Measuring the fitness of mass produced individuals remains a challenge, and such a measure is likely to change depending on the attributes of the aphid species present and parasitoids that are released. Quantifying these parameters should increase the reliability of biocontrol programs and contribute to their acceptance by growers.