Silverleaf Whitefly - Integrated Pest Management - Biological Controls - Natural Enemies

Natural Enemies

Natural enemies are highly effective as biological controls. Parasitoids, predators, and pathogens species of the whitefly keep the population under control. The four Old World species of Eretmocerus (Eretmocerus mundus, Eretmocerus hayati and Eretmocerus emiratus) established in the Western United States are a group of genetic individuals of related taxa that are parasitoids of B. tabaci and serve as biological control agents. Scientists are considering the idea of releasing these parasites in order to be able to control their host’s population growth and save the destruction of important crops. Consequently, not all Eretmocerus can be successfully transplanted into areas where the whiteflies are present due to differences in climate preference. For example, the species Eretmocerus melanoscutus failed to establish in the western United States due to climate issues. Therefore one of the important factors that are involved in successfully implementing specialist parasitoids is the climate. Goolsby (2005) mentions that different species of Eretmocerus are matched with the climate they are able to survive in. The success of the species of Eretmocerus in the USA can be contributed to the smaller host range, better climatic adaptation, and higher attack rate. Other natural species of the B. tabaci include several species of wasps, bigeyed bugs, lacewing larvae, and lady beetle larvae, which all prey on the nymphs of the whitefly.

There are eight different arthropod orders that attack B. tabaci. These include members of the families Phytoseiidae, Coccinellidae, Syrphidae, Anthocoridae, Nabidae, and Miridae, Chrysopidae and Coniopterygidae. There are currently four species that are commercially available; they include Delphastus pusillus, Macrolophus caliginosus, C. carnea, and Chrysoperla rufilabris. D. pusillus are a small, shiny black beetles species that suck-out the inside contents of the silverleaf whitefly by piercing its shell. Adults and larval stages of this beetle agent feed at all life stages of the pest. C. rufilabris is only able to feed on the immature stages or the larval stages of B. tabaci.

Another natural mechanism of controlling the population of B. tabaci is the use of fungal pathogens. The most commonly known pathogens to the whitefly pest are Paecilomyces fumosoroseus, Aschersonia aleyrodis, Verticillium lecanii, and Beauveria bassiana. When spore solutions of V. lecanii are sprayed on eggs, first, second, and third instar nymphs of B. tabaci approximately 89% to 90% of these eggs are killed. There has been resistant traits developed in the whitefly to its fungal pathogens. For example, whitefly pests have grown resistant to infection by V. lecanii.

B. bassiana is only an effective biological control agent at a maximum temperature of 20°C and a humidity level greater than 96%. Not enough studies have been conducted to show the productiveness of fungal pathogen in the real world environment. Much of the success of this biological control on B. tabaci has been conducted in the laboratory. However, it can be concluded though that when the fungal pathogen is combined with an insecticide, the synergistic effect of the two will induce a higher mortality rate of the whitefly. P. fumosoroseus has a broad host range but can attack silverleaf whiteflies at a variety of life stages and these include eggs, nymphs, pupae, and adults stages. On the other hand, A. aleyrodis only infects and destroys nymphs and pupae.

Another natural enemy of the whitefly are parasitoids, which kill their host once their development has been completed. There are three hymenopterous families to which whitefly parasitoids are affiliated with. These families are Platygasteridae, Aphelinidae, and the Eulophidae. The best studied of these whitefly parasitoids are E. formosa and Eretmocerus eremicus, both of which are commercially available. The two most common and well studied parasitoids specific to the silverleaf whitefly include Encarsia formosa ("Beltsville Strain") and Eretmocerus sp. E. formosa "Beltsville Strain" however, is unable to control B. tabaci biotype B in commercial greenhouses; it is only able to control the species in small experimental greenhouses. In an experiment done by the Hoddle laboratory, the release of three or more E. formosa on B. tabaci per week failed to control the pure population of the species on poinsettia plants because wasps that are reared in the B. tabaci are less fecund, have a slower development, and fail to allow immature parasitoids to survive and develop. The species E. formosa works much better at controlling the whitefly species T. vaporariorum rather than B. tabaci. On the other hand, Eretmocerus sp is much better at controlling species of silverleaf whitefly rather than E. formosa "Beltsville Strain." In an experiment done again by the Hoddle laboratory, the release of three female wasps of the species Eretmocerus sp. were able to effectively eliminate patches of the fly nymphs right after discovery. The parasitoid is faster at searching for patches of nymphs of its host species and is consistent with controlling the population. Eretmocerus sp. are bi-parental ecto-endoparasites meaning that parents lay their eggs on the outside of the fly, as the larvae grows it penetrates the fly and continues its growth and development inside the host. Plants growers today have been successfully able to control the population of B. tabaci by using the variable release strategy. In the variable release strategy, six female parasitoids are released per week for the first half of the growing season, while only one female is released per week for the remaining of the season. Thus, the effectiveness of the parasitoid can be improved by releasing varying amounts of the parasitoids per week so that search rate is low. If natural enemies are not able to control the pest population at low levels due to a significant increase in pest, an insecticide compatible with the biological control agent should be used to help keep the population at low levels again.