What are whiteflies?
Greenhouse whiteflies (Trialeurodes vaporariorum) are tiny insects with yellowish bodies and whitish wings (Photo 1). Whiteflies have piercing and sucking types of mouthparts that they use for sucking cell sap mainly from the underside of the leaves. The females lay over 200 eggs on the lower surface of leaves. Eggs hatch within 7 days into pale colored and wingless nymphs that develop through four developmental stages (instars). The first stage individuals are called ‘crawlers’ because they can crawl on plant surfaces with their well-developed legs. These crawlers use their antenna for locating permanent feeding sites on the underside of leaves. After locating a suitable feeding site, crawlers enter into the second stage nymphs that are flatten and legless. These immobile nymphs then attach themselves to the selected feeding site by inserting their mouthparts into leaf tissue where they continuously feed for about 3-4 weeks. While feeding, nymphs molt twice into the third and fourth stages (instars) and then form pupae. After 7-10 days, adult whiteflies emerge from pupae and the life cycle continues. Although adult whiteflies live only for a month, they can complete several generations during the growing season of host crops. There are several species of whiteflies exist in the nature.
Photo 1. Greenhouse whiteflies
Damage caused by whiteflies
Whiteflies cause both direct and indirect damage to their host crops like vegetables, ornamental plants, fruits and several field crops.
Direct damage
Adult and nymphal stages of whiteflies use their piercing and sucking types of mouthparts to suck cell sap from the underside of leaves of their host plants. This type of feeding generally causes symptoms like yellowing (Photo 1), drying and prematurely falling off leaves from the plants. Also, plants that are heavily infested with whiteflies generally look unhealthy and eventually die off prematurely.
Indirect damage
While feeding, whiteflies secrete honeydew that stimulates the growth of black sooty mold on the surface of leaves. This black sooty mold covers the whole leaf and severely affects photosynthesis, a process used by plants to convert light energy from sun to chemical energy for the synthesis of their own food. This black sooty mold also reduces the quality of the produce and aesthetic value of many ornamental plants. In addition, whiteflies are known to transmit different types of diseases causing plant viruses, which are responsible for causing economic loss to the agricultural, horticultural and greenhouse industries.
Control of whiteflies with Amblyseius swirskii
Amblyseius swirskii are pear-shaped tiny predatory mites (Photo 2) with four pairs of legs. Both adults and nymphs of these mites feed on the different stages of whiteflies. These mites are ideal to use for the biological control of both whiteflies because the optimum temperature [between 20°C (68°F) and 29°C (84.2°F)] required for the normal reproduction and development of Amblyseius swirskii is always maintained in the greenhouses for the optimum growth of plants.
Photo 2. An adult of predatory mite Amblyseius swirskii
How predatory mites, Amblyseius swirskii can control whiteflies?
Amblyseius swirskii mites are commercially available as mixed stages of adults and larvae and are currently used as a biological control agent for controlling whiteflies, which are serious pests of greenhouse plants. Both the larvae and adults of Amblyseius swirskii are very active and therefore they can quickly disperse after their application in the greenhouses to seek their hosts including whiteflies. Amblyseius swirskii mites are known to feed voraciously on all the stages of whiteflies. As these mites can eat over 10 whiteflies or eggs or nymphs per day, they can eliminate whitefly populations quickly in the greenhouses.
How many predatory mites should be released?
For the effective control of whiteflies, release over 5000 predatory mites per acre 2-3 times on a bi-weekly basis. Predatory mites can be released as a preventive measure before the occurrence of pests or as a curative measure after the incidence of whiteflies. These warm adapted predatory mites perform better when they are released at optimum temperature mentioned above. The activity of Amblyseius swirskii mites will be reduced if they are released under cooler conditions and they will not survive if released under really cold and frosty temperatures.