What are Whiteflies?
Whiteflies are one of the most damaging insect pests of many greenhouse and field vegetables. They are about 2-3 mm long soft bodied insects with yellowish body, white colored wings and piercing and sucking types of mouthparts. Whiteflies use these specialized mouthparts for sucking cell sap from leaf tissues. While feeding, whiteflies develop through four developmental stages including eggs, nymphs (immature stages), pupae and adults (Photo 1). Whiteflies complete their life cycle within 3-5 weeks and can complete several overlapping generations during the growing season of host crops.
Photo 1. Whitefly adults feeding on the lower surface of leaves
Whitefly Damage
Whiteflies generally cause direct and indirect damage to their host plants including vegetables, ornamentals, fruits and field crops.
Direct damage: Direct damage is caused when both the adults and nymphs (larval stages) of whiteflies actually feed on the leaf tissues and other succulent parts of their host plants. Whiteflies use their piercing and sucking types of mouthparts to suck the cell sap of plant tissues. Symptoms caused by direct feeding of whiteflies include poor plant growth, yellowing and drying of young leaves (Photo 2). As whitefly damage progresses, infested young leaves dry and fall off from the plants that leads to premature death of plants (Photo 2).
Photo 2. Direct feeding damage by whiteflies can cause yellowing and drying of leaves and plants.
Indirect damage: Indirect damage is caused when whiteflies secrete honeydew that promotes the growth of black sooty mold on the surface of infested leaves. As population of whiteflies increases, growing black sooty mold covers the whole surface of a leaf (Photo 3) and badly affects photosynthesis, a process used by plants to make their own food by converting light energy from sun to chemical energy. This black sooty mold also reduces the quality of the produce and aesthetic value of many ornamental plants. In addition, whiteflies can transmit different types of plant viruses that cause diseases like tomato yellow leaf curl virus in tomato and other vegetables. These transmitted virus diseases can cause tremendous economic loss to the agricultural, horticultural and greenhouse crops.
Photo 3. Leaf showing growth of black sooty mold on its lower surface. of lIndirect damage is caused when whiteflies secrete honeydew that promotes the growth of black sooty mold on the surface of infested leaves.
Organic control of whiteflies with beneficial nematodes
Beneficial nematodes like Steinernema feltiae are small microscopic roundworms (Photo 4) that are currently used as biological control agents for targeting and killing mostly soil dwelling stages like larvae (caterpillars or grubs) and pupae of different insects pests such as white grubs, cutworms, armyworms or weevils. Since beneficial nematodes require a film of water for their free movement, they are generally applied to the surface of soil that is often irrigated before and after application of nematodes to maintain optimum moiture level for easy entery and free movement nematodes in the moist soil for searching for their host larvae or pupae. After locating their host, nematodes enter into host’s body cavity via natural openings like anus, breathing pores and mouth and release species-specific symbiotic bacteria from their gut in the insect blood. In the blood, bacteria multiply quickly, cause septicemia and kill larvae or pupae of their host within 48 hours. This reduces future emergence of adult insects thus controlling insect pests.
In contrast to above stated insect pests, whiteflies complete all of their life stages such as eggs, nymphs, pupae and adults only on the surface of leaves or stems. So if you want to use beneficial nematodes for the control of whiteflies then they have to be applied on the aboveground plant parts such as leaves and stems. However, beneficial nematodes are avoided to apply to the foliage for targeting foliage-feeding tiny insects like whiteflies because they are very susceptible to UV light, desiccation and hot temperature that can kill them instantly. Recently, it has been demonstrated that the beneficial nematodes like Steinernema feltiae can be used for the effective killing of whiteflies if they are applied with wetting agents (adjuvants) that allow nematodes to move freely on the surface of leaves to find, infect and kill larvae/nymphs of whiteflies. Wetting agents also reduce direct exposure of nematodes to desiccation and UV light enhancing their chances of survial and finding their host on the surface of foliage.
Photo 4. Beneficial Steinernema feltiae can kill over 80% whiteflies.
how to apply beneficial Steinernema feltiae nematodes to control whiteflies?
As stated above, Steinernema feltiae nematodes will be very effective against whiteflies if they are sprayed at the rate of 10,000 nematodes per ml of 1% solution of horticultural oil (which is not toxic to nematodes or humansor animals) as adjuvant on the foliage. So for application of nematodes, make 1% solution of horticultural oil in water (99 parts of water + 1 part of horticultural oil). Then open nematode package and mix thoroughly nematodes in the 1% solution of horticultural oil using a small stick. Adjust nematode concentration to desired rate by adding more 1% solution of horticultural oil. Then use a hand sprayer or simple watering can (Photo 5) for spraying nematode suspension on the whitefly infested foliage. If Steinernema feltiae are applied at the rate of 10,000 nematodes per mililiter of 1% horticulture solution on the whitefly infested soil, they can provide over 80% of white fly control. For more information on the beneficial entomopathogenic nematodes, visit bugsforgrowers.com
Photo 5. Watering can is easy to use for spraying of beneficial Steinernema feltiae nematodes on the foliage.