What are whiteflies?
Whiteflies are a group of small, winged insects that belong to the family Aleyrodidae. They are called "whiteflies" because they are typically white or yellowish in color, and they are often found on the undersides of leaves.
Whiteflies are not true flies but belong to the order Hemiptera (true bugs). Together with aphids and scale insects, they belong to the division Sternorrhyncha. The most common species are the greenhouse whitefly, Trialeurodes vaporariorum, and the tobacco whitefly, Bemisia tabaci. Both are widespread, polyphagous and are broadly comparable in outline.
Whitefly can be found in many different environments, but they are particularly common in greenhouse settings and in agricultural fields. They feed on the sap of plants, which can cause damage to the leaves and stunting of growth. They can also transmit plant viruses from one plant to another.
Life cycle of whitefly
Whiteflies have six distinct developmental stages: the egg, first, second, third and fourth larval instars, and an adult insect.
The eggs are elliptical and often deposited in circles. The young larvae, which on hatching already have well-developed legs and antennae, are known as, ‘crawlers’. After hatching they are actively engaged for several hours in seeking a suitable place on the leaf where they can feed. Once they have found such a spot, they remain there throughout the period of larval development. In the second and third larval stages, both the legs and the antennae are reduced to one or two segments and are no longer visible. In the fourth larval stage, the insect finally becomes flatter, then subsequently fatter, and the cuticle hardens.
The growth of hairs on the insect depends on the leaf: the longer the hairs on the leaf, the longer the hairs on the insect. In other words, the larva adapts itself to the structure of the leaf on which it lives. On very hairy leaves, the larva can sometimes become deformed if the growth is obstructed by the stiff hairs. As soon as the red eyes of the adult whitefly are visible in the fourth larval instar, the stage is referred to as a ‘pupa’. The different species of whitefly can be most easily distinguished from each other at the pupal stage.
The adult emerges from the hardened cuticle of the pupa through a characteristically shaped opening; the pupa splits on the upper surface along pre-existing seams, to create a T-shaped opening. Newly emerged whiteflies have two pairs of transparent wings; later these two pairs of wings and the body are covered with a white, waxy powder that gives the insect its characteristic appearance. Adult whiteflies are mostly found on the underside of young leaves where they lay their eggs. If the plant is shaken, the adults fly up, and then return to the undersides of leaves.
Whiteflies can be identified by the following characteristics:
- Size and shape: Whiteflies are small, winged insects that are typically about 1-2 mm in length. They have a distinctive, triangular shape and hold their wings in a vertical position over their body when at rest.
- Color: As their name suggests, whiteflies are usually white or pale yellow in color. This is due to the wax produced by the adults, which is covering their body and wings. Some species have dark markings on their wings or bodies.
- Movement: Whiteflies are weak fliers and tend to flutter around plants rather than fly in a straight line. They are often found on the undersides of leaves, where they feed on plant sap.
- Damage: Whiteflies can cause damage to leaves by feeding on plant sap. This can result in yellowing, wilting, or premature leaf drop.
If you suspect that you have a whitefly infestation, it's important to confirm their presence and take appropriate measures to control their population.
The damage caused by whiteflies can be recognized by several visible signs. Here are some of the common signs of whitefly damage:
- Yellowing of leaves: Whiteflies feed on plant sap, which can cause the leaves of infested plants to turn yellow and eventually drop off. This yellowing is often more pronounced on the lower leaves of the plant.
- Stunted growth: Whitefly infestations can also cause plants to grow more slowly or become stunted. This is because the insects are drawing nutrients away from the plant as they feed on its sap.
- Sticky residue: Whiteflies excrete a sugary substance called honeydew, which can coat the leaves and stems of infested plants.
- Sooty mold: Honeydew can also promote the growth of a black, sooty mold that can cover the leaves and stems of infested plants. This mold can further reduce the plant's ability to photosynthesize and grow.
- Wilting or death of plant: In severe cases, whitefly infestations can cause plants to wilt and die. This can occur when the insects are present in very large numbers and are feeding heavily on the plant.
There are many different species of whiteflies, but some of the most common include the greenhouse whitefly (Trialeurodes vaporariorum) and the tobacco whitefly (Bemisia tabaci).
Greenhouse whitefly (Trialeurodes vaporariorum): This species is one of the most widespread and damaging whitefly pests. As the name suggests, it is commonly found in greenhouse environments, where it feeds on a wide range of crops, including tomatoes, cucumbers, and peppers. The greenhouse whitefly is a small, white insect with short antennae and a waxy appearance.
Tobacco whitefly (Bemisia tabaci): This species is a major pest of many crops, including cotton, vegetables, and ornamental plants. It is commonly found in warm climates, and can cause significant damage to crops by feeding on plant sap and transmitting viruses. The tobacco whitefly is a small, white insect with long antennae and a slightly yellowish tint. This species is also called silverleaf whitefly (Bemisia argentifolii), but this name actually refers to the same species.
How to prevent whitefly
To prevent whitefly infestations in crops, there are several measures you can take:
Implement good crop management practices
Maintain plant health by providing proper irrigation, nutrition, and adequate spacing between plants. Healthy plants are less susceptible to whitefly infestations.
Practice strict hygiene
Keep the growing area clean and free from debris, weeds, and plant residues that can serve as reservoirs for whiteflies. Regularly remove and dispose of any infested plant material.
Regularly inspect your crops for signs of whiteflies, including adult insects, nymphs, or their characteristic sticky honeydew. To aid in monitoring, strategically position sticky traps throughout the crop to capture adult whiteflies and provide insight into population levels. Early detection allows for prompt action and prevents the infestation from spreading.
Use physical barriers in greenhouses
Install insect-proof screens on openings such as vents and doors to prevent whiteflies from entering the crop area. This helps create a physical barrier and reduces the chances of infestation.
Introduce biological control agents
Utilize natural enemies of whiteflies such as parasitic wasps, predatory mites, predatory bugs, or entomopathogenic fungi. These beneficial organisms can help control whitefly populations by feeding on them or their eggs.
Rotate crops and plant diversity – outdoor crops
Avoid continuous cropping of susceptible plant species. Rotate crops to disrupt whitefly life cycles and reduce the buildup of infestations. Additionally, planting a diverse range of crops can create an environment less favorable for whiteflies.
Biological control of whitefly
There are several natural enemies that can be used for the biological control of whitefly, including parasitic wasps, bugs and predatory mites. These beneficial insects and mites feed or parasitize on whitefly at different stages of their life cycle, reducing populations and preventing further damage to crops.
The parasitic wasps Encarsia formosa (En-Strip) and Eretmocerus eremicus (Ercal) are commonly used for controlling whitefly populations in greenhouse crops. The female wasps lay eggs inside the whitefly nymphs, where the developing wasp larva consumes the host from the inside out, ultimately killing the whitefly.
Similarly, predatory mites such as Amblyseius swirskii (Swirski-Mite, Swirski-Mite LD, Swirski-Mite Plus, Swirski Ulti-Mite), Amblydromalus limonicus (Limonica) and Amblyseius andersoni (Anso-Mite, Anso-Mite Plus) are also effective natural enemies of whitefly. These predatory mites feed on whitefly eggs and nymphs, and can significantly reduce the populations of whitefly in a short period of time. These predatory mites cannot be used in tomato as they get stuck in the sticky stuff the glandular hairs of the tomato produce.
In tomato the predatory bug Macrolophus pygmaeus (Mirical) is frequently used for the control of whitefly.
Entomopathogenic fungi, such as Lecanicillium muscarium (Mycotal) are also effective for biological control of whitefly. These fungi infect and kill whitefly by penetrating their cuticle and growing inside the insect, eventually causing death.