Tobacco whitefly

General

In a nutshell: Huge range of host plants; sap suckers—excrete sticky “honeydew”, cause leaves to yellow and fruit to ripen unevenly; symptoms can appear even with very low populations; resistant to many pesticides; notorious viral disease vector.

What are tobacco whiteflies?

The tobacco whitefly (Bemisia tabaci) was first described as a pest of tobacco (hence the name) in the late 19th century. The insect has spread worldwide since then and now exists in most tropical and subtropical countries. This species of whitefly is especially feared because it's resistant to many pesticides and transmits many plant diseases. Bemisia tabaci actually consists of at least 24 genetically distinct populations. Experts disagree about whether it’s a single species with several “biotypes” or several different but visually-indistinguishable species.

What kinds of plants do they attack?

Tobacco whiteflies attack a huge range of different plants, but it’s an especially serious pest of tomato and cucumber.

What do they look like?

Tobacco whitefly females are a little over 1 mm in length and the males are slightly smaller. Adults of both sexes have wings set flat against the back that extend past the tail end of the insect’s body. The wings are transparent in newly emerged adults, but within a few hours they become covered with a waxy, yellowish-white powder, as does the body.

Females of this species lay their eggs widely on the underside of leaves over the entire plant. As a result, different life-cycle stages occur together on the same leaf. The eggs are oval shaped and yellow-green when first laid, then they gradually turn a light brown.

During the first larval stage, the tobacco whitefly is about 0.25 mm long, oval shaped and has well-developed legs and antennae. After they hatch, they spend several hours looking for a good place on the leaf to feed. Once they’ve found a nice spot, they stay there for the rest of their larval development. In the second and third stages, the larvae get progressively bigger. They’re flattened on the leaf and have legs and antennae reduced so much that they’re no longer visible. The insects are also virtually transparent at these stages and very difficult to detect.

Tobacco whiteflies in the pupal stage are flat, almost circular and about 0.8 mm in size. The pupa can sometimes have an irregular outline, depending on the leaf structure of the plant. It’s also transparent, and the red eyes and yellowish colour of the developing whitefly inside are visible.

How do I know if my plants are under attack?

Tobacco whiteflies are sap suckers. Similarly to aphids, whitefly larvae consume large amounts of sap. They excrete the excess as a "honeydew" on the leaves.

This honeydew encourages the growth of black fungal moulds on the plant and its fruit. The mould reduces photosynthesis and moisture regulation in the leaves and, in serious cases, can rot fruit.

If the whitefly population is large, the plant won't grow well, and in full sunlight the leaves can wilt and fall off. This in turn causes misshapen fruit, lowers the plant's yield, and ruins the appearance of ornamentals.

Tobacco whitefly larvae inject enzymes into the plant that change its biological processes. Even with low populations, this can cause damage such as irregular fruit ripening (tomatoes and peppers), yellowing flower stalks (gerbera), and severe yellowing of leaves (beans). An infestation can even cause widespread leaf and fruit drop. In various ornamental plants (bouvardia, zinnia) the veins of young leaves turn yellow, leading to “mosaic vein”.

This whitefly is a much feared carrier of over 25 different viruses and many other virus-like diseases. One of the most important of these is Tomato Yellow Leaf Curl Virus (TYLCV).