Tobacco whitefly
Last years it is observed the fast spreading of Bemisia tabaci in the world. Wide genetic flexibility, polyphagia and the high speed of reproduction make it one of the most harmful pests. The tobacco whitefly injures more than 500 plant species from 63 families. Amidst these plants are tomatoes, cucumbers, eggplants and flower-ornamental crops. The pest is also an agent of dangerous plant viruses.
Biology. Life cycle.
The cycle of pest individual development consists of 6 stages: an egg, 4 larva stages and an imago (adult). The tobacco whitefly lays eggs on a leaf underside. The egg is oval-shaped, conical to a distal end. The length of the egg is approximately 0.21 mm. Its width is 0.096 mm. The eggs are nacreous. They darken during development. The egg is attached to a leaf by means of a stemlet (0.024 mm). The female can maximally lay 300 eggs. But its average fertility is considerably less and amounts to 50–70 eggs. The larva hatching from the egg is oval, flat and motile. It has 6 pairs of legs, antennae, pair of eye spots and hairs on the edges of a body. During several hours it creeps on a plant then sticks to the underside of the leaf. Other 2–4 larva stages are not motile. They have rudimental antennae and legs. The larvae are oval-shaped, green-brown. Their length varies from 0.365 mm (the second stage) to 0.662 (the forth stage). In the 4th age the larva becomes prominent and attaches tightly to substratum with a medial part, but the edges remain free. This feature distinguishes Bemisia tabaci from other species of whiteflies. Under this cover, which is called a puparium, the larva ceases to feed and undergoes some changes. After that it is transformed into a winged imago with long antennae and legs, and well developed eyes. The adult whiteflies live at 25°C on an average 14.8 days. (from 1 to 29).
Mode of life
The optimal temperature for Bemicia development is by 3–4°C higher than for the glasshouse whitefly (Trialeurodes vaporariorum). Under the optimal temperature (26.7+1°C) the development from an egg to an imago lasts on an average from 18.6 (on cucumbers) to 29.8 (on tomatoes). In sex ratio females prevail over males in proportion 2:1. In the conditions of glass-covered ground the pest is active during all the year and able to reproduce 8–15 generation. In greenhouses the tobacco whitefly crowds out the glasshouse whitefly because it more flexible and resistant under high temperatures. Bemicia tabaci overwinters in the puparim on fallen leaves. It is able to migrate (drifting flight) at a long distance (several kilometers). The pest prefers to colonize young leaves in shady and wet places. Bemicia tabaci is attracted by a yellow color, but doesn't respond to smells.
Symptoms of injury
Colonizing leaves the pest oppresses growth processes very much and decreases yield. Sooty fungi settle on the sugary excretions of Bemisia tabaci, and due to this process they oppress plants much more. As a result of intensive pest activity merchantable production loses its qualities. The tobacco whitefly is also dangerous because of its ability to be an agent of the virus diseases of vegetable and flower crops.
Polyphagy, the high speed of colonization, the transferring of plant viruses, the resistance to high temperatures and pesticides make this species dangerous and difficulty suppressed.
