B is for biotype, and very bad bug

A nasty sort of whitefly seems bent on world domination, says David Spark
The new strain of a virus-transmitting whitefly that wreaked havoc among California's vegetables in 1991 is spreading further and further round the world. It is rife among tomatoes in Spain. It has reached Australia and threatens Asia, Africa and South America.

The B biotype, as it has been termed, of the tropical whitefly Bemisia tabaci resists insecticides, multiplies five times as fast as other biotypes, transmits up to 60 viruses and can colonise about 600 different crops and weed species.

The British team that studies whiteflies on a world scale is, however, starved of funds to combat the pest. "We are ideally sited to work on tropical problems such as this," says Dr Peter Markham at the John Innes Centre, in Norwich. "We are far enough north to work safely. But we can't get funding."

One reason is that the B biotype is not a British problem, though it presents a potential threat to cabbages, potatoes, tomatoes and French beans grown in other parts of Europe. The Ministry of Agriculture requires that plants brought into Britain be certified free of it.

Peter Markham is applying through the International Cotton Advisory Committee to the Common Fund for Commodities for money to design a cotton resistant to leaf curl virus or its transmission. This is carried by a different, but also aggressive, form of Bemisia tabaci in Pakistan. It has just devastated a million hectares of cotton there.

Here Dr Markham runs into another problem. An American firm, Agracetus, interested in producing better and also coloured cotton, has taken out a patent covering any cotton changed by genetic engineering. The US Patent Office wants to revoke this wide patent but it remains in force in the United States and India. Patent applications are pending in two other big producers, Brazil and China. Agricetus makes money by licensing, but licences can be costly and restrictive.

Whiteflies are tiny, moth-like creatures whose scale-like young feed on plants. Dr Markham and his colleague Ian Bedford have shown that, until the Eighties, each part of the tropical and sub-tropical world had its own type of Bemisia tabaci. This fed on a limited range of plants. Then the less choosy B biotype appeared in the Caribbean, followed by Central America, Florida and California. In the initial attack, California and other southern US states suffered $500m (£316m) losses of vegetables. The flies have also invaded the glasshouses of the Netherlands.

Travelling in their millions, sometimes like a cloud of smoke, they attack a wide range of vegetables and crops, spreading virus diseases that curl the plants' leaves and stunt their growth. Viruses used to present no problem to vegetables of the cabbage family. They do now.

The saliva of the B biotype poisons some plants. Marrow and squash leaves turn silver. As well as suffering from leaf curl, tomatoes ripen unevenly and become pithy and unfit for sale. The Dominican Republic lost nearly 20,000 hectares in a season.

Honduras has suffered heavy losses of cotton, tomatoes and beans, important for the local poor. California, despite its resources for pest control, is still losing $130m of crops, including 4 million acres of melons, per year.

The B biotype's wide range of food plants means that, when one crop dies, it need not die along with it; it can move to another or to a weed.

One question is what will happen when it reaches parts of Africa and Asia populated by other biotypes. In California, the local biotype has now disappeared.

Silverleaf, a characteristic of the B biotype, was spotted in Israel in 1967, and so the B seems to come from the Middle East. Scientists believe that heavy use of pesticides gave an advantage to a biotype which was pesticide-resistant and could multiply fast.

The trade in ornamental plants attractive to the B biotype then provided a means of spreading it to the Americas, Europe, Japan and Australia. A tiny scale can be hard to detect on a plant leaf.

Elsewhere, again under pesticide pressure, local whiteflies have become pesticide-resistant and more fecund. Cassava, a staple food throughout Africa, has been badly damaged in Uganda by mosaic virus, as a result of a new surge of whitefly activity.

In Pakistan, whitefly used to be a late-season pest making cotton sticky with honeydew. By the mid-Eighties, however, it was present throughout the year, including the hot early season: and whitefly transmits virus more efficiently in the heat.

The first important outbreak of leaf curl virus took place at Multan in 1988, when Pakistan introduced a cotton variety susceptible to it. Since then, the virus has spread through the Punjab where small farmers produce most of the cotton on which two-thirds of Pakistan's exports depend.

Peter Markham says: "The misuse of insecticides over 40 years has encouraged supertypes of whitefly which are now uncontrollable."

In Britain, the glasshouse whitefly, which is a different species, is controlled by a parasitic wasp whose larvae grow in whitefly scales and hatch more wasps to attack more scales. Unfortunately, these wasp larvae do not always mature in scales of the tropical whitefly. So there are too few new-generation wasps.

The hunt goes on worldwide for possible predators of the B biotype. If scientists could be sure where it evolved, they might be more successful in this search. The problem remains, however, that only a few whitefly remaining alive can spread viruses through a whole field. This is why Dr Markham wants to develop virus-resistant cotton.

He sees it as part of an integrated pest management strategy, using as little chemical as possible and encouraging natural enemies of cotton pests. It would enable farmers to target whichever pest got out of control without having to worry about eliminating whitefly completely, an almost impossible task. Whitefly is only one of five major pests of cotton.

Although integrated pest management is a fashionable idea, many experts, Dr Markham believes, lack the will to use the whole armoury of control measures. They are wedded to one approach or another.

Rice pests have been controlled in Indonesia by stopping the use of insecticides and encouraging insect-eating spiders. But what works in one place will not necessarily work elsewhere. "A lot more research is needed," says Dr Markham.