Alate production by aphids on sugar beet: how likely is the evolution of sugar beet-specific biotypes?
Aphid clones that feed on the same plant species for many generations often develop biotypes specifically adapted to the particular host-plant. Such biotypes can be particularly damaging to crops. In the case of sugar beet Beta vulgaris, which is not available for colonization for parts of the year, aphids must migrate into and out of the crop each year if host-specific biotypes are to evolve. The likelihood of biotypes evolving to be specifically adapted to sugar beet was examined and the results are discussed in relation to the control of aphids on this crop. The frequency and causes of alate production by aphids on sugar beet were investigated by examining natural field populations, manipulated field populations (in the UK in 1978-81) and laboratory populations. Data from these studies were used in a model that estimated the probability of an aphid clone on sugar beet successfully migrating from the crop, surviving the winter and migrating back on to the crop the following year. In 2 years out of 4, no alatoid nymphs of the aphids Myzus persicae and Macrosiphum euphorbiae were produced on naturally infested crops of sugar beet, and negligible numbers were produced in the other 2 years. In spring, a much greater proportion of the Myzus persicae population was alatoid on oilseed rape Brassica napus than on sugar beet. Similarly in laboratory studies, a greater proportion of Myzus persicae was alatoid on Chinese cabbage Brassica pekinensis than on sugar beet at the same levels of crowding. The colour of the alatoid nymphs reared on Chinese cabbage indicated that it was a more suitable host than sugar beet and this might account for the greater alate production. Infection of sugar beet with beet yellows virus did not increase alate production by M. persicae. A deterministic model showed that the probability of an alate M. persicae migrating from sugar beet and recolonizing sugar beet the following year would, at best, be only 1 in 40 000. As there is negligible transfer of genetic material from year to year, it is suggested that biotypes of M. persicae specifically adapted to sugar beet are unlikely to evolve. As a result, if particular aphicides were only used on sugar beet, resistance to those aphicides would be unlikely to evolve because any new resistance would not be transmitted from year to year.