Resistance to silverleaf whitefly, Bemisia argentifolii (Hem., Aleyrodidae), in Gossypium thurberi, a wild cotton species

Gossypium thurberi Todaro is a wild cotton species native to Mexico and parts of the southwestern USA. Four years of field studies in California's Imperial Valley revealed consistent very high levels of resistance in G. thurberi against silverleaf whitefly, Bemisia argentifolii Bellows and Perring, an important pest of cotton in many regions of the world. Naturally developing field infestations in plots of G. thurberi were significantly lower than in plots of the commercial cotton cultivars DP 5415, Siokra L23, and Stoneville 474. Gossypium thurberi has two morphological traits that, in past research, have been associated with lower levels of whitefly susceptibility: smooth- and okra-leaf; however, the levels of resistance observed in G. thurberi were significantly greater than in the cotton cultivar DP 5415, which is a smooth-leaf cotton, and Siokra L23 which, like G. thurberi, has both smooth- and okra-leaf traits. Therefore, the high level of resistance in G. thurberi seems to be due to factors above and beyond smooth- and okra-leaf. Siokra L23, which is among the least whitefly-susceptible cotton cultivars, developed whitefly populations over 30 times those on G. thurberi in all 3 years that Siokra L23 was tested. The difference in whitefly population development between G. thurberi and the other two cotton cultivars was even more striking, up to a 475-fold difference. In contrast to the clear results on naturally developing field infestations, experiments comparing nymphal survival among G. thurberi and commercial cotton cultivars did not detect antibiosis, and both choice and no-choice oviposition experiments did not detect antixenosis. Thus, the mechanisms of resistance in G. thurberi remain unknown.