In 1995, silverleaf whitefly, Bemisia argentifolii Bellows and Perring, resistance to the widely-used mixture of Danitol^®; (fenpropathrin) + Orthene^®; (acephate) was shown to be severe and widespread in Central Arizona cotton. Thereafter, laboratory experiments were undertaken to identify the other major insecticides that were affected by resistance selected by Danitol+Orthene. Large numbers of whiteflies were collected in November of 1995 from the Gila River Basin (GRB) (highly resistant) and the Yuma (relatively susceptible) areas of Arizona. A mixed colony from GRB and Yuma in a 1:4 ratio was established. After six generations of adult selection, appreciable shifts in the concentration responses for pyrethroid, organophosphate, and carbamate insecticides were witnessed, indicating heritable variation for resistance in the source populations. From this we obtained definitive proof that resistance to Danitol+Orthene confers cross-resistance to Asana^®; (esfenvalerate), Capture^®; (bifenthrin), Danitol, Decis^®; (deltamethrin), Decis+Orthene, and Karate^®; (lambda-cyhalothrin). Additionally, selection with Danitol+Orthene resulted in statistically significant reductions in susceptibility to Curacron^®; (profenofos), Lannate^®; (methomyl), Monitor^®; (methamidophos), and Ovasyn^®; (amitraz).

Studies were performed to assess tolerance of GRB (pyrethroid resistant) and Yuma (pyrethroid susceptible) populations to a diversity of conventional insecticides currently registered for use in Arizona cotton, with the intention of finding compounds that showed promise for overcoming pyrethroid resistance. Of the materials evaluated, Curacron, Lannate, Lorsban^®; (chlorpyrifos), Ovasyn, Supracide^®; (methidathion), and Vydate^®; (oxamyl) were most promising based on reductions in the two strains of insecticide response differences relative to their widely different responses to Danitol+Orthene.

To determine to what degree pyrethroid resistance in cotton influenced resistance in winter vegetables and melons, and vice versa, whitefly populations from spring melons, cotton, and fall melons and cole crops were collected from Western and Central Arizona regions. In most instances, the whiteflies found in Western Arizona were significantly more susceptible to Danitol+Orthene than those found in Central Arizona. Significant decreases were found in susceptibility to Danitol+Orthene during the 1996 season at three of the four locations in which multiple crops were monitored. This emphasizes that pyrethroid resistance levels can be increased in whitefly populations from any of the cotton, melons, or other winter vegetable crops evaluated. Therefore, management of pyrethroid resistance in Arizona cotton will require harmonizing resistance management efforts and specifically limiting pyrethroid use in the entire crop complex.