Chlorpyrifos effect on armored scale (Homoptera: Diaspididae) populations in San Joaquin Valley citrus

Portions of three citrus orchards infested with California red scale, Aonidiella aurantii (Maskell), and with yellow scale, A. citrina Coquillett), were sprayed either with a commercial rate of chlorpyrifos or left unsprayed. The three orchards included a chlorpyrifos-susceptible A. aurantii population, a chlorpyrifos-resistant A. aurantii population, and a chlorpyrifos-resistant A. citrina population. The levels of efficacy of chlorpyrifos in the field trials confirmed the levels of susceptibility of scales in the laboratory bioassays. Susceptible A. aurantii were killed for 60 d after treatment with one application of properly timed chlorpyrifos, Chlorpyrifos-resistant A. aurantii were held at pretreatment levels for 30 d but increased in numbers similar to those of the unsprayed control plots by 60 d after treatment. There was no significant difference in percentage of parasitism between the sprayed and unsprayed treatments. Resistant A. citrina were unaffected by the chlorpyrifos treatment, both in the number of scales per plant structure as well as in the population structure. The percentage of parasitism was significantly greater in the unsprayed compared with the chlorpyrifos treatment in this population