Susceptibility of Cotton Boll Weevil (Coleoptera: Curculionidae) to Spinosyns

Abstract

The control of boll weevil, Anthonomus grandis grandis Boh., relies primarily on synthetic insecticides. Although insecticides are registered to spray cotton fields against boll weevils, only a few offer satisfactory control and most have broad-spectrum action. Alternatively, spinosyns have been recommended against lepidopteran pest species in cotton and are considered comparatively of reduced risk to nontargets. The susceptibility of nine populations of boll weevil to spinosad and spinetoram was determined through dried residue on squares and cotton leaves. Furthermore, control efficacy of spinosad and spinetoram compared with the standard organophosphate malathion was determined for caged adults at two different positions within the plant canopy or treated cotton leaves after different spray intervals. Boll weevil susceptibility varied across the nine populations and tested spinosyns. The lethal concentrations (LC50s and LC90s) varied from 8.62 to 32.25 and 49.86 to 281.70 mg a.i./l for spinosad and from 2.17 to 15.36 and 8.58 to 69.36 mg a.i./l for spinetoram. The location of boll weevil within the plant canopy affected the insecticide efficacy with higher mortality when caged on upper parts rather than on lower parts of cotton plants (>85% vs <45% of control) across all three insecticides. In addition, dried residues of spinosyns resulted in adult mortality >80% until the end of the evaluation period (8 d), while the standard malathion caused only 10% at this last evaluation time. Thus, we can conclude that both spinosyns promoted prolonged adult mortality using recommended field rates compared with the standard malathion.

Dispersal of boll weevil toward and within the cotton plant and implications for insecticide exposure

“…For this reason, to reach weevils located around fruiting structures, low‐volume and high‐pressure insecticide applications are recommended to ensure that insecticides will penetrate deep inside the plant canopy 37,38 . In addition, insecticides that have lasting residues on cotton foliage, such as some spinosyns and fipronil, 24,39 are recommended in the management of the pest to increase the possibility of exposure of walking weevils over treated surfaces. Although spinosad is not registered for boll weevil control in Brazil, when used at the same rate recommended to control lepidopterans in cotton, published data indicate that spinosad provided mortality >80% of adult weevils up to 8 days when caged on field‐collected upper expanded leaves from treated plants.…”

“…Considering the recent reduction in efficacy of pyrethroids against boll weevil, most applications utilize organophosphates because of high toxicity and low cost 21,22 . Among the organophosphates, malathion is by far the most recommended insecticide for boll weevil control, despite the reduced residual control outcome 23,24 …”

“…21,22 Among the organophosphates, malathion is by far the most recommended insecticide for boll weevil control, despite the reduced residual control outcome. 23,24 An insecticide is expected to reach the insect target via contact with the contaminated surface, although ingestion and inhalation are also possible routes depending on the feeding habit and location of the insect. 25 As adult boll weevil usually remains protected from direct exposure to insecticides by plant bracts, immature stages develop inside buds or bolls.…”

Field‐evolved resistance to beta‐cyfluthrin in the boll weevil: Detection and characterization

“…Despite such IPM programs, insecticides still play a major role in cotton pest control in areas with boll weevil infestations. Control of boll weevil requires many insecticides applications 7–10 that generally are non‐selective, which damages natural enemies 11,12 . Such frequent insecticide use results in pest resurgence, secondary pest outbreaks, and selection for resistance.…”

“…Infested squares fall to the ground 5–6 days after oviposition, where the immature stages complete their development. This behavior makes boll weevil a key pest and limits insecticides control to the adult stage 8,10,13,14 . Oviposition occurs over several days, resulting in staggered adult emergence, which requires repeated insecticide applications to limit insect lifecycle 15…”