BACKGROUND Immature stages of boll weevil complete development endophytically leaving only the adult stage accessible for chemical control. We tested the hypothesis that boll weevil colonization of the cotton plants significantly affects their exposure to sprayed insecticides. We determined the adult dispersal toward and within cotton plants, lethal time (LT), and residual control by recommended insecticides (malathion, carbosulfan, thiamethoxam, fipronil, beta‐cyfluthrin, lambda‐cyhalothrin, and thiamethoxam + lambda‐cyhalothrin) through dried residue exposure and residual control regarding the leaf position in the upper and the lower thirds of the treated plant canopy. RESULTS Newly emerged adults from fallen buds reached the cotton plants by walking (80%) and most of the time settling on cotton bolls in the lower part of the plants (78%). Irrespective of sex and mating status, adults released on the upper part of the plant remained longer on the same release site than the lower part, with some individuals remaining up to 50 h on the same flower bud. The shortest LT90 was found with thiamethoxam (106 h). Fipronil and malathion, respectively, provided the longest (>144 h) and shortest (24 h) residual control times and caused boll weevil mortality above 80%. CONCLUSIONS These findings suggest that weevils accessing the cotton plants exhibited within‐plant distribution that minimizes their contact with insecticide residue on plant foliage. Furthermore, short residual control with malathion, the most used insecticide against boll weevil, and the low susceptibility exhibited by the tested population to pyrethroids highlight the current challenges faced for boll weevil control in Brazilian cotton fields. © 2020 Society of Chemical Industry

BACKGROUND Insecticide resistance in arthropods is an inherited trait that has become a major cause of insect pest control failure. Monitoring the level of susceptibility and characterization of the type of resistance of key pest species aims to determine the risk of resistance selection in time to take action to mitigate control failures. Seven populations of the boll weevil, Anthonomus grandis grandis, collected from cotton fields in the Semiarid and Cerrado areas of Brazil, were screened for their resistance to malathion and beta‐cyfluthrin, insecticides widely recommended for control of boll weevil and other pests. RESULTS The levels of adult mortality were variable for beta‐cyfluthrin (0–82%) but invariant (100%) for malathion. Bioassays of concentration‐mortality were used to determine lethal concentrations (LCs) for each insecticide. The LC‐values corroborate the lack of resistance to field rates of malathion but high levels of resistance to beta‐cyfluthrin from 62.7‐ to 439.7‐fold. Weevils resistant to beta‐cyfluthrin were found through genome sequencing to possess a kdr mutation through the L1014F substitution in the voltage gated‐sodium channel gene. CONCLUSIONS This study found boll weevil resistance to beta‐cyfluthrin to be not mediated by carboxylesterases, but with cross‐resistance to DDT and carbaryl, and kdr mutation as the major mechanism of the resistance in our samples. Caution is recommended in further use of beta‐cyfluthrin against boll weevil due to potential resistance. Monitoring studies using other boll weevil populations are recommended to determine the geographic pattern and extent of pyrethroid resistance. © 2021 Society of Chemical Industry.

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.

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Immature stages of the boll weevil, Anthonomus grandis grandis Boh. (Coleoptera: Curculionidae), develop protected inside cotton fruiting structures. Therefore, the adult beetles have become the main target of insecticide applications. The use of insect growth regulators (IGRs) is recommended against immatures, even though they may also affect the survival and reproductive traits of adult insects. The present study evaluated the impact of a juvenile hormone analog (pyriproxyfen), an ecdysteroid agonist (methoxyfenozide), and a chitin biosynthesis inhibitor (lufenuron) on adult cotton boll weevils, a key cotton pest. Mated and virgin beetles were treated by feeding them contaminated squares and cotton leaf discs that were previously immersed into pyriproxyfen, methoxyfenozide, and lufenuron solutions at field-rate concentrations. After exposure, treated couples were caged onto cotton plants, and survival, fecundity, and egg viability were evaluated. The IGRs neither affected the survival nor fecundity of adult boll weevils. On the other hand, egg viability was significantly reduced by lufenuron, regardless of whether the females were treated premating or postmating or whether their pairs were either treated or untreated. However, egg viability increased as the females aged since the initial exposure date to lufenuron, indicating a potential transovarial effect of this insecticide. Our results indicate that pyriproxyfen and methoxyfenozide do not affect adult boll weevils, whereas lufenuron temporarily reduces the egg viability of this key cotton pest.

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Synthetic insecticide application is one tactic for reducing boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), infestations during the cotton, Gossypium hirsutum L., reproductive stage. We assessed the susceptibility of the boll weevil and its natural enemies to ethiprole (mode of action 2B), a phenylpyrazole insecticide, and diagnostic concentrations of ethiprole indicative of boll weevil susceptibility. Differences in the lethal concentrations of ethiprole were calculated with susceptibility ratios based on LC50 ranging from 2.89- to 10.34-fold relative to a natural susceptible population. The lowest and the highest recommended field rates of ethiprole, 100 and 200 g a.i./ha, produced residues that caused 83.3% and 93.7% mortality of weevils caged with cotton leaves from field-treated plants for 8 d. We found that ethiprole was less toxic than fipronil to the boll weevil parasitoid Bracon vulgaris Ashmead (Hymenoptera: Braconidae) and to the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), while fipronil was highly toxic to both. Adult earwigs, Euborellia annulipes Lucas (Dermaptera: Anisolabididae), were relatively tolerant to ethiprole and fipronil at the highest field rates. Pooled LC50-and LC95-concentrations of ethiprole calculated from studied populations were used as diagnostic for boll weevil mortality, and the outcome fitted to the expected mortality for boll weevil populations from different locations serving for further control failure assessment. Ethiprole appears to be suitable for boll weevil control with low impact on natural enemy communities.

The departure and arrival of boll weevil in cotton fields have fostered major control decisions against this pest over time. Field colonization and distribution of boll weevil were evaluated using cotton fields from 93.7 to 154 ha each, located in Serra da Petrovina and Campo Verde, both in the Cerrado biome of Mato Grosso State, Brazil, as a function of cotton field bordering vegetation and crop phenology. The monitoring of adult weevils was carried out using traps containing sex and aggregation pheromone. The traps were set up considering the four coordinates of the field, bordering vegetation, and distances from the field margin. Six traps were set in each direction with three representing the field margin (0 m, 60 m, and 120 m from the borderline) and the other three set up equidistant from the center of the field (> 200 m). Traps were evaluated weekly beginning 10 days after plant emergence (DAE) throughout the cotton phenology until harvesting. Boll weevils were caught in the first evaluation 10 DAE, irrespective of the vegetation bordering the cotton field and distance of trap from field margin. The average weevils per trap was relatively low throughout the crop development but increased significantly through the maturation and harvesting periods. Furthermore, the bordering vegetation was not the only determinant factor for boll weevil colonization and distribution throughout the cotton field. Based on these data, boll weevil exhibited early colonization, already occurring beyond the border of the field.