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

Assessment of Boll Weevil Susceptibility to Isocycloseram and Ethiprole and Differential Toxicity to Natural Enemies

Insecticides efficient against the target species while conserving natural enemies in the agroecosystem are required for IPM. With the imminent discontinuation of fipronil, a broad-spectrum insecticide, ethiprole, which belongs to the same group as phenylpyrazole (2B), and isocycloseram, a novel isoxazoline insecticide with distinct mode of action (30), provide options for controlling boll weevil. The susceptibility of the boll weevil, Anthonomus grandis grandis (Boh.), and two natural enemies [Eriopis connexa (Germar) and Bracon vulgaris Ashmead] to these insecticides were studied. Furthermore, the survival and biological traits of the lady beetle, E. connexa, exposed to fipronil, isocycloseram, and ethiprole were assessed. The LC50s values for fipronil, ethiprole, and isocycloseram for A. grandis grandis were 2.71, 0.32, and 0.025 mg a.i./L, respectively; 0.86, > 200, and 3.21 mg a.i./L for E. connexa; and 2.31, 592.94, and 0.18 mg a.i./L for B. vulgaris, respectively. The recommended rates of ethiprole did not cause mortality in adult lady beetles, although fipronil and isocycloseram were highly toxic. Lady beetle larvae and adults survived more than 80% when exposed to dried residues of ethiprole, but less than 10% when exposed to fipronil and isocycloseram. Lady beetle larvae development, reproduction, and predation rates of adults were similar between ethiprole and the control group. Although fipronil and ethiprole belong to the same insecticide group, the difference in toxicity to boll weevils and natural enemies is presented and discussed. Ethiprole was more toxic to boll weevils than to its parasitoid and lady beetle, and isocycloseram was highly toxic to all three species.

Monitoring cotton squares with opened and/or yellowed bracts to determine the need and efficency of chemical control against the cotton boll weevil

The technique of terrestrial sampling of boll weevil (BW) populations is expensive and inefficient over large areas, but may be cheaper and more efficient without involving the manipulation of cotton squares. The aim of this study was to develop a technique to sampling cotton squares based on the observation of opened and/or yellowing bracts to determine the need and efficacy of chemical control of BW in cotton crops. The first experiment aimed to estimate the ratio between the number of cotton squares with opened and/or yellowed bracts and that of squares with BW oviposition punctures. The second experiment, aimed to determine the efficacy of chemical control for BW by sampling cotton squares with opened and/or yellowed bracts. The ratio between the number of opened and/or yellowed bracts and the number of cotton squares with oviposition punctures was 2:1. The level and efficiency of chemical control of BW, based on the percentage and sampling of cotton plants with opened and/or yellowed bracts, was 5% and did not differ from the one based on the observation of cotton plants with 10% cotton squares with oviposition punctures by BW females. The control level based on sampling cotton plants with open and/or yellowing bracts was 5%. The efficiency of chemical insecticides using this economic threshold against the BW did not differ from that based on sampling cotton plants with 10% of cotton squares with oviposition punctures by BW females. This indicates that the chemical control of cotton boll weevil can be carried out based on cotton squares with open and/or yellowed bracts.

A novel picorna-like virus identified in the cotton boll weevil Anthonomus grandis (Coleoptera: Curculionidae)

The cotton boll weevil (CBW; Anthonomus grandis; Coleoptera: Curculionidae) is considered the major insect pest of cotton, causing considerable losses in yield and fiber quality. An increase in the boll weevil population due to increasingly inefficient chemical control measures is of great concernamong cotton producers. The absence of conventional or transgenic cultivars with minimal resistance to CBW has stimulated the search for new molecular and biological tools for efficient control of this insect pest. In this study, we used a metagenomic approach based on RNA deep sequencing to investigate the presence of viruses and coding viral RNA in apparently healthy native adult CBW insects collected from cotton crops in Mato Grosso state, Brazil. Using an Illumina HiSeq 2000 paired-end platform, 138,798 virus-related reads were obtained, and a consensus sequence of a putative new virus, 10,632 nucleotides in length, was assembled. The sequences of the 5' and 3' untranslated regions (UTRs) were determined by rapid amplification of cDNA ends (RACE), followed by Nanopore sequencing. The complete genome sequence included a 5'-UTR (1,158 nucleotides), a 3'-UTR (561 nucleotides), and a single ORF of 8,913 nucleotides encoding a large polyprotein. Sequence analysis of the putative polyprotein showed several regions with high sequence similarity to structural and non-structural proteins of viruses of the family Iflaviridae. Pairwise alignments of polyprotein amino acid sequences showed the highest sequence identity (32.13%) to a partial polyprotein sequence of a putative iflavirus (QKN89051.1) found in samples from wild zoo birds in China. Phylogenetic analysis based on full polyprotein sequences of different iflaviruses indicated that this new picorna-like virus is most closely related to iflaviruses found in lepidopteran insects, and it was therefore tentatively named "Anthonomus grandis iflavirus 1" (AgIV-1). This is, to our knowledge, the first complete viral genome sequence found in CBW, and it could provide a basis for further studies about the infectivity and transmission of this virus and its possible association with symptoms or acute disease. AgIV-1 could potentially be used to develop biological or molecular tools, such as a viral vector to carry interfering RNA molecules for CBW control.

Insecticides in Use and Risk of Control Failure of Boll Weevil (Coleoptera: Curculionidae) in the Brazilian Cerrado

Growers rely on synthetic insecticides to control the boll weevil throughout the reproductive cotton stage. An average of 19.6 insecticide applications (range: 10 to 30) for control of boll weevil were found in a survey with growers in the Brazilian Cerrado, covering an area of 494,100 hectares of cotton. Twenty-one insecticides were applied, with 64.8% of the applications made with malathion, fipronil, carbosulfan, and thiamethoxam + lambda-cyhalothrin. These four insecticides were used by 100, 76, 70, and 62% of the growers, with respectively 7.2, 2.1, 1.8, and 1.6 applications. Growers classified their boll weevil control achieved into four categories (fair, good, very good, or excellent), without correlation between these categories with the number of insecticide applications. Control of cotton regrowth and volunteer cotton plants were the major obstacles for effective boll weevil management, followed by the low efficacy of insecticides. Five registered insecticides to spray cotton against other pests than boll weevil were enlisted by growers with potential for recommendation. A boll weevil standard population for susceptibility was assayed with 27 insecticides and the results presented within a failure risk quotient (FRQ). The FRQ of eight, six, and 13 of the 27 tested insecticides was high, intermediate, and low, respectively. The high FRQ included 7 of 10 pyrethroid formulations, pymetrozine, and methomyl. On the opposite end, fipronil had the lowest FRQ value.

Susceptibility of Boll Weevil (Coleoptera: Curculionidae) to Ethiprole, Differential Toxicity Against Selected Natural Enemies, and Diagnostic Concentrations for Resistance Monitoring

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.

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

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.

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

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.

Selective insecticides secure natural enemies action in cotton pest management

Cotton hosts a variety of arthropod pests requiring intensive control mostly with insecticides, which in turn may impact beneficial insects and the environment. Therefore, insect control in cotton fields preconizes the use of selective insecticides that offer pest control but conserve natural enemies. In this work, we measured the impact of recommended insecticides on the abundance of predatory insects and predation upon sentinel preys in the field. Further, the survival of four key selected predatory insects of cotton ecosystem, representing chewing and sucking feeding habits and different pest species attacked [Chrysoperla externa Hagen, Eriopis connexa (Germar), Podisus nigrispinus (Dallas) and Orius insidiosus (Say)], were assessed when exposed to the dried residues of the tested insecticides. Mortality of sentinel prey caused by natural enemies was higher in areas treated with selective insecticides relative to the non-selective ones, and most of time similar to the untreated areas. Furthermore, areas treated with non-selective insecticides experienced prolonged impact between sprays depending on the insecticide applied. Seasonal abundance of predatory insects was 2× greater in fields under selective and untreated fields compared to those under non-selective recommendation. Survival of predators exposed to the dried residues of the selective insecticides pymetrozine, chlorantraniliprole, pyriproxyfen, and cyantraniliprole were greater than when exposed to the non-selective lambda-cyhalothrin, malathion, dimethoate, and thiamethoxam. Among the non-selective insecticides, malathion and dimethoate exhibited shorter residual time compared to the thiamethoxam and lambda-cyhalothrin + thiamethoxam. Therefore, the recommendation of selective insecticides provides benefits for cotton pest management by maintaining the action of the natural enemies present in the field.

Sublethal Effects of Insect Growth Regulators on Boll Weevil (Coleoptera: Curculionidae)

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.