No abstract

BACKGROUND: Knowledge of the spatio-temporal distribution of pests is important for the development of accurate management approaches. The boll weevil, Anthonomus grandis grandis Boh., is a deleterious cotton, Gossypium hirsutum L., pest in the western hemisphere. The spread of boll weevils across cotton fields remains poorly understood. We assessed the dispersal pattern of adult weevils through cotton fields cultivated in a tropical area during dry and wet seasons using geostatistics for the number of adults and infested reproductive structures (buds, bolls and total). RESULTS: Adult weevils and infested reproductive structures increased across both seasons despite the prevailing climatic variables. In both seasons, boll weevil adults and infested reproductive structures followed an aggregated distribution. The distances over which samples maintained spatial dependence varied from 0.7 to 43.4 m in the dry season and from 6.0 to 614.4 m in the wet season. Boll weevil infestations started at field borders and the infested reproductive structures (oviposition and/or feeding punctured) were greater than the adults regardless of cotton growth stage.CONCLUSION: Sampling for boll weevils in cotton fields should start at the field borders and focus on total infested reproductive structures (buds + bolls) and as cotton plants develop, sampling should focus on the field as a whole. Distances among samples will vary from 6 to 470 m. Thus, despite the cotton phenological stage or growing season, monitoring of boll weevil should be done by sampling total infested reproductive structures with a minimum distance of 6 m among samples.

The advent of genetically modified crops expressing insecticidal proteins of the bacterium Bacillus thuringiensis (Bt) revolutionized the management of insect pest species. Nonetheless, such disruption does not come free from controversies and concerns. The quick spread of Bt resistance in targeted pest species is an example, and the impact in non‐targeted arthropod species is another. The former is a continuous focus of attention, while the latter faces knowledge gaps and methodological shortcomings particularly when arthropod communities are concerned. If Bt crops such as cotton are aimed at drastically reducing a key community component, such as targeted caterpillars in the Neotropical region, a significant community‐level impact is expected and deserves assessment. However, the subject is neglected with existing studies focusing on arthropod assemblages or low‐level taxa compromising their resolution. Thus, food web analysis was used here to assess the short‐term impact of Bt cotton on the associated arthropod community in a Neotropical scenario. Bt cotton and related non‐Bt cotton were cultivated for two years, and the arthropods were sampled using sweep net and whole plant collections throughout the cotton phenological cycle. The gathered data were used to build (plant–consumer–predator) trivariate networks using food web analysis. Thirty‐six arthropod species were sampled with the prevalence of whiteflies, cotton aphids, boll weevils and green leafhoppers, the main pest species in the region. Cotton genotype and cultivation year significantly affected the abundance and diversity of the associated arthropod assemblage (Fapp ≥ 2.86, p < 0.01). However, the Bt cotton‐associated food webs were similar to those of the non‐Bt cotton, indicating no significant impact of this technology on arthropod food webs. This result would be welcome, if it not indicating that Bt cotton did not provide a significant control of the main target pest species of the crop raising questions on the usefulness of the technology in the area.

Although the boll weevil (BW), Anthonomus grandis grandis (Coleoptera: Curculionidae) has been attributed to the significant losses caused to cotton yield in the Americas, the categorization as a quarentenary pest in places where it is still not occurring has increased its relevance worldwide. In areas where it is widespread, pest suppression relies on many broad-spectrum insecticide applications. However, other control tactics are sought. Considering that early-flowering cultivars escape from boll weevil infestation, we investigated if three different planting dates (November, December, and January) could alter the plant life cycle, allowing the plants to escape from boll weevil infestation. Field trials were run in two seasons (2014/2015 and 2017/2018), and variables (days required to reach each flowering stage, fruiting plant structures—undamaged and damaged by the BW, and totals—number of boll weevils on plants and that had emerged from fallen structures on the ground) were assessed over 29 and 33 weeks, respectively. Based on the number of days required to initiate and terminate the flowering stage, the time to reach the economic threshold (ET), the number of undamaged, damaged, and the total reproductive structures, we concluded that planting dates in December for the Central Cerrado of Brazil should be preferred over the other two tested dates. Cultivations run at this planting date, anticipating the flowering period initiation and termination, reduced infested flowering structures, and delayed the decision making to control the pest, when compared to the other two planting dates.