Landscape-level variation in Bt crops predict Helicoverpa zea (Lepidoptera: Noctuidae) resistance in cotton agroecosystems

BACKGROUND

Helicoverpa zea (Boddie) damage to Bt cotton and maize has increased as a result of widespread Bt resistance across the USA Cotton Belt. Our objective was to link Bt crop production patterns to cotton damage through a series of spatial and temporal surveys of commercial fields to understand how Bt crop production relates to greater than expected H. zea damage to Bt cotton. To do this, we assembled longitudinal cotton damage data that spanned the Bt adoption period, collected cotton damage data since Bt resistance has been detected, and estimated local population susceptibility using replicated on-farm studies that included all Bt pyramids marketed in cotton.

RESULTS

Significant year effects of H. zea damage frequency in commercial cotton were observed throughout the Bt adoption period, with a recent damage increase after 2012. Landscape-level Bt crop production intensity over time was positively associated with the risk of H. zea damage in two- and three-toxin pyramided Bt cotton. Helicoverpa zea damage also varied across Bt toxin types in spatially replicated on-farm studies.

CONCLUSIONS

Landscape-level predictors of H. zea damage in Bt cotton can be used to identify heightened Bt resistance risk areas and serves as a model to understand factors that drive pest resistance evolution to Bt toxins in the southeastern United States. These results provide a framework for more effective insect resistance management strategies to be used in combination with conventional pest management practices that improve Bt trait durability while minimizing the environmental footprint of row crop agriculture. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Estimations of Fine-Scale Species Distributions of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in Eastern Florida

Many species distribution maps indicate the ranges of Aedes aegypti (Linnaeus) and Aedes albopictus (Skuse) overlap in Florida despite the well-documented range reduction of Ae. aegypti. Within the last 30 yr, competitive displacement of Ae. aegypti by Ae. albopictus has resulted in partial spatial segregation of the two species, with Ae. aegypti persisting primarily in urban refugia. We modeled fine-scale distributions of both species, with the goal of capturing the outcome of interspecific competition across space by building habitat suitability maps. We empirically parameterized models by sampling 59 sites in south and central Florida over time and incorporated climatic, landscape, and human population data to identify predictors of habitat suitability for both species. Our results show human density, precipitation, and urban land cover drive Ae. aegypti habitat suitability, compared with exclusively climatic variables driving Ae. albopictus habitat suitability. Remotely sensed variables (macrohabitat) were more predictive than locally collected metrics (microhabitat), although recorded minimum daily temperature showed significant, inverse relationships with both species. We detected minor Aedes habitat segregation; some periurban areas that were highly suitable for Ae. albopictus were unsuitable for Ae. aegypti. Fine-scale empirical models like those presented here have the potential for precise risk assessment and the improvement of operational applications to control container-breeding Aedes mosquitoes.

Sunshine versus gold: The effect of population age on genetic structure of an invasive mosquito

The genetic diversity and structure of invasive species are affected by the time since invasion, but it is not well understood how. We compare likely the oldest populations of Aedes aegypti in continental North America with some of the newest to illuminate the range of genetic diversity and structure that can be found within the invasive range of this important disease vector. Aedes aegypti populations in Florida have probably persisted since the 1600-1700s, while populations in southern California derive from new invasions that occurred in the last 10 years. For this comparison, we genotyped 1,193 individuals from 28 sites at 12 highly variable microsatellites and a subset of these individuals at 23,961 single nucleotide polymorphisms (SNPs). This is the largest sample analyzed for genetic structure for either region, and it doubles the number of southern California populations previously analyzed. As predicted, the older populations (Florida) showed fewer indicators of recent founder effect and bottlenecks; in particular, these populations have dramatically higher genetic diversity and lower genetic structure. Geographic distance and driving distance were not good predictors of genetic distance in either region, especially southern California. Additionally, southern California had higher levels of genetic differentiation than any comparably sized documented region throughout the worldwide distribution of the species. Although population age and demographic history are likely driving these differences, differences in climate and transportation practices could also play a role.

Patterns of genetic divergence among populations of Aedes aegypti L. (Diptera: Culicidae) in the southeastern USA

Background

The yellow fever mosquito, Aedes aegypti is a public health concern in the USA, especially in the wake of emergent diseases such as Zika and chikungunya. Aedes aegypti populations dwindled after the invasion of Aedes albopictus in the 1980s and many populations were extirpated. However, in some areas Ae. aegypti persisted in small populations and there are reports of recent resurgences of Ae. aegypti in Florida, Louisiana, Nevada and California. We assessed the population genetic structure of Ae. aegypti in Florida and Georgia, which has concomitant consequences related to mosquito dispersal, pesticide resistance and vectorial capacity.

Methods

We collected Ae. aegypti across Florida and in Georgia using ovitraps. We hatched the eggs and reared them to adults, and after sacrifice we extracted their DNA. We then probed each individual for variation in 6 microsatellite markers, which we used to address population genetic characteristics.

Results

We collected Ae. aegypti and genotyped seven Florida populations and one Georgia population using microsatellite markers. We found evidence of isolation by distance model of gene flow supported by driving distance among cities within Florida and two theoretic genetic clusters.

Conclusions

Significant genetic structure between some populations with substantial gene flow between geographically distant cities suggests regional genetic structuring of Ae. aegypti in Florida. This study provides information on the genetic exchange between populations of Ae. aegypti in the southeastern USA and suggests potential routes of spread of this species.

Surveillance for Immature Mosquitoes in Windshield Wash Basins at Gas Stations

Gas stations often provide windshield wash basins (WWBs) that customers may use to clean their windshields. Motivated by casual observations, we conducted a survey of WWBs in and around Raleigh, NC, to determine whether these WWBs also serve as larval habitats for mosquitoes. We found that 27.7% (95% CI: 12.4-43.14%) of the 36 surveyed gas stations had mosquito larvae in their WWBs, and 22.4% (95% CI: 15.07-29.1%) of the 152 WWBs surveyed were positive for mosquito larvae. Two species were identified inhabiting these containers: Aedes albopictus (Skuse) and Culex quinquefasciatus Say. Aedes albopictus was associated with clear, unturbid water, whereas Cx. quinquefasciatus did not have any significant association with water characteristics. Pupae of both species were observed, suggesting these habitats could be sources of pest mosquitoes. Gas stations may be a convenient surveillance target for vector control specialists and may provide insight into human-aided mosquito dispersal.

Mosquitoes of field and forest: the scale of habitat segregation in a diverse mosquito assemblage

Knowledge of the distribution of arthropod vectors across a landscape is important in determining the risk for vector-borne disease. This has been well explored for ticks, but not for mosquitoes, despite their importance in the transmission of a variety of pathogens. This study examined the importance of habitat, habitat edges, and the scale at which mosquito abundance and diversity vary in a rural landscape by trapping along transects from grassland areas into forest patches. Significant patterns of vector diversity and distinct mosquito assemblages across habitats were found. The scale of individual species' responses to habitat edges was often dramatic, with several species rarely straying even 10 m from the edge. The present results suggest blood-seeking mosquito species are faithful to certain habitats, which has consequences for patterns of vector diversity and risk for pathogen transmission. This implies that analysts of risk for pathogen transmission and foci of control, and developers of land management strategies should assess habitat at a finer scale than previously considered.

Recent Changes in the Local Distribution of Aedes aegypti (Diptera: Culicidae) in South Florida, USA

Disease transmission is directly tied to the spatial distribution of disease vectors. The distribution of Aedes aegypti (L.) in the United States has diminished since the introduction of Aedes albopictus (Skuse) in the 1980s. However, Ae. aegypti persists in some urban areas, particularly in south Florida. The pattern of habitat segregation of these two species is well documented, but the consistency of this phenomenon over time is unknown. To examine the dynamics of the local distributions of these two species, we studied the spatial pattern of Ae. aegypti and Ae. albopictus over time at a fine landscape scale in Palm Beach County, FL. We compared patterns from 2006-2007 with their distributions in 2013, taking into account abiotic factors of microclimate and land cover. We found evidence for a local shift in Ae. aegypti distribution, but could not attribute this to changes in measured abiotic factors. Alternatively, the interaction between Ae. aegypti and Ae. albopictus that initially resulted Ae. aegypti decline may be being attenuated through natural selection. This study confirms the importance of monitoring the changing ranges of these two important vector species.