Crop and Semi-Natural Habitat Configuration Affects Diversity and Abundance of Native Bees (Hymenoptera: Anthophila) in a Large-Field Cotton Agroecosystem.

The cotton agroecosystem is one of the most intensely managed, economically and culturally important fiber crops worldwide, including in the United States of America (U.S.), China, India, Pakistan, and Brazil. The composition and configuration of crop species and semi-natural habitat can have significant effects on ecosystem services such as pollination. Here, we investigated the local-scale effect of crop and semi-natural habitat configuration in a large field (>200 ha in size) cotton agroecosystem on the diversity and abundance of native bees. The interfaces sampled included cotton grown next to cotton, sorghum or semi-natural habitat along with a natural habitat comparator. Collections of native bees across interface types revealed 32 species in 13 genera across 3 families. Average species richness metrics ranged between 20.5 and 30.5, with the highest (30.5) at the interface of cotton and semi-natural habitat. The most abundant species was Melissodes tepaneca Cresson (>4000 individuals, ~75% of bees collected) with a higher number of individuals found in all cotton-crop interfaces compared to the cotton interface with semi-natural habitat or natural habitat alone. It was also found that interface type had a significant effect on the native bee communities. Communities of native bees in the cotton-crop interfaces tended to be more consistent in species richness and abundance. While cotton grown next to semi-natural habitat had higher species richness, the number of bees collected varied. These data suggest that native bee communities persist in large-field cotton agroecosystems. Selected species dominate (i.e., M. tepaneca) and thrive in this large-field cotton system where cotton-crop interfaces are key local landscape features. These data have implications for potential pollination benefits to cotton production. The findings also contribute to a discussion regarding the role of large-field commercial cotton growing systems in conserving native bees.

A Native Bee, Melissodes tepaneca (Hymenoptera: Apidae), Benefits Cotton Production.

The cotton agroecosystem is one of the most intensely managed, economically and culturally important cropping systems worldwide. Native pollinators are essential in providing pollination services to a diverse array of crops, including those which have the ability to self-pollinate. Cotton, which is autogamous, can potentially benefit from insect-mediated pollination services provided by native bees within the agroecosystem. Examined through two replicated experiments over two years, we hypothesized that native bees facilitated cross-pollination, which resulted in increased lint of harvested bolls produced by flowers exposed to bees and overall lint weight yield of the plant. Cotton bolls from flowers that were caged and exposed to bees, flowers that were hand-crossed, and bolls from flowers on uncaged plants exposed to pollinators had higher pre-gin weights and post-gin weights than bolls from flowers of caged plants excluded from pollinators. When cotton plants were caged with the local native bee Melissodes tepaneca, seed cotton weight was 0.8 g higher on average in 2018 and 1.18 g higher on average in 2019 than when cotton plants were excluded from bees. Cotton production gains from flowers exposed to M. tepaneca were similar when measuring lint and seed separately. Cotton flowers exposed over two weeks around the middle of the blooming period resulted in an overall yield gain of 12% to 15% on a whole plant basis and up to 24% from bolls produced from flowers exposed directly to M. tepaneca. This information complements cotton-mediated conservation benefits provided to native pollinators by substantiating native bee-mediated pollination services provided to the cotton agroecosystem.

Field Edge and Field-to-Field Ecotone-Type Influences on Two Cotton Herbivores: Cotton Fleahopper, Pseudatomoscelis seriatus (Hemiptera: Miridae), and Verde Plant Bug, Creontiades signatus.

In the United States, the average field size has roughly doubled from the 1980s to the mid-2000s, while average cropland has stayed the same. This will likely influence how semi-natural habitats and edges affect local patterns and processes such as natural pest control or pest densities. We hypothesized that densities of two cotton pests, cotton fleahopper (Pseudatomoscelis seriatus) and verde plant bug (Creontiades signatus) (Hemiptera: Miridae), and corresponding cotton injury in a cotton agroecosystem were affected by field edge, ecotone type (described by the neighboring habitat), and the influence of ecotone type on edge effects. Studies over 2 yr using transect and random point sampling indicated that densities of both insects declined significantly and in a linear fashion from the cotton field boundary (0 m) to field interior (200-300 m from field edge). The decline was influenced by ecotone type for cotton fleahopper. Pest densities in cotton at the interface with semi-natural habitat were higher but declined at a greater rate into the cotton field interior compared to densities seen at the interfaces with sorghum or another cotton field. These effects were also observed for verde plant bug and the cotton boll injury it causes. Regardless of the pest densities near the field edge and the rate of decline into the field interior, densities beyond 100 m into the field were up to 70% less than field edges for both insect species and for boll injury. Potential for land managers to improve sampling efficiency when scouting is apparent. For example, pest species may be at economic threshold in certain parts of the field but not others, leading to different management decisions in larger fields. Therefore, for cotton fleahopper and verde plant bug, edges should be the focus of initial pest detection and sampling, and interior field sampling may only be required when edges are above the economic threshold.

Africanization of a feral honey bee (Apis mellifera) population in South Texas: does a decade make a difference?

The arrival to the United States of the Africanized honey bee, a hybrid between European subspecies and the African subspecies Apis mellifera scutellata, is a remarkable model for the study of biological invasions. This immigration has created an opportunity to study the dynamics of secondary contact of honey bee subspecies from African and European lineages in a feral population in South Texas. An 11-year survey of this population (1991-2001) showed that mitochondrial haplotype frequencies changed drastically over time from a resident population of eastern and western European maternal ancestry, to a population dominated by the African haplotype. A subsequent study of the nuclear genome showed that the Africanization process included bidirectional gene flow between European and Africanized honey bees, giving rise to a new panmictic mixture of A. m. scutellata- and European-derived genes. In this study, we examined gene flow patterns in the same population 23 years after the first hybridization event occurred. We found 28 active colonies inhabiting 92 tree cavities surveyed in a 5.14 km(2) area, resulting in a colony density of 5.4 colonies/km(2). Of these 28 colonies, 25 were of A. m. scutellata maternal ancestry, and three were of western European maternal ancestry. No colonies of eastern European maternal ancestry were detected, although they were present in the earlier samples. Nuclear DNA revealed little change in the introgression of A. m. scutellata-derived genes into the population compared to previous surveys. Our results suggest this feral population remains an admixed swarm with continued low levels of European ancestry and a greater presence of African-derived mitochondrial genetic composition.

Stage-structured matrix models for organisms with non-geometric development times.

Matrix models have been used to model population growth of organisms for many decades. They are popular because of both their conceptual simplicity and their computational efficiency. For some types of organisms they are relatively accurate in predicting population growth; however, for others the matrix approach does not adequately model growth rate. One of the reasons for the lack of accuracy is that most matrix-based models implicitly assume a specific degree of variability in development times for the organism. Because the variability is implicit, the implied variances are often not verified with experimental data. In this paper, we shall present extensions to the stage-classified matrix models so that organisms with arbitrary means and standard deviations of development times can be modeled.

Africanization in the United States: replacement of feral European honeybees (Apis mellifera L.) by an African hybrid swarm.

The expansion of Africanized honeybees from South America to the southwestern United States in <50 years is considered one of the most spectacular biological invasions yet documented. In the American tropics, it has been shown that during their expansion Africanized honeybees have low levels of introgressed alleles from resident European populations. In the United States, it has been speculated, but not shown, that Africanized honeybees would hybridize extensively with European honeybees. Here we report a continuous 11-year study investigating temporal changes in the genetic structure of a feral population from the southern United States undergoing Africanization. Our microsatellite data showed that (1) the process of Africanization involved both maternal and paternal bidirectional gene flow between European and Africanized honeybees and (2) the panmitic European population was replaced by panmitic mixtures of A. m. scutellata and European genes within 5 years after Africanization. The post-Africanization gene pool (1998-2001) was composed of a diverse array of recombinant classes with a substantial European genetic contribution (mean 25-37%). Therefore, the resulting feral honeybee population of south Texas was best viewed as a hybrid swarm.

Temporal pattern of africanization in a feral honeybee population from Texas inferred from mitochondrial DNA.

The invasion of Africanized honeybees (Apis mellifera L.) in the Americas provides a window of opportunity to study the dynamics of secondary contact of subspecies of bees that evolved in allopatry in ecologically distinctive habitats of the Old World. We report here the results of an 11-year mitochondrial DNA survey of a feral honeybee population from southern United States (Texas). The mitochondrial haplotype (mitotype) frequencies changed radically during the 11-year study period. Prior to immigration of Africanized honeybees, the resident population was essentially of eastern and western European maternal ancestry. Three years after detection of the first Africanized swarm there was a mitotype turnover in the population from predominantly eastern European to predominantly A. m. scutellata (ancestor of Africanized honeybees). This remarkable change in the mitotype composition coincided with arrival of the parasitic mite Varroa destructor, which was likely responsible for severe losses experienced by colonies of European ancestry. From 1997 onward the population stabilized with most colonies of A. m. scutellata maternal origin.