Distribution and abundance of parasites of the rhodesgrass mealybug, Antonina graminis: reassessment of a classic example of biological control in the Southeastern United States.

Control of the rhodesgrass mealybug, Antonina graminis Maskell (Hemiptera: Pseudococcidae), by the encyrtid wasp Neodusmetia sangwani is considered a textbook example of classical biological control. However, recent evidence suggests that A. graminis is abundant in the southeastern United States and no recent surveys have been conducted to determine the status of N. sangwani or other A. graminis parasites. A survey was conducted and it was found that N. sangwani was uncommon overall, occurring at only 20 percent of survey sites. In addition, N. sangwani exhibited a patchy geographic distribution. Possible causes for these results are that N. sangwani has not dispersed widely since its introduction, or that the imported fire ant, Solenopsis invicta, is interfering with biological control. These results suggest that a reevaluation of the efficacy of biological control may be necessary. The survey also found two other encyrtid wasps utilizing A. graminis as a host. One, Acerophagus sp., is apparently native and was nearly as frequent as N. sangwani, while the other, Pseudectroma sp., is apparently introduced and relatively rare.

Plant resources and colony growth in an invasive ant: the importance of honeydew-producing Hemiptera in carbohydrate transfer across trophic levels.

Studies have suggested that plant-based nutritional resources are important in promoting high densities of omnivorous and invasive ants, but there have been no direct tests of the effects of these resources on colony productivity. We conducted an experiment designed to determine the relative importance of plants and honeydew-producing insects feeding on plants to the growth of colonies of the invasive ant Solenopsis invicta (Buren). We found that colonies of S. invicta grew substantially when they only had access to unlimited insect prey; however, colonies that also had access to plants colonized by honeydew-producing Hemiptera grew significantly and substantially ( approximately 50%) larger. Our experiment also showed that S. invicta was unable to acquire significant nutritional resources directly from the Hemiptera host plant but acquired them indirectly from honeydew. Honeydew alone is unlikely to be sufficient for colony growth, however, and both carbohydrates abundant in plants and proteins abundant in animals are likely to be necessary for optimal growth. Our experiment provides important insight into the effects of a common tritrophic interaction among an invasive mealybug, Antonina graminis (Maskell), an invasive host grass, Cynodon dactylon L. Pers., and S. invicta in the southeastern United States, suggesting that interactions among these species can be important in promoting extremely high population densities of S. invicta.

Experimental manipulation of queen number affects colony sex ratio investment in the highly polygynous ant Formica exsecta.

In polygynous (multiple queens per nest) ants, queen dispersal is often limited with young queens being recruited within the parental colony. This mode of dispersal leads to local resource competition between nestmate queens and is frequently associated with extremely male-biased sex ratios at the population level. The queen-replenishment hypothesis has been recently proposed to explain colony sex ratio investment under such conditions. It predicts that colonies containing many queens (subject to high local resource competition) should only produce males, whereas colonies hosting few queens (reduced or no local resource competition) should produce new queens in addition to males. We experimentally tested this hypothesis in the ant Formica exsecta by manipulating queen number over three consecutive years in 120 colonies of a highly polygynous population. Queens were transferred from 40 colonies into another 40 colonies while queen number was not manipulated in 40 control colonies. Genetic analyses of worker offspring revealed that our treatment significantly changed the number of reproductive queens. The sex ratio of colonies was significantly different between treatments in the third breeding season following the experiment initiation. We found that, as predicted by the queen-replenishment hypothesis, queen removal resulted in a significant increase in the proportion of colonies that produced new queens. These results provide the first experimental evidence for the queen-replenishment hypothesis, which might account for sex ratio specialization in many highly polygynous ant species.

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios.

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.

Effects of brood manipulation costs on optimal sex allocation in social hymenoptera.

In eusocial Hymenoptera, queens and workers are in conflict over optimal sex allocation. Sex ratio theory, while generating predictions on the extent of this conflict under a wide range of conditions, has largely neglected the fact that worker control of investment almost certainly requires the manipulation of brood sex ratio. This manipulation is likely to incur costs, for example, if workers eliminate male larvae or rear more females as sexuals rather than workers. In this article, we present a model of sex ratio evolution under worker control that incorporates costs of brood manipulation. We assume cost to be a continuous, increasing function of the magnitude of sex ratio manipulation. We demonstrate that costs counterselect sex ratio biasing, which leads to less female-biased population sex ratios than expected on the basis of relatedness asymmetry. Furthermore, differently shaped cost functions lead to different equilibria of manipulation at the colony level. While linear and accelerating cost functions generate monomorphic equilibria, decelerating costs lead to a process of evolutionary branching and hence split sex ratios.

Colony sex ratios in the facultatively polygynous ant Pheidole pallidula: a reanalysis with new data.

A recent study by Fournier et al. (2003) provides important new information on sex allocation in the ant Pheidole pallidula, and proposes a new scenario for sex-ratio evolution in P. pallidula and similar species. However, Helms proposed to the authors that two important conclusions of the study were questionable because of potential problems with the analyses. Here we provide new data and a reanalysis that strengthens the conclusion that colony sex ratio is associated with breeding system (i.e., polygyny or monogyny). However, the proposal that colonies shift from monogyny to polygyny when they become larger and more productive is weakened because there is substantial overlap in productivity between monogynous and polygynous colonies.

Temporal and spatial variations of gyne production in the ant Formica exsecta.

Social insects have become a general model for tests of sex allocation theory. However, despite tremendous interest in the topic, we still know remarkably little about the factors that cause dramatic differences in sex allocation among local populations. A number of studies have suggested that environmental factors may influence sex allocation in ant populations. In polygynous (multiple queens per nest) populations of the ant Formica exsecta, sex allocation is extremely male biased at the population level, with only a small proportion of nests producing any gynes (female reproductive brood). We analysed the proportion of gyne-producing nests in 12 F. exsecta populations during three successive breeding seasons and found considerable temporal and spatial variability in the proportion of gyne-producing nests. The populations differed in a number of characteristics, including elevation, nest density, size of the nest mound, and number of nests per population. However, the proportion of gyne-producing nests was not associated with any of these geographic and demographic variables. Moreover, differences between populations in the production of gynes were not consistent between years. Thus, the proportion of gyne-producing nests appears to vary stochastically, perhaps because of stochastic variations in environmental factors. For example, year-to-year variations in the proportion of gyne-producing nests were associated with differences in spring weather conditions between years. The finding that gyne production varies greatly between years suggests that it may not always be adaptive at a local scale.

COLONY SEX RATIOS, CONFLICT BETWEEN QUEENS AND WORKERS, AND APPARENT QUEEN CONTROL IN THE ANT PHEIDOLE DESERTORUM.

Sex-ratio conflict between queens and workers was explored in a study of colony sex ratios, relatedness, and population investment in the ant Pheidole desertorum. Colony reproductive broods consist of only females, only males, or have a sex ratio that is extremely male biased. Colonies producing females (female specialists) and colonies producing males (male specialists) occur at near equal frequency in the population. Most colonies apparently specialize in producing one reproductive sex throughout their life. Allozyme analyses show that relatedness does not differ within male-specialist and female-specialist colonies and they do not appear to differ in available resources. In the population, workers are nearly three times more closely related to females than males; however, the investment sex ratio is near equal (1.01, female/male), which is consistent with queen control. Selection should be strong on workers to increase investment in reproductive females, so why do workers in male-specialist colonies produce only (or nearly only) males? One hypothesis is that queens in male-specialist colonies prevent the occurrence of reproductive females, perhaps by producing worker-biased female eggs. An earlier simulation study of genetic evolution of sex ratios in social Hymenoptera (Pamilo 1982b) predicts that such mechanisms can result in the evolution of bimodal colony sex ratios and queen control. Results on P. desertorum are generally consistent with that study; however, information is not currently available to test some of the model's predictions and assumptions.