Abstracts of Current and Previous Research

1. Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus

In many ant species, multiple modes of founding new colonies occur in the same population. These modes include dependent founding, independent founding by haplometrosis (single queen), and independent founding by pleometrosis (multiple queens). In several cases, a dimorphism in queen size has been found, such that each morph specializes in a particular nest-founding behavior. I investigated queen size in the ant Temnothorax longispinosus in several southern Wisconsin populations and found three distinct queen morphs: small queens with very low fat content and short wings, large queens with low fat and long wings, and large queens with high fat and very long wings. Several traits associated with founding behavior correlated with these queen sizes. Small queens were produced in lower numbers, were more common in polygynous nests, and returned to the nest in higher proportions than both large queen morphs. The size ranges and fat levels of each queen morph were similar to those of other species that specialize in either haplometrosis (very large, high fat), pleometrosis (large, low fat), or dependent founding (small and low fat). However, there was extensive overlap in several of the founding behaviors, suggesting that the morphs in these populations have some flexibility in founding behavior. The queen morphs in these populations of T. longispinosus may resemble early stages in the evolution of more specialized dispersal polymorphisms found in other ant species.

Howard, K.J. 2006. Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus. Insectes Sociaux 53: 480-488.

1. Small female alate	2. Large, low fat female alate
3. Large, high fat female alate	4. Acorn with a colony of Temnothorax longispinosus inside. Coleopteran or Lepidopteran larvae make the small hole, the ants move in later.
5. View inside a typical nest of Temnothorax longispinosus inside of an acorn.	6. Female alate taking flight within a mating swarm
7. Female alate perched with her wings held up within a mating swarm	8. Workers inside a lab nest biting a female larva.

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2. Foraging strategies in colonies of the social wasp Polybia occidentalis (Hymenoptera, Vespidae).

Social insect colonies should forage at rates that maximize colony fitness. Foraging at higher rates increases the rate of worker production, but decreases adult survival. This trade-off has particular significance during the pre-emergence period, when adults lost are not replaced. Earlier work found that independent-founding wasps rear the first workers more quickly by foraging at high rates. Foraging rates decreased after those individuals pupated, presumably to reduce the risk of foundress death. Swarm-founding wasps have many workers, making colony death by forager attrition less likely. Do swarm-founding wasps show similar shifts during the pre-emergence period? We measured foraging rates of the swarm-founding wasp, Polybia occidentalis, by counting wasps arriving at the nest at four stages of colony development, before and during the pre-emergence period. Foraging rates at each stage correlated with the number of larvae present, which in turn correlated with the number of cells in the new nest. The foraging rates per larva changed over the pre-emergence period, increasing initially and then decreasing later. Thus, foraging rates appear to be demand-driven, with the level of demand set early in the pre-emergence period by the size of nest that is constructed. The changes in foraging rates per larva suggest that colonies minimize the development times of larvae early in the pre-emergence period. Later in the period, foraging rates decrease, likely to reduce worker mortality until new workers eclose. This pattern likely responds to conflicting pressures to maximize colony growth and minimize the risk of colony death by forager attrition.

Howard, K.H. and R.L. Jeanne. 2005. Shifting foraging strategies in colonies of the social wasp Polybia occidentalis (Hymenoptera, Vespidae). Behav Ecol Sociobiol 57: 481-9. (PDF version available.)

1. Mature nest of Polybia occidentalis.	2. The same P. occidentalis nest undergoing reproductive fissioning
3. A day 28 pre-emergence nest of Polybia occidentalis

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3. Rates of brood development in a social wasp: effects of colony size and parasite infection

Colonies of social insects are at their most vulnerable during the founding stage. Because the colony's population is at its smallest during this period, comprising one or a relatively small group of adults, attrition of members of the founding unit can lead to failure of the colony. It is not surprising, therefore, that many eusocial species have evolved means to minimize the length of the founding, or pre-emergence, stage by accelerating the rate of development of the first worker offspring. The sooner a colony can begin producing workers, the less the risk of colony failure, the steeper the growth curve of the colony during the ergonomic stage, and the larger the colony will be at reproductive maturity. Swarm-founding species, whose founding units consist of hundreds or thousands of workers and whose per-capita population increases are less, may face less selection pressure to minimize the duration of the founding stage than independent-founding species. This consideration predicts that within a swarm-founding species, small founding swarms should have shorter founding stages than large swarms, by rearing a small group of precocious brood. On the other hand, evidence that large social groups organize colony labor more efficiently, gather resources more predictably, and homeostatically maintain physical conditions inside the nest within narrower ranges, predicts that larger groups should have shorter founding stages by rearing all brood more rapidly. We tested these predictions in colonies of Polybia occidentalis, a Neotropical swarm-founding wasp, by measuring brood development rates in colonies collected after 28 days of development, just short of the minimum egg-to-adult development time. We found that as size increased across colonies, pre-emergence times decreased, mean age of pupae in the nest increased, and age of the median brood (larvae + pupae) increased. That is, brood developed significantly faster in large colonies compared to small ones. Using these same measures, we also found that infection by a gregarine parasite increased brood development time, independently of colony size.

Howard, K.H. and R.L. Jeanne. 2004. Rates of brood development in a social wasp: effects of colony size and parasite infection. Insectes Soc. 51: 179-185.

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4. Novel method of swarm emigration by the epiponine wasp, Apoica pallens (Hymenoptera Vespidae)

Wasps in the Neotropical genus Apoica engage in swarm emigration behavior unlike any other swarm-founding wasp. We observed three emigrations by a colony of A. pallens in Costa Rica. Following absconding, the colony clustered on leaves as individuals flexed the terminal segments of the gaster dorsally. This posture exposed the bases of the 5th and 6th metasomal sternites, suggesting that the wasps were releasing a pheromone from the sternal glands on these segments. At sunset, wasps left in an explosion of departures and formed a diffuse swarm as they flew to a new site. No wasps marked trails by gaster dragging nor did scouts leave prior to departure of the whole swarm. Individuals at the new site ran along the substrate with the gaster held in the position of the calling display. We conclude that emigrating Apoica wasps use the calling display to guide the swarm to new nest sites.

Howard, K.H., A.R. Smith, S. O'Donnell, and R.L. Jeanne. 2002. Novel method of swarm emigration by the epiponine wasp, Apoica pallens (Hymenoptera Vespidae). Ethology, Ecology, and Evolution 14: 365-371.

1. Typical Apoica pallens nest next to a grapefruit. (photo taken by Lucille Howard)	2. Apoica pallens nest constructed on its side.
3. Black and white photo of a small cluster of Apoica pallens after the nest was destroyed. Lines indicate individuals performing the stereotypical calling display.	4. Individual Apoica pallens performing the stereotypical calling display.

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