If you ever caught a slow housefly with your bare hand, you most probably culled out a weak or sick member of the species. If you had a little bit of curiosity about your prey, and maybe a magnifying glass handy, you might have discovered that it was infested with parasites. Yes, little tiny insects get sick too.
Some people, like University of Wisconsin-Madison doctoral candidate Andrew Bouwma, make it their job to study the parasites that infest insects. In an August 12 lecture at the Smithsonian Tropical Research Institute’s Tupper Auditorium in Ancon, he describe his studies of how infections by protozoa of the Gregarinasina subclass affect the lives of a Polybia occidentalis, a species of wasp that is found down here.
These wasps are social animals, living proof that the Social Darwinist depiction of nature as a struggle between individuals to eat, mate and produce the most viable offspring is but a crude caricature of the Law of the Jungle. Wasps, like ants, termites, bees and other social insects, have a division of labor in which a large number of individuals are sterile, but work in harmony with a single or small number of reproductive female queens and a relatively few sexually active males to ensure the survival and propagation of the swarm, whose genes the sterile workers share. “These workers who are slaving away for the queen are doing so to propagate their genes indirectly,” Bouwma noted.
But of course, if all the individuals in a hive are closely related, an inherited susceptibility to parasite infections could spell doom for the genetic line. Yet in fact, parasite infections of wasps are common and typically do not cause extinction of the lineages that contract them.
Bouwma explained that few studies have been done on the importance of infection in the lives of social insects, and observed that at first blush the nests of such species would seem to be wonderful breeding grounds for parasites. In a wasp nest, a parasitic protozoan finds a lot of related individuals concentrated in a small space, a great invitation to the spread of disease.
It turns out, however, that Polybia occidentalis use “age polyethism” to defend itself against infection. Young worker wasps tend to the queen and her brood, and as they age they are consigned to tasks farther and farther from the queen’s chamber. Only the oldest workers leave the nest to forage for food or building materials, and by that stage they aren’t allowed anywhere near the queen. Parasite infections are typically picked up outside the nest, so the age hierarchy among the sterile worker wasps tends to prevent infections of the colony’s reproductive members and increases the mean lifespan of all of its individual members.
Polygymy, the tendency of queens to mate with several males, also increases the hive’s genetic diversity and thus limits the effect of inherited special susceptibilities to diseases.
But if it is established that certain social behaviors act as deterrents against parasites, are there effects in the other direction? Do parasites affect wasps’ social behavior?
Bouwma posited three hypotheses. First, that parasites represent a cost to social behavior. Second, that parasites represent a cost to the physical fitness of the wasps, but are neutral in their effects on social behavior. Third, that they actually favor workers’ social behavior and play a role in the evolution of the social structure.
To test these possibilities, the biologist dissected a lot of wasps and examined them for infections by gregarine protozoa, taking notes on the prevalence and extent of such infections.
This species of wasp multiplies over the course of Panama’s rainy season, until at the end of the rains, colonies divide up into separate swarms that buzz off in search of places to found their new colonies. Typically, these individual wasps live about one month, so that in the life of a colony several generations live between the establishment of a nest and its division into new swarms. In July and August, there is a heavy infection rate among the insects, but by swarming time in November and December, few infections are found.
Bouwma noted some significant gaps in scientific knowledge here. The wasps have not been studied in the dry season. The means by which gregarine infections are acquired are unknown. Research would be advanced if somebody could develop an assay to detect the presence of infection to replace the laborious task of mass dissections in search of parasites.
Nevertheless, even with the unknown factors and the slow study methods that must be employed --- knocking down a nest, which makes its inhabitants swarm, then following the swarms and taking a census, then performing lots of dissections in search of infections --- scientists have been able to discover certain trends. Highly infected colonies build smaller nests, have a lower brood weight per capita and, curiously, have a lower mortality rate.
It appears that higher infection rates reduce foraging activities, possibly because the affected workers are just too weak to forage or maybe because the infections cause hormonal changes that inhibit foraging behavior. In any case, infected wasps hardly ever go out to forage. Thus a heavy infection rate means less food and fewer building materials being brought back to the nest. It also means fewer of the older workers going outside, where fatal accidents and death due to predation are ever-present possibilities.
In any case, the observed data have been sufficient to rule out the third hypothesis stated above. Parasites definitely do not favor the expression of worker behavior.
Bouwma is in Panama as a short-term Smithsonian fellow in an attempt to find out how infections are transmitted the wasps. He came here in large part because of a 1978 study of wasp nests in the tree trunks in Gatun Lake near the railroad tracks documented lots of nests, many of them with gregarine infections present. Now, he says, there are very few nests along that route.
Just why THAT might be, especially given the several changes that the railroad and its maintenance methods have undergone since 1978, deteriorating water quality due to increase pollution of tributary streams, changes in the lake’s wildlife populations, two and one-half decades worth of the canal’s evolving weed control strategies and who knows what other variables, presents another topic for another researcher at another time.
Meanwhile, Andrew Bouwma must change his research strategy, given the unexpected change in the place where he intended to do most of his study. But isn’t investigation of the unknown and unexpected the greater part of what science is all about?
