Red Imported Fire Ant - Queen and Worker Behavior

Queen and Worker Behavior

Studies have been conducted on the sex ratios exhibited within colonies of Solenopsis invicta. More specifically, it was observed that the queen actually predicts the sex ratios. In an experiment, 24 field colonies were selected with highly biased sex ratios in a monogyne population. Eleven of these colonies were male specialists (numerical proportion of males, range: .77 to 1.0), and 13 were female specialists (numerical proportion of males, range: 0.0 to .09). After exchanging queens, twenty-two of the 24 colonies accepted the foreign queen, and 21 of these colonies produced a new batch of reproductive 5 weeks later.

Luc Passera and colleagues “found that the sex ratios produced in a colony post-switching was predicted by the colony from which the queen came.” For example, post-switching, a colony produced predominantly males if the queen came from a male-producing colony, and vice versa, regardless if the host colony originally produced mainly females. “By contrast, no significant change in sex ratio occurred in control colonies in which male-specialist and female-specialist colonies were given a queen from the same colony type.”

Another study compared the inhibition of the number of sexuals (male and female) produced in a single queen colony and a queenless colony. Pheromones were tested to have an impact—freshly killed corpses of functional (egg-laying) queens were added daily to queenless colonies. These effectively inhibited the production of sexuals, although not as effectively as living queens. Conversely, corpses of non-egg-laying queens did not inhibit the production of sexuals. Also, the addition of queens to previously queenless colonies, which had already developed large sexual larvae, resulted in the execution of most of these larvae by workers. This indicated that the queen’s control over the production of sexuals can act retroactively, even after the larvae are sexualized. The results provide evidence that functional queens exert control over the production of sexuals in S. invicta through pheromones that influence the behaviors of workers toward both male and female larvae.

The fire ant Solenopsis invicta also presents a paradox for kin selection theory. In multiple-queen (polygyne) colonies, the egg-laying queens are on average unrelated to one another, so the workers appear to raise new sexuals that are no more closely related to them than are random individuals in a population. This was tested by removing worker/queen pairs engaged in trophallaxis with foreceps, and then sampling the allele frequency to estimate for the reference population. Frequencies of the most common allele at each locus in workers from the study colonies are .89, .92, .78, .71, and .81 respectively—the frequencies of these markers have been found to conform to Hardy-Weinberg expectations in past studies. Genotypic data were used to estimate relatedness between the workers and the winged-queens they tended, and it was indistinguishable from zero. The results indicate that S. invicta workers tending queens in polygyne nests do so without respect to the relatedness of those queens.

It has been observed that S. invicta workers not only tend to queens indiscriminately, but they also indiscriminately attack them. After temporary cooperation associations end between queens, a queen who produced more workers gained no advantage over the less productive queens. Queens producing diploid males reared fewer offspring but were as likely to survive as queens producing only workers. It would have been assumed that if workers controlled queen mortality, they would be expected to discriminate in favor of their mother, therefore increasing their inclusive fitness. This however should favor the queen with the greatest number of daughters during the period of queen execution. The data actually shows that the fights among queens themselves have a strong role in determining which queen survives—the heavier co-foundress was more likely to win. Thus, queen survival is enhanced by high fighting ability relative to co-foundresses, rather than by the number of offspring she has. Workers respond to these queen differences by attacking the previously injured queen to reinforce the effects of competition among the queens.