Potential causes and consequences of colony-level variation in foraging behavior of the red imported fire ant (Solenopsis invicta)

Background/Question/Methods

Among social insects, maintaining genetic variation in a behavior can allow a colony to exhibit more flexible, rapid responses to environmental changes and promote homeostasis. In addition, genetic variation in traits enables adaptive evolution. Our preliminary behavioral assays demonstrated significant and persistent colony-level variation in fire ant foraging behavior including variation in discovery and recruitment to resources, resource preferences, and extra-nest activity and exploration. In a number of social insects, variation in foraging behaviors and division of labor has been associated with variation in expression of the foraging gene (for), a gene encoding cGMP-dependent protein kinase G (PKG). We used a degradation resistant cGMP analog, 8-Br-cGMP, to artificially activate PKG and address the question: How does variation in PKG activity affect fire ant foraging behavior and what are the potential ecological consequences of this variation? Thirty fire ant colonies were collected from the field and divided into standardized experimental colonies. Each laboratory sub-colony was provided with either an 8-Br-cGMP treated diet or a control diet for ten days, during which colonies were assayed for discovery and recruitment to carbohydrate and protein resources as well as ant predation of caterpillars on cotton plants.

Results/Conclusions

Behavioral assays revealed that treatment with 8-Br-cGMP had a significant effect on fire ant recruitment and predation compared to control colonies. As expected, treated colonies showed significantly decreased recruitment to both a carbohydrate resource (p=0.008) and a protein resource (p=0.02), indicating that increased PKG activity decreases fire ant foraging activity. Treated fire ants also killed fewer caterpillars on cotton plants over a thirty hour interval (p=0.07). Notably, this alteration in behavior had significant effects on herbivore damage to plants. Plants placed in treated colonies had significantly higher mean damage per leaf (p=0.028), an increase from a mean of 4% to 12% damage per leaf, standardized for plant size. Currently, we are assaying fire ant colonies in the field and lab for colony level variation in PKG activity and expression of for. This study suggests that significant variation in foraging behavior exists among fire ant colonies and may underlie variation in the ecological effects of fire ants and variation in methods necessary for their control.