With a 1350 km coast scattered with seaports, California is a major trade hub, but flourishing imports also permit uninvited guests to hop ashore. Argentine ants (Linepithema humile) first landed in the Golden State in the 1890s in shipments of sugar and coffee. With no natural predators, the invaders thrived, establishing a vast megacolony – stretching more than 1000 km from San Diego to northern California – comprising thousands of nests. The secret behind the Argentine ants’ invasive success lies in their relaxed reactions to visitors from other megacolony nests. Coated in similarly scented wax blends, megacolony visitors smell unthreatening, in contrast to ants from other colonies, which are attacked. As well as defining friend from foe, the waxy coating protects ants from dehydration. However, Neil Tsutsui from University of California, Berkeley (UCB), USA, explains that selecting specific combinations of waxes for their aroma might impair the mixture's waterproofing, so might megacolony residents from distant nests have fine-tuned their waxy coatings to focus on protection from dehydration in drier climes?
Gathering Argentine ants from megacolonies from locations in northern and southern California, as well as nests from three unrelated smaller supercolonies in southern California, Tsutsui and his team transported the ants back to his Berkeley lab where they constructed tiny homes for mini ant colonies of 20 individuals, to test how well the insects from all across the state withstood desert-like conditions, normal humidity air and high humidity.
Sure enough, all of the ants in humid homes survived well, but the insects in the simulated desert succumbed quickly (87% died within 16 h). And when Brian Whyte (UCB) analysed the ants’ survival rates in air with normal hydration levels, those from megacolony nests in northern California coped better than megacolony ants from southern California, while the ants from the three southern supercolonies struggled most. However, when the air was super dry, all of the ants from the megacolony are able to withstand arid conditions better than the other Argentine ants, possibly contributing to their extraordinary success. But what was the source of their resilience? Had the megacolony residents fine-tuned their waxy waterproof cocktail for better protection?
Jan Buellesbach (currently at University of Münster, Germany) washed individual ants in solvent to collect their unique wax blends and then analysed the components, suspecting that the waterproofing cocktails of the ants from the driest nests in southern California would contain more solid waxes and fewer oily compounds than those of ants from the north. However, when Whyte analysed the composition of the waterproof mixtures from the different nests, he was surprised that the ants from drier regions had not adjusted their waxy cocktails to improve protection. Smelling the same as other members of the megacolony is probably a higher priority. Some other factor must be responsible for the ants’ ability to withstand drought. Might body size contribute more to the invaders’ desiccation resilience?
Collecting ants from the laboratory simulated desert conditions, Rebecca Sandidge (UCB) weighed the insects and compared their size with their drought resilience. The largest ants – all of which originated from megacolony nests – coped better than those from the supercolonies obtained from southern California, probably because the megacolony ants have a smaller surface area relative to their body size – reducing water loss – than the supercolony residents.
Body size, rather than the composition of their waterproof coating, is a key factor in the ability of invasive Argentine ants to survive dry environments and the team is keen to find out whether size also contributed to the ants’ successful takeover of the state of California.