Angry wasp versus hungry ant

Scientists at Victoria University were amazed to witness a hitherto unknown behaviour among introduced wasps (Vespula vulgaris) – when competing with native ants (Prolasius advenus) for protein-rich food, the wasps will pick up their competitors with their mandibles, fly off backwards with them and drop them. As the number of ants on the food increases, the wasps become more frantic with their ant-tossing activities and take the ants further from the food.

The surprising behaviour was captured on film during a recent study of the wasp pests in South Island beech forests where 48 bait stations with canned tuna (a protein-rich food) had been laid to attract the insects.

The researchers captured more than 60 instances in which individual wasps physically removed the competition, carrying the 2mm ants upwards of 8cm before dropping them. Scientists call this type of behaviour by the social wasps ‘interference'.

During the course of the study, the interference behaviour was also seen in the natural environment between a wasp and a group of ants foraging in leaf litter for mealworms.

Scientists Dr Phil Lester and Dr Julien Grangier say the experience for the ants is the human equivalent of being thrown up to half the length of a football field. The ants were usually not physically hurt but appeared stunned or confused by the drop and often did not return to the bait station.

Vespula vulgaris, English wasp or common yellowjacket is a native of northern Eurasia and the northern Americas, but is now widely found throughout the world. It arrived in New Zealand sometime in the 1970s and was firmly established by 1978. The pest is on the list of the world’s 100 worst invasive species and reaches the highest known density in South Island beech forests. The wasps compete for sugar resources with nectar-feeding birds and insects and for invertebrate prey with insect-eating birds and other predacious invertebrates.

Both the ants and the wasps feed on carbohydrate-rich honeydew excreted by scale insects that infest the trunks and branches of beech trees, but both species also need some protein-rich foods (usually caterpillars or dead bugs such as cicadas).

In a David and Goliath-like battle, the researchers say that, despite being 200 times smaller, the ants are able to hold their own by rushing at the wasps, spraying them with acid and biting them.

“Eventually the wasps get so angry they pick up the ant, take it away and return to eat the food. The strategy works. It’s giving the wasp access to resources it wouldn’t otherwise have,” says Dr Lester.

“To the best of our knowledge, this behaviour has never been observed before. Our results suggest that these insects can assess the degree and type of competition they are facing and adapt their behaviour accordingly,” says Dr Grangier. “It’s a new interaction between a native and an invasive species and a wonderful example of behavioural plasticity.”

He says the wasps’ ability to tune their behaviour according to the abundance and identity of competitors could help explain why they are so widespread and invasive.

Dr Lester says other data gathered during the research suggests that ants may actually attract wasps in the first place.

“Wasps seem to hear ants ‘talking’. They have nerves in their antennae that pick up pheromones or communication chemicals given out by the ants, so it could be the foraging ants that bring wasps to the food resource. Once there, they adjust their behaviour according to the level of competition imposed by these ants.”

The research findings were published in the Royal Society journal Biology Letters on 30 March 2011.