Yellow crazy ants could threaten butterflies

Yellow crazy ants can form huge colonies, force out native ants, and kill other animals such as crabs. Now scientists have found evidence that these invasive bugs may have another target: butterflies.

The ants have taken over hundreds of hectares of Australia’s Wet Tropics World Heritage Area, a rainforest region rich in butterfly species. Butterfly larvae are often found on plants, where yellow crazy ants also like to forage.

A team of Australian scientists wanted to find out whether the crazy ants were more likely than native ants to attack butterflies. So the researchers set up experiments with two types of plants, a weedy vine and a native vine. The team placed containers of the plants in the rainforest and put caterpillars — larvae of the native cruiser butterfly — on the leaves. Then the scientists observed yellow crazy ants and native green tree ants on the plants for the next few hours.

The yellow crazy ants were much more active than the green tree ants, the team reports in Biotropica. The invaders foraged on 77 percent of the weedy vines and 88 percent of the native vines. Sometimes, 20 ants swarmed a single plant. Meanwhile, the green tree ants foraged on 40 percent of the weedy vines and 30 percent of the native vines, and no more than four ants occupied one plant.

Similarly, the caterpillars were more likely to be attacked by the invasive ants. Yellow crazy ants attacked 53 percent and 88 percent of the caterpillars on the weedy and native vines, respectively. The green tree ants attacked only 3 percent and 20 percent of the caterpillars on those plants, respectively.

Other types of butterflies also could be at risk. For instance, the Seychelles — another site of yellow crazy ant invasion — harbors more than 500 butterfly and moth species. Scientists haven’t studied the effect of invasive ants on butterflies much, but these new results suggest that yellow crazy ants may be even more of a menace than people thought. Roberta Kwok | 3 December 2015

Source: Lach, L. et al. 2015. High invasive ant activity drives predation of a native butterfly larva. Biotropica doi: 10.1111/btp.12284.

Image © Steve Shattuck | Flickr (CC BY-SA 2.0)