Even the sweetest relationships can turn sour. New research shows butterflies use nectar to drug unsuspecting ants with mind-altering chemicals. The Japanese oakblue butterfly (Narathura japonica) enlists the help of ants (Pristomyrmex punctatus) to stand guard and protect its growing caterpillars (pictured) from predators such as wasps and spiders. Sweetening the deal, the caterpillars pay for their bodyguards with a sugary reward secreted from nectar glands on their backs. But a study published online before print in Current Biology suggests this nutritious gift may be a Trojan horse. The team found that ants that consumed the nectar moved less and defended the caterpillars more aggressively, charging and running around them when they inflated specialized tentacles to signal distress. The researchers also found lower levels of dopamine—a signaling molecule that regulates movement and aggression in other insects—in the brains of nectar-eating ants. Ants that spent time near the caterpillars but didn’t eat the nectar were unaffected, suggesting that caterpillars manipulate the ants with chemicals dissolved in their nectar, not visual cues or airborne chemicals. This study calls into question whether the ants are willing participants in a collaborative arrangement or merely slaves to the whims of their caterpillar masters. The authors say the caterpillars may resort to manipulation because the relationship is unbalanced; ants can live without the nectar, but caterpillars are completely helpless without their ant guards.