Changes in dopamine and octopamine levels caused disordered behaviour in red imported fire ants exposed to cinnamon essential oils

Some plant essential oils (EOs) are environmentally friendly insecticides because they can kill insects by manipulating the social behaviour of the red imported fire ants. In this study, we found that the main component of cinnamon oil, trans-cinnamaldehyde, which is extracted from the bark and leaves of Cinnamomum loureirii Nees and Cinnamomum cassia Presl, could cause leg-antennal and double-antennal grooming, as well as fighting behaviors in red imported fire ants. In particular, the bark oils not only produced strong fighting behaviour but also accelerated the death of the worker ants. Analysis of the contents of neurotransmitters (octopamine and dopamine) in red imported fire worker ants treated with trans-cinnamaldehyde, the main component of cinnamon oil, showed that trans-cinnamaldehyde destroyed the antennal receptor structure, causing a decrease in octopamine content and in the ants' ability to recognise their nest mates. Trans-cinnamaldehyde stimulated the synthesis of dopamine, thereby enhancing the aggression of the worker ants. Indeed, the relative mRNA expression levels of two genes needed for dopamine synthesis, tyrosine hydroxylase and DOPA decarboxylase, were significantly upregulated compared with those of the control group (CK) from the beginning of the fight to the peak of the fight. The ratio of octopamine to dopamine reached 1:86 during the peak of the fight. The findings revealed that trans-cinnamaldehyde acts on the nervous system of red imported fire ants to interfere with the behaviour of the workers, thus providing a strategy to control the spread of red imported fire ants using cinnamon EOs.