Kyoto University discovers evasive ability of caterpillars not seen in rearing containers

In rearing containers, larvae of the diamondback moth (Plutella xylostella) jump backward when they encounter ants on leaves but are eventually cornered. Graduate Student Kazuya Ito (at the time of the study, currently at Sumitomo Chemical), and Assistant Professor Shuichi Yano of the Graduate School of Agriculture at Kyoto University, have discovered that the larvae of the diamondback moth, when on a leaf, dangle by a thread from the edge of the leaf after jumping backward, and when on the underside of a leaf, they escape from ant attacks by hanging there in place. The success rate of these evasion strategies is over 80%. The work was published online in Annals of the Entomological Society of America.

Ants roam widely on the ground and prey almost indiscriminately. Therefore, some insects hide in shelters, use the nectar as a reward to tame ants, or defend themselves with thorns or poisonous substances. Among them, there is an amazing caterpillar that rivals ants in terms of athletic ability.

The larvae of the diamondback moth, which is an insect pest of plants in the family Brassicaceae, are top "athletes" in the caterpillar world. They are known to acrobatically jump backward like shrimp when they are attacked by a predator. However, when observing the diamondback moth larvae and ants placed on the bottom of rearing containers, the larvae jump backward and successfully dodge several ant attacks, but they are always caught by the ants within a few minutes. Thus, the larval movements appear to be completely futile resistance. Perhaps the rearing environment prohibits the larvae from performing the evasive action that they would in their natural habitats.

Stink bugs that faint from their own scent, predators that cannibalize their own offspring, and prey animals that feign death and expose themselves before the predator may not be exhibiting their inherent behaviors. For example, there is a small insect that falls and then freezes when it is attacked by a predator. This is an effective predator avoidance technique in a three-dimensional habitat where the insect can fall to hide, but it appears to be merely feigning death in a two-dimensional plane where it cannot fall.

The research group came up with the idea that the diamondback moth larvae are caught by the ants in rearing containers because they cannot take the inherent evasive action, but the situation must be different on the three-dimensional plants that they inhabit naturally. They placed a leaf of komatsuna (Brassica rapa var. perviridis) right next to a hole of a colony of Formica japonica in the field and individually observed the avoidance behavior taken by the larvae when they were attacked by the ants on the upper and lower surfaces of the komatsuna leaves.

The results showed that the diamondback moth larvae attacked by ants on the upper surface of the leaves jumped backward and then hung from the leaf edges by spinning threads. The diamondback moth larvae on the lower surface of the leaves hung from there immediately after the ants touched them. No ants were able to pursue the hanging larvae at the distal end of a thread. Once the ants abandon the pursuit, the larvae can follow the thread back to the feeding leaf, avoiding falling onto the ground where there are dangerous ants roaming. Although the probability that a single ant attack succeeded was not more than 20% on both the upper and lower surfaces of the leaves, this technique may be adequate to subdue the diamondback moth larvae in a natural ecosystem, given that many ants repeat the attacks endlessly. The ants attack after they touch the diamondback moth larvae with their antennae to identify their targets. This is the moment during which the diamondback moth larvae must initiate the avoidance behavior to successfully evade the ant attacks.

Previously, the backward-jumping and hanging behaviors of the diamondback moth larvae have been reported separately. In the present study, the research group showed for the first time that the sequence of the two movements is effective to evade the ant attacks and why the diamondback moth larvae use the two movements differently depending on the conditions of their location. If you observe a living organism and obtain an inexplicable result, you might want to doubt the result once as it could be because you observed a confined laboratory animal.

Yano said, "In this study, the larval movements were discovered through observation and verified through experiments by Mr. Ito, the first author. Nowadays, even graduate students spend a lot of time in passive classroom lectures, but no professors can give lectures about facts that have yet to be discovered, and no description about such facts can be found in textbooks. In biology, nature around us, which has been created by natural selection, is the best textbook for making new discoveries. Go easy with classroom learning."

Journal Information
Publication: Annals of the Entomological Society of America
Title: Rapid evasive behaviors of diamondback moth larvae against ants
DOI: 10.1093/aesa/saae031