“You should avoid this plant with the yellow flowers!” The expert view of birds obviously goes beyond recognizing poisonous caterpillars, a study shows: They learn to identify and exclude plant species that typically harbor these inedible insects from catching prey. This is a new aspect of insect deterrence strategies, say the researchers.
To avoid predators, they do the opposite of camouflage: some insects or their larvae draw attention to themselves with bright colors or patterns. The reason: You are announcing the message “Don’t eat me, because I am poisonous and taste bad!” A striking example of this concept are the striking yellow and black striped caterpillars of the butterfly species Tyria jacobaeae. They eat Jacob’s herb, which is widespread in Europe. It is a notoriously poisonous plant that can cause problems for grazing cattle. But the Jacob wort bear caterpillars do not mind the toxin, on the contrary: They accumulate it in their bodies and use it to protect themselves from predators. Because a bird usually only attacks one of these caterpillars once. He learns to combine the bad taste and the unpleasant poisonous effect with the striking yellow-black stripe pattern of the insects.
What do experienced birds pay attention to?
As part of their study, the researchers led by Callum McLellan from the University of Bristol have now investigated the question of whether the avoidance behavior goes beyond the appearance of the caterpillars. Specifically: do the birds also learn to recognize the characteristics of the yellow blooming St. James’ cabbage and to associate them with the negative experience? To investigate this, the researchers carried out experiments in the field in the Bristol area. They made caterpillar dummies to attract insectivorous birds from the area. One version showed the yellow-black stripe pattern of the Jakobskraut bear caterpillars. Purely brown prey dummies served as controls. In both versions, the researchers used Jacob’s weed and, for comparison, plants on which caterpillars and other poisonous insects are not normally found: blackberry bushes. They then evaluated which combination was associated with the most bird attacks on the dummies.
It turned out: the baits with the natural warning colors of the caterpillars – but also the “harmless”-looking versions were picked up significantly less often by insect-eating birds such as blackbirds, titmouse and co. On the other hand, on the blackberries, the birds dared to use the yellow and black striped bait comparatively often. The researchers concluded that the predators apparently also associated the plants themselves and not just the appearance of the bait with danger or safety. In order to substantiate this further, they included what is probably the most striking feature of St. James’ s herb in their experiments: They removed the yellow flowers from some of the test plants and used them to decorate the blackberry bushes.
Yellow flowers indirectly warn of poisonous caterpillars
Through these experiments, the researchers were able to confirm the warning signal effect of the yellow flowers: By removing these features, the Jacob’s herb clearly lost its deterrent effect and more dummies were attacked. Conversely, “decorating” the blackberry bushes with the St. James cabbage flowers meant that the birds in the neighborhood tended to avoid the potential prey on these plants, according to the evaluations. “The results show that birds learn that this plant characteristic is a sign of the presence of poisonous prey,” says McLellan. According to him, this behavior also makes sense: “It is probably more efficient for the birds to avoid the entire plant than to make decisions about individual caterpillars,” says the biologist.
As his colleague Nick Scott-Samuel from the University of Bristol finally points out, there is also an interesting additional benefit for the caterpillars: “Our results indicate that herbivorous insects, which specialize in easily recognizable host plants, unite regardless of their warning signal enjoy better protection from predators, ”says the scientist.
Source: University of Bristol, Article: Current Biology, doi: 10.1016 / j.cub.2021.09.048