Green food with a hearty mushroom side dish

A sponge spider caterpillar greedily eats the spores of a rust fungus that has infested a poplar leaf. (Photo: MPI for Chemical Ecology / Franziska Eberl)

They sometimes eat whole trees bare – but in addition to the plant food, the dreaded sponge caterpillars also destroy a previously unknown concentrated feed: the pests eat spores that form on leaves infested with harmful fungi, researchers report. The smell of leaves with the mushroom supplement also magically attracts the caterpillars. Apparently with good reason: the nutritious mushroom feed makes them grow particularly quickly. The results once again illustrate how complex the relationships between plants, insects and other organisms such as fungi can be, the researchers say.

They are four to seven centimeters long, hairy and voracious: the larvae of the moth Lymantria dispar have caused great damage to trees and shrubs in forests and in settlement areas, particularly in recent years. Because the sponge spinner likes it warm and dry. The current conditions therefore lead to fears that, in 2020, hosts of caterpillars will again plague the already stressed trees in Germany. In the forests, the oaks in particular suffer from the loss of their leaves, but sponge-moth caterpillars are not very picky: They look for many types of woody plants from May to July, including the leaves of the black poplar (Populus nigra). Using this tree species, the researchers led by Sybille Unsicker from the Max Planck Institute for Chemical Ecology in Jena are investigating the interactions of trees with their pests.

Favorite dish: mushroom spores served on the leaf

As they report, they had noticed that the trees are particularly susceptible to sponge caterpillar infestation if they suffer from rust fungus or mildew at the same time. These fungal pathogens form meshes in the leaf tissue, through which they rob the plant of nutrients and damage it. In the advanced stage, these fungi then form spore carriers on the leaf surface in order to spread. “We could see that caterpillars are attracted to the scents of poplar infested with fungi, so we asked ourselves why this is so: Would the caterpillars prefer to eat sick poplar leaves? Do you benefit from this? And if so, what chemical substances are responsible for it? ”Says the first author of the study Franziska Eberl.

Experiments in which sponge spider caterpillars were offered with and without fungal attack confirmed the clear preference of the caterpillars for the infected leaves. In the early stage of development, they even ate the fungal spores on the leaf surface before they gnawed on the leaf tissue. “Regardless of whether it is rusty fungus or powdery mildew, young caterpillars in particular have attacked the fungi and preferred to eat leaves contaminated with fungi,” reports Eberl. Chemical analyzes suggest that the caterpillars especially like the mannitol contained in the mushrooms. It is a substance that is also used as a sweetener in human food.

Special kind of concentrate

Further experiments then showed that the mushroom-based diet apparently provides the caterpillars with important nutrients: “Caterpillars that had fungi in their food develop faster and pupate earlier. This gives them an advantage over their siblings, which eat healthy leaves. Important nutrients such as amino acids, nitrogen and B vitamins probably play a role here, which were more concentrated in sick leaves, ”reports Eberl. For the caterpillars previously classified as herbivores, fungal food is also of considerable importance, the scientists conclude.

This raises the question of how widespread this mushroom eating could be. “Our results suggest that microorganisms living on plants could have a much larger role in the common developmental history of plants and insects than previously thought,” says study leader Unsicker. “It is quite conceivable that herbivorous insects have adapted to the additional resource of fungus.” Further studies are now to clarify how widespread the fungal food is in other herbivorous insect species and the importance of this additional income for the potential pests.

Source: Technical article: Max Planck Institute for Chemical Ecology, Ecology Letters, doi: 10.1111 / ele.13506

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