Ground-cherries boost caterpillar’s immune system despite being toxic to other insects

Scientists have long been interested in the medicinal properties of ground-cherries, a curious orange fruit wrapped in a lantern husk. That’s because the fruit contains withanolides, a class of steroidal lactones that possess anticancer and anti-inflammatory properties. The plant, a member of the nightshade family that includes the tomatillo, produces withanolides as a defense against herbivores. The chemicals can, for example, disrupt insect molting. But these fruits are not toxic to Heliothis subflexa larva, a moth caterpillar that feeds exclusively on the ground-cherry species Physalis peruviana. Researchers led by Hanna M. Heidel-Fischer of the Max Planck Institute for Chemical Ecology wondered why. The team discovered that, besides helping the caterpillar gain weight, withanolides stimulate the caterpillar’s immune system by activating the expression of genes that help it fight bacterial pathogens (Nat. Commun. 2016, DOI: 10.1038/ncomms12530). This is opposite to the effect these compounds have on all other insects studied to date, the researchers note. “Beyond simply countering the plant-produced antiherbivore compounds or using them for its own defenses (for example, by sequestration), the insect has succeeded in converting the inhibitory effects of withanolides into activation effects for its own advantage,” they write.