Not just candy: A plant protein in insect honeydew boosts a parasitic wasp’s fitness

Pablo Urbaneja-Bernat, Cesar Rodriguez-Saona, M. Luz Valero, Joel González-Cabrera, Alejandro Tena

This is a plain language summary of a Functional Ecology research article which can be found here.

When herbivores attack, plants often activate defense mechanisms, leading to the accumulation of secondary metabolites in the plant’s phloem. These metabolites, induced by herbivory, can have detrimental effects not only on the herbivores themselves but also on their natural enemies. These plant defensive metabolites may also be present in the honeydew excreted by phloem-feeding insects, potentially reducing the survival and reproductive success of the herbivores’ natural enemies that feed on honeydew. This is notable because honeydew serves as a primary and easily accessible carbohydrate source for natural enemies across various agroecosystems, potentially influencing tri-trophic interactions.

In this study, we investigated the impact of honeydew from a mealybug (Planococcus citri) on the longevity and fecundity of its main parasitoid (Anagyrus vladimiri). Additionally, we collected honeydew from seven phloem-feeding insects and analyzed their protein content. Finally, we examined the effect of superoxide dismutase (SOD), an antioxidant enzyme associated with plant defenses commonly found in the analyzed honeydews, on the longevity and fecundity of the parasitoid.

Contrary to the conventional understanding, our findings reveal that honeydew from the mealybug enhanced the fecundity of its parasitoid, suggesting that honeydew contains proteins with high nutritional value. The proteomic study confirmed that honeydew from all the tested phloem feeders contained proteins related to plant defenses. Our final experiment demonstrated that the addition of SOD to a carbohydrate diet enhanced the fecundity of the parasitoid.

Schematic diagram summarizing key findings of our study (Credit: Lindsay Erndwein and the authors)

This study uncovers a previously undocumented interplay within ecological food webs, wherein plants may confer benefits to the natural enemies of phloem feeders through induced proteins present in excreted honeydew.