Floral feeding increases diet breadth in a polyphagous mirid

Abstract

Herbivorous insect species vary significantly in the number of host plant taxa they attack. Previous studies suggested host range expansion to be more likely to occur in herbivores whose immatures feed on flowers due to flowers containing lower levels of plant defense compounds than vegetative tissues. We tested whether the polyphagous mirid bug Apolygus lucorum, an important pest of cotton and fruit crops, could increase its diet breadth by feeding on flowers. In field plots, the density of A. lucorum nymphs was higher at the flowering stage than at the vegetative stage in the eight plant species included in this project during 2009–2016. In two laboratory studies, adults preferred to lay eggs on the plants at flowering stage over those at vegetative stage and immatures failed to develop on the vegetative tissues of five of the eight species. Flowers of the same plant species, however, sustained successful development of the mirid nymphs. Therefore, we compared the levels of secondary compounds and main nutrients in flowers and vegetative tissue of the eight plant species. Six out of the eight plants tested contained lower levels of secondary metabolites in flowers than in vegetative tissue, and all contained more sugars in flowers than in vegetative tissue. In a manipulative assay, A. lucorum nymph survival increased when fed on vegetative tissues with sucrose. Hence, a higher sugar content of flowers, not just a lower content of plant defense compounds, allows A. lucorum to adapt its diet to a wider variety of plant species.