Most years the hatch is very synchronized with peak emergence at early petal fall. However, a cold snap during this time may result in split hatch, making chemical control more difficult. Nymphs will initially feed on plant sap attained from leaf veins, but will also sting developing fruitlets. Several weeks after petal fall the nymphs become predaceous and begin feeding on small prey such as European red mite and aphids. Nymphs with red bellies are an indication that they have been feeding on mites.
Nymphs progress through five instars before becoming adults.
The adult insect migrates to mullein plants, common along ditch banks and the sides of roadways in Ontario, where they feed throughout the summer months. Season-long monitoring in apply orchards does indicate that some mullein bugs will remain in the orchard during the summer. During that time they are considered important, but often overlooked, predators of aphids and mites. In late fall, female mullein bugs return to apple trees and lay overwintering eggs into young wood. McIntosh and other cultivars seem to be largely unaffected but it is not known whether mullein bug does not generally attack fruit of these other cultivars, or if they are immune to mullein bug stings.