Pyrrhalta viburni, an invasive chrysomelid native to Europe and first detected in the USA in 1994, is becoming a major landscape pest in the Northeast and poses a serious threat to a large portion of the U.S. Larvae and adults feed on shrubs in the genus Viburnum, and plants in both managed landscapes and natural areas are at risk. Plants belonging to susceptible species are often killed within a few years, but plants belonging to resistant species are rarely heavily defoliated. P. viburni adult females lay eggs in groups in small cavities they excavate in the twigs. Females prefer to lay their eggs on branches already infested, and will lay their eggs in close proximity to egg masses already present, forming aligned clusters of egg masses along infested twigs. Two potential benefits of this aggregative oviposition behavior were examined: larval group feeding and overcoming plant defense. In the laboratory, survivorship of P. viburni larvae was reduced when larvae were reared individually rather than in groups of 5 and 10, and resultant adults were smaller when reared in groups of 5 rather than in groups of ten. These findings were consistent for all the viburnum species used as hosts. Analysis of egg mass densities on field-collected Viburnum dentatum twigs revealed that plant defense rate was negatively correlated with magnitude of infestation, suggesting that egg survivorship might be higher on heavily infested twigs. Results of this research shed light a new aspect of P. viburni biology, and add to our understanding of the ecology of this important new pest.