Halyomorpha halys (Stål), commonly known as the brown marmorated stink bug, is an invasive and economically damaging insect pest in U.S. agriculture. To date, H. halys has been found in 42 U.S. states and two Canadian provinces. The stink bug feeds on more than 300 plant species, many of which are important agricultural crops including apples, peaches, persimmons, blackberries, sweet corn, field corn, and soybeans. As a species that has the potential to cause significant economic damage, having H. halys colonies maintained in a laboratory setting for research purposes is critical. While cannibalism occurs in both predatory and phytophagous insects, only two phytophagous pentatomids have been reported to be cannibalistic. After observing cannibalistic oophagy within our H. halys colony, we sought to identify the effects of this behavior on the hatch rate of eggs. Laboratory-reared H. halys egg masses were exposed to either second- or fourth-instar H. halys nymphs for varying lengths of time, after which the proportion of eggs hatched was determined. Both the number of days an egg mass was exposed to nymphs and the age of the nymphs affected egg mortality and hatch rate. This knowledge is useful to researchers attempting to maintain healthy, stable populations of H. halys in laboratory colonies and aids in developing a more successful rearing protocol. Those managing a H. halys colony should be aware that cannibalism may occur in H. halys and take appropriate measures to minimize the impact of this behavior.