Bivoltinism
Guides
Bessa harveyi
Bessa harveyi is a tachinid fly parasitoid that attacks the larch sawfly, Pristiphora erichsonii, in North American larch forests. The species exhibits bivoltinism, with a partial second generation emerging in fall after host larvae have completed feeding; these fall-emerging adults rarely reproduce successfully due to scarcity of alternate hosts. Despite being a common parasite, it is ineffective at preventing or terminating outbreaks of its host. Population dynamics are strongly influenced by photoperiod and temperature, with warm, prolonged summers under long-day conditions increasing bivoltinism-related mortality.