Precocene-I mediated reduction of juvenile hormone titers and ovarian activity is ephemeral in a bumble bee

Abstract

Juvenile hormones (JHs) are key insect endocrine signals regulating pre-adult development and adult life history strategies, physiology, and behavior. The importance of this endocrine signal has led to the development of insecticides targeting JH signaling pathways. While effective against target pests, they can also affect beneficial insects, such as pollinators and natural pest enemies, many of which are hymenopterans. Research on JH, which is crucial for understanding the physiology, behavior, and organization of social insects, requires effective means to manipulate JH signaling pathways. A common method for reducing JH titers includes applying the phytotoxin precocene-I (P-I), which abolishes JH biosynthesis in the corpora allata (CA). However, achieving consistently effective topical treatments has proven challenging. Here, we investigated the dynamics of the effect of P-I manipulations on JH titers and reproductive development in orphan worker groups of a key pollinator bumble bee. A single P-I treatment effectively reduced circulating JH levels in newly emerged workers, even after 12 days. The treatment delayed but did not prevent ovarian activation. Multiple treatments failed to further reduce JH titers or ovarian activity, and were overall less effective than surgically removing the CA. Finally, we suggest guidelines for developing P-I manipulation studies, and more generally, compare methods for reducing circulating JH levels in insects.