Rosemary (Salvia rosmarinus), an evergreen shrub native to the Mediterranean, is widely recognized for its robust, pine-like fragrance. This powerful scent comes from a high concentration of volatile oils stored within the plant’s needle-like leaves. While the herb’s aroma is often used to repel a wide range of common garden pests, it is a misconception that rosemary is universally avoided by insects. The plant actively attracts two distinct groups of insects: those seeking nectar and pollen, and a smaller, highly specialized set of insects that have evolved to overcome its chemical defenses.
Primary Attractants: Pollinators
The small, often pale blue to purple flowers that bloom on rosemary plants are a notable source of forage for beneficial insects. These flowers are structurally designed to be easily accessible to generalist pollinators, providing a reward of both pollen and nectar. The plant is particularly attractive to these insects because it frequently blooms during the cooler months of late winter and early spring.
During these periods, other flowering plants may be scarce, making the rosemary bush a reliable early-season food source. Various species of bees, including domestic honeybees and numerous solitary bee species, are the primary visitors. The bees rely on the plant’s nectar to fuel their colonies and the pollen to feed their developing young.
The aroma itself acts as a long-range chemical beacon, guiding these foraging insects directly to the flower clusters. Other generalist pollinators, such as certain species of butterflies and flies, will also visit the rosemary flowers.
Specialized Pests of Rosemary
Although rosemary is a natural insect repellent for many garden varieties, a few specialized insects are specifically attracted to it as a host plant for feeding and reproduction. The most recognized of these pests is the Rosemary Beetle (Chrysolina americana), which is visually striking with its metallic green and purple stripes. This beetle, despite its name, is native to Southern Europe and has become established in other regions. Both the adult beetles and their larvae feed voraciously on the plant’s foliage.
The larvae of the Rosemary Beetle are soft-bodied, greyish-white grubs that cause significant damage by skeletonizing the leaves. Another common pest is the spittlebug, which creates a characteristic frothy white mass, often called “cuckoo spit,” on the stems and leaf axils. The nymph hides within this foam as it pierces the plant tissue to suck out the sap.
Other sap-sucking insects, such as scale insects and certain species of thrips, can also be drawn to rosemary, especially when the plant is stressed. The Southern purple mint moth (Pyrausta laticlavia) is also documented as a host-specific insect, with its caterpillars feeding along the stems. These specialized pests have evolved mechanisms to detoxify or ignore the compounds that deter other herbivores.
The Dual Nature of Rosemary’s Aroma
The powerful fragrance of rosemary is derived from a complex blend of volatile organic compounds (VOCs), primarily monoterpenes, which serve as the plant’s primary defense mechanism. The concentration of these potent chemicals repels generalist insect herbivores that lack the physiological tools to process them. This is why rosemary is often used in companion planting.
Major Volatile Compounds
- 1,8-cineole (eucalyptol)
- Camphor
- Alpha-pinene
- Camphene
This chemical profile creates a paradox, as the same compounds that repel most insects are used as specific cues by others. For pollinators, the VOCs act as an unambiguous scent trail, signaling the presence of a nectar reward. Specialized pests like the Rosemary Beetle have adapted to these chemicals, which now serve as host-location signals, identifying a safe food source free from competition.
Scientific studies have demonstrated that certain extracts of rosemary can attract specific moths, such as the grape berry moth (Lobesia botrana). Furthermore, individual components, like 1,8-cineole, have been shown to attract pests such as the western flower thrips (Frankliniella occidentalis). Rosemary’s aroma is a sophisticated chemical signature that selectively attracts beneficial insects and highly adapted pests while broadly deterring everything else.