Visual and olfactory sensory systems employed by monarch butterflies (Danaus plexippus) to locate their milkweed host plants

Abstract

Monarch butterflies (Danaus plexippus) are well known to depend almost exclusively on milkweed (genus Asclepias) host plants for oviposition sites, as their larvae need to ingest compounds critical for adult butterfly chemical defense against predators. Many phytophagous insects make fast and accurate decisions on the quality of host plants, and whether or not to accept them as oviposition sites. We were interested in the cues that attract monarchs (males and females) to milkweed, while in flight, before contact stimulation can occur. We developed a novel flight apparatus to test monarch attraction behaviour, under a number of different experimental protocols, to both milkweed and control stimuli in order to identify the cues necessary or sufficient for monarch attraction to milkweed. Monarchs were found to be attracted to milkweed stimuli when the visual image alone of milkweed was available or when olfactory cues alone were available. Attraction behaviour was maximized when both cues were available. It was also discovered that attraction to milkweed was significantly diminished when ultraviolet reflection from milkweed leaves was prevented from being detected by monarchs in flight.