On the evolutionary arms-race between Utetheisa ornatrix moth and its Florida host, Crotalaria pumila: chemical attraction, and mechanical defense

Authors: SOURAKOV, ANDREI - University Of Florida; Alborn, Hans

Submitted to: Tropical Lepidoptera
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2017
Publication Date: 6/22/2017
Citation: Sourakov, A., Alborn, H.T. 2017. On the evolutionary arms-race between Utetheisa ornatrix moth and its Florida host, Crotalaria pumila: chemical attraction, and mechanical defense. Tropical Lepidoptera. 27:16-18.

Interpretive Summary: Crotalaria spectabilis, C. lanceolata and C. pallida are non-native legumes introduced to the New World from Asia and Africa in the 20th century. All three have spread rapidly throughout southeastern United States, and while originally they were introduced as cover crops, they are toxic to birds and large mammals including cattle. They are highly invasive in many natural habitats and thus need to be controlled. Fortunately, since plants’ introduction, the native moth, Utetheisa ornatrix, has adopted them as hostplants. The moth’s caterpillars effectively feed on exotic Crotalaria as well as its native host C. pumila, destroying not only their foliage, but also, and more importantly, their seeds. Thus this is a great example of natural control of invasive organisms by a native species. We have been trying to understand plant-moth interactions and especially what signals the moth uses to find and select suitable hostplants. We identified volatile compounds released by whole plants as well as by seed pods. The latter are of special interest as caterpillars search out and feed on alkaloid-rich seeds to gain extra protection from predators. Caterpillars also sequester alkaloids, so that adult moths are also protected and synthesize their pheromones from these chemicals. We found that despite a great variation in the volatile profiles among the four Crotalaria species mentioned above, their pods had several compounds in common of which a subset might function as attractants.

Technical Abstract: While Utetheisa ornatrix larvae are able to develop through feeding only on foliage of its hostplants in the genus Crotalaria, in later instars they are attracted to seeds as a richer source of alkaloids. Recently, it was demonstrated that seeds receive different degrees of mechanical protection from the larvae as provided by the surrounding pericarps. In the present paper we demonstrate that pods of C. pumila, a host native to the moth’s range, attract larvae away from the foliage, which in turn slows down their development as they expand time and energy on breaking through the pericarps instead of feeding. Hence in this closer-to-natural scenario, gaining extra alkaloids through seed-feeding, while has many demonstrated advantages for the adult moths, can also place negative selective pressure on caterpillars. In Crotalaria pumila, pods are small and scattered, so to understand how larvae locate them on a plant, we analyzed the volatile chemicals that are produced by these pods and compared them to those produced by the foliage and by pods of three other (non-native) Crotalaria: C. spectabilis, C. lanceolata and C. pallida, which too widely are utilized by moths as hosts in Florida. The volatiles coming from pods are a much more diverse and complex mixture of chemicals than those coming from foliage. Pods of Crotalaria species we tested produce species-specific chemical profiles, yet they had several compounds in common: , <(E)-beta-ocimene>, , , <2-Pentanone, 4-methyl-, oxime>, <2-Methylpropanal oxime>, and . It is likely that one of these compounds or a combination of several of them contain cues that are used by larvae while searching for pods.