Creature Feature: Charoxus Rove Beetle

Rove beetles are insects within the family Staphylinidae which includes around 63,000 species and thousands of genera. One of these genera in particular, Charoxus, appears to take advantage of the very tight mutualistic relationship between fig trees and their pollinating wasps.

We know a lot about the fig-wasp mutualism, yet remarkably little is known about what role Charoxus rove beetles play in this system. The adult beetles lay their eggs within fig fruit and the larva that develop eventually consume the adult wasps that also developed in the same fruit. [Taken by: Sydney Schalk]

Fig trees, those in the genus Ficus, are pollinated by wasps in the family Agaonidae. Most, but not all, fig tree species are pollinated by only one species of wasp. The adult female wasp will climb into the fig fruit and lay its eggs inside, losing its wings in the process and dying inside the fig. When the eggs hatch, the new wasps will mate inside the fig. The males will never leave the fig in which they hatched. The females, on the other hand, will  exit the fig in which they developed to lay its eggs inside of another fig, bringing along pollen with it in the process and fertilizing the new fig by crawling around inside. This is possible because a fig itself is made up of many small flowers that face inward. In this kind of relationship, or mutualism, each species involved benefits from the association. A relationship like this, however, creates the opportunity for other species to take advantage of it, and a few do. Parasitic wasp species are well known to lay their eggs inside of the figs without ever having to enter and fertilize the flowers inside [1]. In the neotropics, rove beetles in the genus Charoxus have also been observed to take advantage of this relationship.

An adult Charoxus rove beetle seen under a microscope. The genus consists of at least 6 species, distributed from Florida to Central America. [Taken by: Sydney Schalk]

The genus Charoxus contains 6 species that have been described, and these species are found from Florida all the way into Central America [2],[3]. Adult Charoxus beetles are thought to enter the figs to lay eggs and subsequently block the hole through which they entered so the wasps cannot escape [4]. The beetle larva hatch within the fig and feed on the adult fig wasps inside. They have also been observed to display cannibalistic tendencies in a laboratory setting. The larva will then drop from the fig and pupate in the soil [5]. The pupa are able to move independently, potentially to avoid light and move under the soil. Adult rove beetles have also been observed preying on adult fig wasps when given the opportunity, but it is unknown whether they are a significant part of their diet in nature.

Watch as a Charoxus beetle attacks an adult fig wasp. This predatory activity has been observed in laboratory settings but has not yet been confirmed in the wild. [Taken by: Sydney Schalk]

The rove beetle life cycle appears to temporally synchronize  with the life cycle of the fig trees, and individual figs tend to only contain adults or larva. However, in another study done in Miami, researchers found the beetles to be far more abundant in May and rare the rest of the year [5]. In the research project I conducted in Monteverde, Costa Rica as part of a University of California Education Abroad Program (UCEAP), the beetles were found in the vast majority of figs I observed in May of 2018, however, samples were not taken at any other time.

A Charoxus larva seen under a microscope; the rove beetle life cycle is thought to be synchronized with fig fruiting patterns, although this has not yet been examined extensively across Charoxus range. [Taken by: Sydney Schalk]

A lot is known about the relationship between figs and fig wasps, however there is still a lot to learn about the role these Charoxus rove beetles play in the system.  It is unclear how common these beetles are and what impacts they might have on fig wasp populations. It would also be interesting to study the other life stages of these beetles, as it seems possible that they are only associated with the fig trees during reproduction. It is also possible that more species in this genus are still undiscovered throughout the neotropics and potentially beyond. Hopefully future research will uncover more about these interesting creatures.

Written by: Sydney Schalk, a 2nd year transfer student studying Wildlife, Fish and Conservation Biology (WFCB) at UC Davis. Sydney studied rove beetles as part of UCEAP’s study abroad program in Monteverde, Costa Rica. I am interested in insects and hope to eventually pursue a career in field biology.


References:

  1. Cook, J. M., & Rasplus, J. Y. (2003). Mutualists with attitude: coevolving fig wasps and figs. Trends in Ecology & Evolution, 18(5), 241-248. https://doi.org/10.1016/S0169-5347(03)00062-4
  2. Frank, J. H., & Thomas, M. C. (1996). A new species of Charoxus (Coleoptera: Staphylinidae) from native figs (Ficus spp.) in Florida. Journal of the New York Entomological Society, 70-78. https://www.jstor.org/stable/25010202
  3. Kistner, D. H. (1981). The reclassification of the genus Charoxus Sharp with the description of new species (Coleoptera: Staphylinidae). Journal of the Kansas Entomological Society, 587-598. https://www.jstor.org/stable/25084197
  4. Bronstein, J. L. (1988). Mutualism, antagonism, and the fig‐pollinator interaction. Ecology, 69(4), 1298-1302. https://doi.org/10.2307/1941287
  5. Frank, J. H., & Nadel, H. (2012). Life cycle and behaviour of Charoxus spinifer and Charoxus major (Coleoptera: Staphylinidae: Aleocharinae), predators of fig wasps (Hymenoptera: Agaonidae). Journal of natural history, 46(9-10), 621-635. https://doi.org/10.1080/00222933.2011.651641

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