Processionary Weevil   Phelypera  distigma                                                                                  

Weevils in the genus Phelypera are found only in Central and South America.  P. distigma is the most northerly distributed of the 14 described species.   In Jalisco, Mexico there is a single generation per year.  Overwintered adults appear in late June and early July and lay eggs on the leaves of their sole host tree Guazuma ulmifolia Lam. (Sterculiaceae).  The eggs hatch shortly thereafter and the larvae feed gregariously on the young leaves, typical aligned side by side in contact with one another.    Development is rapid; larvae reach maturity in as little as a week, then pupate on the leaves in silk covered cocoons.  When building cocoons the larvae raise the hind part of their body high above the substrate to allow the silk to play out of the anus.  Adults eclose about a week later and then, after spending a week or two feeding on leaves, disappear until the following year.

 
      

They larvae of Phelypera are highly social and are always in close contact.  When they rest between bouts of feeding, they commonly arrange themselves in circular formations.   It has been assumed that these "cycloalexic" arrangements function as antipredator defensive formations.  The larvae of P. distigma readily bite and regurgitate when disturbed whether isolated or in groups.  When cycloalexic formations consist of sufficient numbers of participants, the larva are protected from lateral attack but their lateral flanks are exposed in smaller aggregations and stink bugs are able to pierce larva and drag them from the assemblage.  There appears to be no other significant predators or parasitoids and populations of the weevil are often very large. 

Cycloalexic formations in P. distigma may have coevolved with processionary behavior and side-by-side leaf-margin feeding, both of which are orderly arrangements of larvae. Indeed, it has been shown that circular formations readily arise from and collapse into feeding aggregations. 

The adoption of a circular resting pattern places every member of the colony in intimate lateral contact with other individuals and maximizes the total amount of body contact possible in a two dimensional aggregation.  The arrangement allows a tactile signal arising anywhere within the group to be transmitted from one individual to the next and to rapidly radiate through the group.  Thus, the entire resting assemblage can be simultaneously alerted to the potential onset of a bout of foraging or to the imminent formation of a procession by even the smallest of  tactile disturbances that precede these events.  Sawflies in the genus Perga are processionary and also exhibit cycloalexy during the first three larval instars when they are small enough for the whole colony to be accommodated on the flat surface of a leaf.  Cycloalexy and processionary behavior, however, are not invariably linked, since the processionary larvae of Hylesia. lineata do not arrange themselves in circles even though they often rest in the open on the surfaces of leaves.  For more information on cycloalexic formations see references by Jolivet, et al.

 

Weevil larvae, aligned head to tail in processions, physically stimulate the tips of the abdomens of precedent individuals with their heads to promote the forward locomotion of the assemblage.  Tactile or chemotactile stimuli found at the tips of the abdomens of larvae in processions also serve to stimulate following behavior.  Thus,  tactile stimuli appear to promote processionary behavior both by drawing larvae forward from the front and by nudging them forward from behind. However, since isolated larvae of  P. distigma readily follow lures made from the eviscerated abdomen of a larva, tactile stimuli applied to the tip of the abdomen are not essential to the elicitation of locomotion.

When the larvae of P. distigma move from an old to a new site they typically travel in subgroups with stragglers sometimes not arriving at the new site until as long as an hour after the arrival of the initial contingent.  Thus, trail pheromones subserve processionary behavior by allowing stragglers to catch up with colony mates that have gotten ahead of them.  Fragmentation of colonies into permanently isolated subgroups would likely occur in the absence of the trail pheromone.

Costa, J. T., Fitzgerald, T. D., Pescador-Rubio, A., Mays, J., and Janzen, D. H. 2004. Group foraging behavior of larvae of the Neotropical processionary weevil Phelypera distigma (Boheman) (Coleoptera: Curculionidae: Hyperinae).   Ethology in press.

Fitzgerald, T. D., A. Pescador-Rubio, M. Turna, and  J. T.  Costa. 2004.  Trail making and processionary behavior of the larvae of the weevil Phelypera distigma (Coleoptera: Curculionidae). J. Insect Behavior in press.

Jolivet, P., Vasconcellos-Neto, J., and Weinstein, P. (1991). Cycloalexy: A new concept in the larval defense of insects. Insecta Mundi 4: 133-141.

Jolivet, P., and Maes, J. M. (1996). A case of cycloalexy in a curculionid: Phelypera distigma (Boheman) (Hyperinae) from Nicaragua. L’ Entomologiste. 52: 97-100.