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Processionary Weevil   Phelypera  distigma                                                                                  

 
                                                                                                 
Weevil adult

Overview and Life History
Circular Formations
Trail Making and Processionary Behavior
References







The P. distigma larvae to the left are traveling in a loose procession along a branch of their host tree. They are moving unusually fast because the branch is exposed directly to the sun and their body temperatures are in excess of 35 degrees Celsius. When the larvae lose tactile contact with the caterpillar immediately ahead of them, they rely on the trail pheromone to maintain their place in the procession.



Overview and Life History



Processionary behavior is a form of locomotion in which larvae travel together in single file, often in head-to-tail contact.  While recent studies show that processionaries employ trail pheromones, contact stimuli associated with the bodies of siblings are essential to the initiation and maintenance of processions.  Within the Insecta,  processioning is known only from the social caterpillars and from the single species of weevil larva described here.  In the Lepidoptera, it occurs most commonly in the subfamily Hemileucinae of the Saturniid and has been investigated in the genera Hemileuca and Hylesia.  It is also particularly well developed in the Notodontidae genera Thaumetopoea and Ochrogaster.  Sawfly caterpillars in the genus Perga are the only non-lepidopteran species previously reported to move together in procession-like formations but their behavior has yet to be studied.
 While processionary behavior is uncommon in the Insecta, many species of social insects engage in trail-based foraging behavior. Trail marking is ubiquitous among the ants (Formicidae) and termites (Isoptera) and has been reported from the social caterpillars of a number of lepidopteran and symphytan species as well.  Despite the vastness of the Coleoptera and the great diversity of behaviors found among its members, it is noteworthy that the larvae of the curculionid weevil, Phelypera distigma (Boheman)  is the first beetle shown to mark and follow chemical trails and to engage in processionary behavior.


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.
larva spinning cocoon

  

cocoons
Weevils
Emerged adults
 
      

Circular Formations

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. 

formation of circular resting aggregation


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 in circle
Weevils in circle
Stinkbug attacking weevils

 



Trail Marking and Processionary Behavior

trail following


Larvae deposit trails as they advance over the substrate that stimulate and orient the locomotive behavior of conspecifics.  The marker appears to be secreted from ventral surface in the region of abdominal segments 5 and 6, but a definitive secretory site has not been identified. Although the prepupal larvae of P. distigma secrete silk from anal glands to create protective shells when they pupate, the earlier instars do not produce silk and the material is never used as a component of the larval pathway as it is in the Lepidoptera.  Trails laid down on leaves can not be washed away with water indicating that they are largely water insoluble. Pathways of larvae moving in tandem elicit trail following by other larva placed at their origins for approximately two hours after their deposition. 


Weevils in procession



 


 

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.
Following model


                                                                                                    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. 
Weevils following model



References

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.

This material is based upon work supported by the National Science Foundation under Grant No. INT 0205841. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).



 
 


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