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...hence the name parsnip webworm

Parsnip webworms spin copious amounts of silk throughout their larval development.  The amount of silk produced is impressive; of the food ingested by the webworm, nearly a quarter of the biomass and 18% of the nitrogen are destined for silk.  And, it is time-consuming--webworms spend a third of their time spinning silk.

webbing.JPG (102783 bytes) Where is the silk used?
Webworms employ their silk to  tie together their host's reproductive parts.  The larva at left has used its silk to bring together all of the umbellets in an umbel.  Umbels at the bud stage can be securely bound into a dense mass of plant tissue, completely hiding the larva or larvae within.

The larvae typically remain inside their webs, feeding on the plant tissues.  As they grow, the caterpillars must move and begin construction of new, larger webs to accomodate their increased size.

Late instar larvae are rather proprietary when it comes to their webs.  Roving larvae that attempt to enter another webworm's lair are vigorously rebuffed.  The conflicts involve strikes with the head.  In most encounters, the occupant of a web is successful at repelling an intruder.  If, however, the owner of the web is removed for a time, and another larva is inserted into the web, a fierce battle will ensue between the original owner and the new one.  Clearly, a web is an important possession and is not easily replaced.   If portions of a web are cut away but left nearby, a webworm will retrieve the silk and reincorporate it into its web.  The question that remains is why do webworms spin these costly webs in the first place? 

The adaptive value of webs.
There are several non-mutually exclusive hypotheses for the adaptive significance of webs.  One is that they protect the larvae from predators--this one has not been well-studied.  Another is that the webs and the plant material inside the webs serve to shelter the larva from UV, which activates several of the toxic furanocoumarins contained in the host plant.  A third hypothesis is that the web serves to stake out a portion of food and make it unavailable to other webworms. 
Occasionally, food can become limiting on parsnip plant.  Under those circumstances it may be beneficial for a larva to protect its food supply from conspecifics by enclosing it in a web.  To test this idea, we placed four ultimate instar larvae into cups containing artificial diet.  Some of the cups were partitioned into four quadrants with clear plastic sheeting.  In these cups the larvae were prevented from interacting.  The remaining cups were not partitioned.   Half of these cups were left alone, while the webs in the other half were removed daily.  Although food was not limiting, the webworms that had their webs removed daily gained the least body mass, while the larvae that were partitioned from one another gained the highest body mass.  Silk production was inversely related to body mass gain.  The larvae with webs removed daily produced the greatest amount of silk and those that were separated produced the least.  These results suggest that web investment is flexible and is dictated by interactions with other webworms. 

last updated 3/11/1999