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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.
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 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?
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| 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.
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