There are many methods which
plants can use to deter herbivores, one way to defend from butterflies is to
appear as though the plant is already ‘occupied’. A number Passiflora species have independently developed structures on their
leaves resembling the eggs of their main herbivore, the Heliconus butterfly. Their structures are a golden colour closely
resembling the colour of butterfly eggs just before hatching, and they tend to
cluster at meristems; where the female prefers to oviposit. It is believed that
these structures have evolved specifically to mimic the Heliconus butterfly because; they are the primary defoliating
herbivore, and the females exhibit great care when assessing a plant for
oviposition sites when eggs are already evident. When Williams and Gilbert (1981) tested the validity of this
mimicry they found that Heliconus is
significantly less likely to oviposit on plants with eggs, regardless of if
they were mimic eggs or real eggs (Williams and Gilbert, 1981).
Although the butterflies did
oviposit on some of the egg mimic plants, it was substantially less often than
without the mimic eggs. This is because the female Heliconus prefer not to overcrowd a plant to reduce the chance of
caterpillar cannibalism if the Passiflora
is completely defoliated. Heliconus larva
can have a serious detrimental effect on the health of the host plant. In this
study, the Passiflora plants
subjected to herbivory exhibited suppressed flowering and an increased
incidence of root disease (Williams and Gilbert, 1981).
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| Passiflora exhibiting egg mimicry. (Gilbert, L http://www.mobot.org) |
Another slightly different
example of what can be categorised as ‘occupation mimicry’ in the butterfly
context, is that of the Desmodium
motorium plant. This plant exhibits large leaves that perform slow, deep elliptical
movements typically at night, and small stipule movements during the day. The leaves
partake in elliptical circles every few minutes, and until recently there was
no hypothesis as to why the plant had acquired this movement ability. Lev-Yadun (2013) proposes that this movement is the mimicry of a
butterfly resting (slow movements) or ovipositing (faster movements) on a leaf.
This mimicry, similar to the one mentioned above, is believed to trick passing
butterflies into thinking the plant is already being occupied and is
unavailable for oviposition (Lev-Yadun, 2013).
This mimicry technique may also
be a method of attracting butterfly predators to deter females from landing.
The elliptical movements, which are very similar in timing to that of a
butterfly, may serve as an attractant to birds, reptiles and arthropods why may
subsequently eat Desmodium motorium’s herbivores
or serve as a successful deterrent (Lev-Yadun, 2013).
This video shows the (very slow) rotation of a stipule.
Hi Emily,
ReplyDeleteIn regards to Desmodium motorium have they been able to identify the stimulus for the movement ie. light, sound, touch? A very cool trick regardless!
That example of stipule movement is a really great one demonstrating plant behaviour! This idea of “occupied mimicry” is really an intriguing one. Do other species of butterfly oviposit on Passiflora flowers? How common is occupied mimicry in the plant world and is there a higher incidence in areas such as the tropics? Great post!
ReplyDeleteSneaky plants! Do you know where the butterflies end up laying their eggs if not on these plants? Does some other poor plant get to host all the hungry, hungry caterpillars?
ReplyDeleteHarley researchers are still unsure of the exact stimulus. The leaf rotation is significantly slower at night than during the day, leading to the conclusion that light availability plays a part, and the movement has also been measured as much faster at high temperatures above 35 degrees Celsius. The assumption is that is has something to do with thy hydraulics involved in the leaf movement, it may only be optimum under certain environmental conditions regardless of if the butterflies are present.
ReplyDeleteTasmin other species of butterflies do oviposit on the Passiflora plant, but the Heliconus butterfly is believed to be the driving factor of the mimicry because its larvae have the most devastating effect on the plant and the spots most resemble the Heliconus eggs. Despite this the egg mimicry seems to work just as well on other butterfly species as it does on the Heliconus. 'Occupied' mimicry seems to pop up frequently in the literature, with the main objective to avoid herbivory, not to aid in pollination as most other cases of mimicry do.
Vanessa the Heliconus larvae can only feed on Passiflora plants, so the mimicry can only work so well. The butterflies prefer to lay on plants that are not already occupied and will continue searching for other hosts if they can. If only occupied hosts are present the butterfly will lay its eggs on an occupied Passiflora anyways. While it is an imperfect method of mimicry it still reduces herbivory enough to ensure that the Passiflora is not eaten to extinction, and left with a few tendrils to recover