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We examined the oviposition preference of 23 eastern
and 25 western North American monarchs on different species of Asclepias
(milkweed), their host plant. We predicted that monarchs would preferentially
lay more eggs on milkweed species that are present in their native
habitat. Mated female monarchs were placed in a cage with four milkweed
species. Preference was quantified by counting the number of eggs
per plant, and was standardized relative to the weight of each stalk
and the total number of eggs laid by each female. Two western milkweed
species (Asclepias fascicularis and Asclepias speciosa davis) and
two eastern species (Asclepias incarnata and Asclepias syriaca)
were used in experiments. Results showed that monarchs exhibited
strong preferences for particular milkweed species, but the rank
order preferences did not vary between eastern and western populations.
Thus, although some minor quantitative differences were measured,
we found no apparent local adaptation or preferential use of endemic
host plants. However, within both populations we observed that siblings
had very similar preferences and that family effects were statistically
significant. This suggests that oviposition preference may be a
heritable or maternally determined trait.
Background
Monarch butterflies (Danaus plexippus) in North America
have different regional populations that appear to be separated
by both geographic barriers and migratory divides. Monarchs east
of the Rocky Mountains migrate to central Mexico for the winter.
Western monarchs migrate to the California coast for the winter.
There is little quantitative data that indicates how biologically
distinct these populations are. Comparing regional differences in
monarch behavior and other traits will provide vital information
towards understanding and protecting this well known butterfly species,
as recent controversies have surrounded the commercial sale and
long-distance transfer of captive reared monarch butterflies.
Specific goals of this study were to
- Determine if there is a difference in host plant preference
between eastern and western monarchs
- Examine variation in preferences within and among family groups
to assess the potential role of genetic or maternal effects
Milkweed Species We presented monarch butterflies
with a choice of 4 species of greenhouse reared milkweeds native
to eastern and western North America. All species have relatively
low cardenolide content. (Cardenolides are toxic chemicals monarchs
sequester to deter predators) Cage Design Each mating cage contained
a food source and clippings from each of the four species. The position
of the plants was rotated daily, and all stalks were removed after
100 or more eggs were laid.
Eastern and western monarchs did not show significant
differences in host plant preference. Instead, females from these
two populations preferred species in the same rank order (but note
small quantitative differences in response to A. fascicularis and
A. incarnata). Eastern Families Fig. 2. The preference of 5 eastern
families for each milkweed species measured by the average proportion
of eggs laid per gram of plant material. Each family contains 2
to 6 siblings. The overall trend shows low preference for fascicularis
and high preference for incarnata. Eastern families had a varied
response to syriaca. Western Families Fig. 3. The preference of
6 western families for each milkweed species measured by the average
proportion of eggs laid per gram of plant material. There is a high
degree of variation for davis in this population. Again, the overall
trend shows low preference for fascicularis and high preference
for incarnata.
- Monarchs showed strong rank order preference for particular
milkweed species
- There was no significant preferential difference between populations
- Preference of individual monarchs was highly varied
- Preference of siblings were similar and significantly affected
oviposition patterns
These conclusions show that eastern and western monarchs
are not highly specialized to their native host plants. They also
establish an intriguing link between monarch families and oviposition
preference. More work will be needed to investigate whether this
is primarily caused by maternal or genetic factors. Also, it will
be important to determine whether larval feeding preferences and
performance on milkweed species show similar patterns as differences
in oviposition behavior.
There are two monarch populations in North America.
one group lives on the east coast and migrates to Mexico, the other
group lives on the west coast and migrates inland in California.
I wanted to see if Eastern monarchs prefer eastern host plants and
Western monarchs prefer western host plants (plants that monarchs
lay eggs on). So, I put each female into a cage with 2 eastern and
two westerm milkweed host plants. I counted the number or eggs per
plant to determine preference. It turned out that the monarchs liked
certain species better than others, but there was no significant
difference between the eastern and western populations. However,
related (sibling) monarchs had laid a very similar number of eggs
on each species of milkweed. This shows that oviposition (egg laying)
is either a heritable triat (geneticaly determined) or it's a maternal
effect (only true for one generation).
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