Biology Reference
In-Depth Information
1992). A second species, the goldenrain tree (
Koelreuteria paniculata
), has
been introduced farther north, where the native host is the soapberry tree.
Soapberry bugs feeding on this tree now have beaks averaging about 8%
longer than those feeding on the soapberry tree. Another alien plant, the
heartseed vine (
Cardiospermum halicacabum
), has also been introduced to
areas where the native plant host is the soapberry tree. Its fruits are still
larger, and soapberry bugs feeding on this plant exhibit beaks averaging
almost 17% longer than those on the soapberry tree. Laboratory experi-
ments have shown that beak length has a genetic basis (Carroll and Din-
gle 1996). Thus, the shift to new plant hosts has been accompanied by
rapid evolutionary adjustment of beak length to fruit wall thickness.
Additional studies of soapberry bugs on the native balloon vine and
the alien small-fruited flamegold tree have shown that fecundity and juve-
nile survivorship are now greater for each population on its own plant
host (Carroll et al. 1998). Furthermore, soapberry bugs living on the
flamegold tree mature more rapidly and lay twice the number of eggs as
those reared on the balloon vine (Carroll et al. 2001). Although the eggs
laid by soapberry bugs on flamegold trees are smaller than those of ances-
tral bugs living on balloon vine, they achieve a much greater lifetime
reproductive effort on this host than do ancestral bugs on the balloon
vine. Thus, physiological adjustments as well as morphological changes,
have occurred in the evolutionary shift of soapberry bugs to their new
hosts. These adjustments have occurred over a period of about 100 gen-
erations.
Further studies have revealed still other life history adjustments by
soapberry bugs on balloon vine and flamegold tree (Carroll et al. 2003a).
These are related to the seasonal pattern of seed production and availabil-
ity of the two host plants. Balloon vines tend to produce small seed crops
at various times throughout the year, with considerable variability and
asynchrony from plant to plant. Soapberry bugs are thus able to move
from plant to plant and to remain active throughout the year. Flamegold
trees simultaneously produce larger crops of seeds in late November and
December, with seeds persisting only until March. On flamegold trees,
soapberry bugs are forced to enter a diapause for much of the year
because of seed unavailability.
Because of these striking differences in seed production by the
native balloon vine and introduced flamegold tree, Carroll et al.
(2003a) investigated reproductive patterns of soapberry bugs on the
two hosts. On both hosts, long-winged females, capable of flight, and
flightless, short-winged female morphs existed in about the same fre-
