Questions 22-32 are based on the following
passage.
This passage is adapted from Elsa Youngsteadt, “Decoding a Flower’s Message.” ©2012 by Sigma Xi, The Scientific Research Society.
Texas gourd vines unfurl their large, flared
blossoms in the dim hours before sunrise. Until they
close at noon, their yellow petals and mild, squashy
aroma attract bees that gather nectar and shuttle
5 pollen from flower to flower. But “when you
advertise [to pollinators], you advertise in an
open communication network,” says chemical
ecologist Ian Baldwin of the Max Planck Institute for
Chemical Ecology in Germany. “You attract not just
10 the good guys, but you also attract the bad guys.” For
a Texas gourd plant, striped cucumber beetles are
among the very bad guys. They chew up pollen and
petals, defecate in the flowers and transmit the
dreaded bacterial wilt disease, an infection that can
15 reduce an entire plant to a heap of collapsed tissue in
mere days.
In one recent study, Nina Theis and Lynn Adler
took on the specific problem of the Texas
gourd—how to attract enough pollinators but not
20 too many beetles. The Texas gourd vine’s main
pollinators are honey bees and specialized squash
bees, which respond to its floral scent. The aroma
includes 10 compounds, but the most
abundant—and the only one that lures squash bees
25 into traps—is 1,4-dimethoxybenzene.
Intuition suggests that more of that aroma should
be even more appealing to bees. “We have this
assumption that a really fragrant flower is going to
attract a lot of pollinators,” says Theis, a chemical
30 ecologist at Elms College in Chicopee,
Massachusetts. But, she adds, that idea hasn’t really
been tested—and extra scent could well call in more
beetles, too. To find out, she and Adler planted
168 Texas gourd vines in an Iowa field and,
35 throughout the August flowering season, made half
the plants more fragrant by tucking
dimethoxybenzene-treated swabs deep inside their
flowers. Each treated flower emitted about 45 times
more fragrance than a normal one; the other half of
40 the plants got swabs without fragrance.
The researchers also wanted to know whether
extra beetles would impose a double cost by both
damaging flowers and deterring bees, which might
not bother to visit (and pollinate) a flower laden with
45 other insects and their feces. So every half hour
throughout the experiments, the team plucked all the
beetles off of half the fragrance-enhanced flowers and
half the control flowers, allowing bees to respond to
the blossoms with and without interference by
50 beetles.
Finally, they pollinated by hand half of the female
flowers in each of the four combinations of fragrance
and beetles. Hand-pollinated flowers should develop
into fruits with the maximum number of seeds,
55 providing a benchmark to see whether the
fragrance-related activities of bees and beetles
resulted in reduced pollination.
“It was very labor intensive,” says Theis.
“We would be out there at four in the morning, three
60 in the morning, to try and set up before these flowers
open.” As soon as they did, the team spent the next
several hours walkingfrom flower to flower,
observing each for two-minute intervals “and writing
down everything we saw.”
65 What they saw was double the normal number of
beetles on fragrance-enhanced blossoms.
Pollinators, to their surprise, did not prefer the
highly scented flowers. Squash bees were indifferent,
and honey bees visited enhanced flowers less often
70 than normal ones. Theis thinks the bees were
repelled not by the fragrance itself, but by the
abundance of beetles: The data showed that the more
beetles on a flower, the less likely a honey bee was to
visit it.
75 That added up to less reproduction for
fragrance-enhanced flowers. Gourds that developed
from those blossoms weighed 9 percent less and had,
on average, 20 fewer seeds than those from normal
flowers. Hand pollination didn’t rescue the seed set,
80 indicating that beetles damaged flowers directly
—regardless of whether they also repelled
pollinators. (Hand pollination did rescue fruit
weight, a hard-to-interpret result that suggests that
lost bee visits did somehow harm fruit development.)
85 The new results provide a reason that Texas gourd
plants never evolved to produce a stronger scent: “If
you really ramp up the odor, you don’t get more
pollinators, but you can really get ripped apart by
your enemies,” says Rob Raguso, a chemical ecologist
90 at Cornell University who was not involved in the
Texas gourd study.