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When plants attract bugs, it may be their own fault

May 27, 2020 – If you're debating about what
impatiens to
plant in your yard, a recent study at the University of Illinois
suggests that
you go with ‘Cajun Carmine,’ that is if you want fewer insects in your
garden.

May 27, 2020 – If you're debating about what
impatiens to
plant in your yard, a recent study at the University of Illinois
suggests that
you go with ‘Cajun Carmine,’ that is if you want fewer insects in your
garden.

Why some varieties of the popular bedding plant impatiens attract more thrips
than others was one of the questions graduate student Katie Yu investigated.

"The fragrances given off by flowers
are actually complex compounds known as plant volatiles, some of which cannot
be detected by humans," Yu said.

"Volatile compounds act as a language
that they use to communicate and interact with the surrounding environment.
It's a defence mechanism against herbivores and it's a means to attract
pollinators. As of today, there have been over 1,000 plant volatiles reported.
But, none have yet to be reported in impatiens."

Impatiens, one of the top-selling bedding
plants in America, is very durable, relatively easy to grow and has wholesale
sales exceeding over $170 million per year. Yu chose two the popular varieties
‘Dazzler White’ and ‘Cajun Carmine’ to study their resistance to western flower
thrips.

Yu's initial research in greenhouses showed
that ‘Cajun Carmine’ had significantly less damage from thrips than ‘Dazzler
White.’ So she set out to prove the reason why, suspecting that impatiens may
emit volatiles that attract the thrips.

Thrips are very tiny, sliver-like insects
that are native to northwestern North America. They are a worldwide pest,
causing problems in field crops and greenhouses. They are attracted to a wide
range of host plants including impatiens, fuscia, hibiscus, chrysanthemum,
begonias, ivy, petunias, and major food crops. They feed on the plants' leaves
and petals and transmit devastating plant viruses.

For her research, Yu used a tiny glass
apparatus shaped like the letter "Y." The thrips are inserted one at
a time into the base of the Y. When the thrip reaches the junction in the Y, it
has the opportunity to choose to continue one way or the other. In this
experiment, one of the choices was toward purified air, while the other was
toward volatiles from an impatiens plant.

"Because we want to know if the thrips
are choosing based on a non-visual cue, the apparatus is contained in a black
box so the thrips cannot see if they're going toward the plant or not," Yu
said.

In the experiment, thrips chose ‘Dazzler
White’ over the purified air 64 per cent of the time. The thrips chose ‘Cajun
Carmine’ only 53 per cent of the time. "Because the thrips are blinded to
the plants, it's easy to infer that they are responding to the volatile
compounds," Yu said. "Thrips did not choose the ‘Cajun Carmine’
preferentially over the purified air. What this implies is that ‘Cajun Carmine’
does not produce a volatile attractive to thrips."

Although the percentages don't appear
significantly different, Yu said that it is conclusive. "As a control, we
also did the Y test with purified air only, and the thrips were choosing one
side over the other 50-50. Because the thrips were choosing ‘Cajun Carmine’
basically 50-50, choosing ‘Dazzler White’ 64 per cent of the time shows they
were definitely choosing the plant [‘Dazzler White’] over the purified
air."

While preliminary, these findings are
exciting to researchers seeking to minimize damage to impatiens in commercial
greenhouses.

When specific plant volatiles are identified
as attractant or repellents to specific insect pests, these volatiles can then
be used as selection factors in plant breeding programs or by producers seeking
to limit insect damage," said Yu's U of I advisor Daniel Warnock.
"The development of commercially acceptable cultivars of impatiens that
are not attractive to or recognized as a food source by western flower thrips
will reduce insecticide usage in greenhouses as the thrips will choose to feed
elsewhere. Attractant volatiles may also be used as lures to trap insects as a
control method. Repellents may be used as a deterrent to feeding if formulated
for application on other crops."

Future research to be conducted at the
University of Illinois will focus on identifying the presence of volatile
compounds in impatiens germplasm lines that show higher levels of resistance to
western flower thrips feeding than ‘Cajun Carmine.’ Once the presence or
absence of volatiles are confirmed, researchers plan to begin fractioning the
mass volatiles into specific compounds that are candidates for use in reducing
insect attractiveness to greenhouse crops.

Funding for the study was provided by the
University of Illinois Research Board.

Debra Levey Larson is a communications
specialist with the University of Illinois.