Researchers have found that with greenhouse plants, applying
pesticides to the bottom, not the top, works better and saves
labor costs, too.

A University of Georgia study compared using subirrigation
to apply pesticides in commercial greenhouses to putting
pesticides
on the potting soil surface. Subirrigation, also known as
ebb-and-flow,
is widely used in Europe.

“It’s a closed system where the growing area is flooded
with water or fertilizer from a holding tank,” said Marc
van Iersel, a horticulturist with the UGA College of Agricultural
and Environmental Sciences.

“After the potting medium absorbs the water,” he
said, “the flow reverses and the water flows back into the
holding tank.”

This system is known to work well for applying fertilizers.
But there had been no research into whether it would work for
applying pesticides.

The Pesticide Becomes Part of the
Plant

“We used systemic pesticides for the study,” Van
Iersel said, “as they are much more effective for much
longer
than spray pesticides. Plants take up the systemic pesticides,
and when insects feed on the plants, they feed on the
pesticide.”

For the study, Van Iersel and CAES entomologist Ron Oetting
used poinsettia plants that were infested with whiteflies.
Whiteflies
are major pests of poinsettias, pansies, petunias and many other
greenhouse plants.

The pesticide used for the study was imidacloprid, the active
ingredient in Marathon.

“This pesticide is traditionally applied to the surface
of the potting medium, either as a drench or as granules,”
he said. “The water carries the pesticide down into the pot.
Using subirrigation, the pesticide is delivered from the bottom
… by capillary action to the surface of the potting
medium.”

The scientists found subirrigation much better at maintaining
the pesticide levels in the plants. Nine weeks after the
application,
the hand-watered plants showed far lower pesticide concentrations
than the subirrigated plants.

It Kills Adult and Immature
Insects

“This suggests the pesticide is leached from the potting
medium with repeated hand-watering,” Van Iersel said.
“With
subirrigation, the pesticide isn’t leached from the pots and
remains
available to the plants for much longer.”

Subirrigation also proved better at killing both adult and
immature insects. Killing immature insects is an essential part
of a control program, Van Iersel said.

“Preventing reproduction is much more important than just
controlling the adult insects,” he said. “After all,
a few adults won’t cause much damage. But if they reproduce
rapidly,
populations can quickly build up where they can be a serious
problem.”

Van Iersel says the bottom line is to apply the pesticides
the same way you water the plant.

Besides being an effective way to apply pesticides,
subirrigation
is also a huge labor-saver.

Kills Pests and Saves
Time

“Normally, a greenhouse worker would have to apply 2
ounces
of pesticide to each plant,” Van Iersel said. “Using
subirrigation, you just mix up the pesticides and apply them all
at one time.”

With subirrigation, the pesticides are always contained, which
cuts down on the amount of pesticide runoff from greenhouses.

“When applying pesticides from the top of the plant,
anytime
you overwater, the pesticides leak out onto the ground,”
he said.

Subirrigation is becoming more prevalent in greenhouses in
the northern United States and Canada. But so far, few greenhouse
growers in Georgia have installed the system.

“In Georgia, everyone is waiting for their neighbor to
do it first,” Van Iersel said. Subirrigation costs about
$7 per square foot to install.

“It’s expensive initially,” he said. “But
growers
can earn back the cost in labor savings in just a few
years.”

As a result of this research, Olympic Horticultural Products,
manufacturers of Marathon, now lists subirrigation as a legal
method of applying their product to greenhouse plants.