February 2022 - 11

Thrips hit onions; foliar disease boosts risk
By Dean Peterson
VGN Correspondent
Onion thrips are a major challenge to
organic growers and to growers who grow
onions with conventional practices. Foliar
diseases like stemphylium are also a major
challenge.
New research from Michigan State
University (MSU) is finding a link
between the two - with more onion thrips
in fields that have more foliar disease.
That's the message from Zsofia Szendrei,
associate professor in MSU's Entomology
Department. Szendrei talked about the
onion thrips research at the recent
Great Lakes Fruit, Vegetable & Farm
Market EXPO held in Grand Rapids,
Michigan.
Fifteen fields of commercial onion
production - seven organic and eight
conventional - were regularly visited by
researchers over the summer. The thrips
populations were counted and the amount
of foliar disease measured.
growing onions in the field longer so
more disease pathogens may be present, "
Szendrei said.
The study also tested organic pesticides
for onion thrips control.
In 2020, researchers compared Neem
Oil, Entrust, Neem Oil plus Kocide 3000,
Entrust plus Kocide 3000 and Kocide 3000
alone to an untreated control. The study
was repeated in 2021 with a change that
Neemix 4.5 was used instead of Neem Oil.
The products are all OMRI-listed (Organic
Materials Review Institute).
Neem is an extract of an ornamental
shade tree in India.
" It has many, many formulations and
trade names, " Szendrei said. Neemix
4.5 was selected in 2021 to help ensure
product consistency.
Entrust (spinosad) is produced through
the fermentation of a naturally occurring
soil bacterium. Kocide 3000's active
ingredient is copper hydroxide. All three
products were applied with a surfactant.
In comparison, all of the organic
pesticides performed similarly to each
other, but only managed onion thrips
slightly better than the untreated control.
" The organic products did not control
onion thrips, " Szendrei said. " They helped
but did not get the thrip populations
below threshold. We don't yet have a silver
bullet for thrips if you're trying to grow
organic onions. " VGN
Stemphylium symptoms on an onion
plant. Photo: Michigan State University
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Zsofia Szendrei, associate professor in
MSU's Entomology Department, has
conducted research confirming there is
a relationship between onion thrips and
other pathogens. The onion thrips were
more prevalent on diseased plants. Photo:
Dean Peterson
In general, there was no difference in
the number of onion thrips per leaf found
between the organic and conventional
farming systems. Onion thrips
populations in both systems also never
reached numbers requiring treatment.
" The pressure from onion thrips was
essentially the same in each farming
system and stayed below threshold over
the whole season, " Szendrei said.
However, some of the conventional
fields had about twice as much foliar
disease as the organic fields. The fields
with more foliar disease also had more
onion thrips per leaf and thrip populations
were slightly more than the treatment
threshold.
" There is a relationship between onion
thrips and other pathogens, " Szendrei
said. " Onion thrips were more prevalent
on diseased plants. "
Thrips damage onions by puncturing
one cell after another.
" Thrips could be opening entry points
for disease and could be helping the
pathogens spread by carrying them from
plant to plant, " Szendrei said.
Field history could be another reason.
" Conventional farmers may have been
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February 2022

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