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Onion thrips, disease
management strategies
Recently, onion thrips have been shown
NATALIE CONSTANCIO &
ZSOFIA SZENDREI
Michigan State University
Onion thrips are a major insect pest
in onions. They feed on onion leaves,
resulting in silvery scarring, reducing the
amount of green leaf tissue (Fig. 1A).
A reduction in green tissue negatively
affects the onion's ability to photosynthesize,
and results in smaller bulbs and decreased
yield. Additionally, when onion thrips feed,
they leave open wounds on the onion tissue,
creating an entryway for plant pathogens.
Onion thrips spread many kinds of plant
pathogens, including viral, bacterial and
fungal pathogens. While onion thrips are
primarily known to spread tospovirus,
such as Tomato spotted wilt virus, they
also contribute to the spread of bacterial
pathogens such as center rot, and fungal
pathogens such as Stemphylium leaf blight.
In Michigan, Stemphylium leaf blight is
the main pathogen in onion fields and
presents many challenges to growers.
to spread Stemphylium leaf blight by
picking up fungal spores on the outside of
the body and moving the spores between
onion plants. Symptoms of Stemphylium
leaf blight include tan water-soaked lesions
on the leaves, and premature leaf die back,
resulting in severe yield losses (Fig. 1B).
Once an onion is infected with
Stemphylium leaf blight, it is also
vulnerable to infection by secondary
pathogens, which can cause storage rot.
Growers manage onion thrips and plant
pathogens in different ways. Onion thrips
are managed through scouting programs
prior to insecticide application.
Scouting for onion thrips is critical
to successful control because they
are resistant to multiple insecticide
classes. To prevent further insecticide
resistance, growers must rotate
insecticide classes and implement
action thresholds. The action threshold
indicates the pest population level a
grower should start treatment to prevent
significant crop loss and is necessary for
reducing insecticide resistance.
Additionally, using an action threshold
Figure 1: (A) An onion infested with onion thrips larvae. Onion thrips feed on plant tissue,
which results in silvery leaf scarring and reduces the plants' ability to photosynthesize.
Severe onion thrips infestations result in smaller bulbs and decreased yield. Onion thrips
also contribute to disease spread in onion fields. (B) An onion leaf infected with a plant
pathogen, Stemphylium leaf blight. Stemphylium leaf blight symptoms include tan, watersoaked
lesions on the onion foliage. In Michigan, Stemphylium is the main pathogen
found in onion fields, and can result in severe yield loss.
also reduces the amount of insecticide
applied throughout the growing season
and may save the grower time and money.
In contrast, plant pathogens are
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controlled on a calendar schedule and
are advised to apply fungicides early
in the season. After the first fungicide
application, treatments are applied
at regular intervals as a preventative
measure. Increasingly, research provides
evidence that fields infested with onion
thrips experience higher disease pressure,
indicating that the management of both
pests should be considered simultaneously.
However, because of the differences
in spray recommendations for onion
thrips and plant pathogens, it can be
difficult for growers to implement a spray
program that effectively combines both
management approaches.
In the 2022 growing season,
researchers at Michigan State University
tested the combination of synthetic
conventional insecticides and fungicides
at varying action thresholds to evaluate
their ability to simultaneously manage
onion thrips and Stemphylium leaf
blight. The spray trial was conducted in a
commercial onion field in Michigan and
consisted of 8 different spray programs,
with varying action thresholds. The
treatments were: 1) Control - no spray,
2) Fungicide only applied weekly, 3)
Low threshold + Fungicide, 4) Medium
threshold + Fungicide, 5) High threshold
+ Fungicide, 6) Low threshold, 7)
Medium threshold without fungicide
and 8) High threshold without fungicide.
All the treatments with fungicides began
with Miravis Prime and were alternated
with Bravo Weather Stik weekly. For the
low threshold treatments, either with or
without a fungicide, treatments began
when the action threshold reached 0.5
thrips/leaf and consisted of Movento,
Minecto Pro, and Radiant each applied
twice 7 days apart before rotating to the
next insecticide. The medium threshold
treatments began at an action threshold
of 0.6 thrips/leaf and consisted of
Minecto Pro and Radiant, applied twice
7 days apart before rotating to the next
insecticide. The high threshold was
set at 1.0 thrips/leaf and consisted of 2
applications of Radiant applied 7 days
apart. The date of insecticide applications
began varied depending on the
thresholds, with low threshold application
starting on July 5, the medium on July 26
and the high threshold starting on August
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30 | VegetableGrowersNews.com
Figure 2: In 2022, we evaluated onion thrips control in a plot trial in a commercial onion
field in Michigan. The goal was to examine if starting the insecticide program at different
thrips action thresholds, we could control thrips and reduce foliar disease development on
plants. We compared thrips numbers in no-spray (gray) control plots to fungicide program
only (solid red) or an insecticide program tank-mixed with fungicides (red and blue hash)
or an insecticide program alone without fungicides (solid blue). All pesticide applications
were applied with a CO2 pressurized backpack sprayer with a fan nozzle. Bars with the
same letter indicate no significant differences among the number of thrips.

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November 2022

Table of Contents for the Digital Edition of November 2022