PUTNAM COUNTY — Farmers have to be prepared every year for insect
damage in their growing crops. Weather often determines which insect
pest will be the most problematic during the growing season. One may
believe that dry weather would cause egg deposits to dry up and decrease
the survival rate of larvae, or make it more difficult for adults to
find host plants. This may be true for some insects, but others are
quite adaptive. Dry weather generally favors insects or insect-like
pests that we do not see in normal years, such as spider mites. They
will often show up in field borders first as they move in from other
habitats, such as nearby ditches that have recently been mowed or the
vegetation has become dry from moisture stress.
Spider mites are
difficult to see. Look for injury signs – small yellow spotting on the
upper side of leaves. Stippling is another word that entomologist may
use for the yellow spotting. In soybeans, this damage usually begins in
the lower canopy and progresses upward as the mite population increases.
Heavily infested leaves may also have light webbing similar to spider
There are no number-based thresholds available for mites, in
part because it is impractical to count them in a scouting context
(they are almost impossible to see with the naked eye and tough to see
with a hand lens). During drought, populations can increase rapidly so
farmers should scout fields every 4 to 5 days during dry conditions.
or crop consultants will walk a broad pattern in the field and examine
at least two plants in each of 20 locations. Plants will be given a 0 to
5 point value following an integrated pest management scale developed
by the University of Minnesota. A miticide is recommended when fields
reach level 3. Since the two-spotted spider mite is an arachnid and not
an insect (eight legs versus six), standard insecticides are generally
not effective. Common choices for spider mite control in soybeans are
products containing chlorpyrifos, dimethoate, or bifenthrin.
is the point system: Zero points for no spider mites of injury observed.
One point for minor stippling on lower leaves, no premature yellowing
observed. Two points for stippling common on lower leaves, small areas
on scattered plants with yellowing. Three points when heavy stippling on
lower leaves with some stippling progressing into middle canopy. Mites
present in middle canopy with scattered colonies in upper canopy. Lower
leaf yellowing common and some lower leaf loss. Begin Spraying at this
time. Four points start when lower leaf yellowing readily apparent. Leaf
drop common. Stippling, webbing and visible mites are common in middle
canopy. Mites and minor stippling present in upper canopy. Economic loss
has occurred and will continue without an insecticide application.
Level five starts when lower leaf loss common, yellowing or browning
moving up plant into middle canopy, stippling and distortion of upper
leaves common. Mites present in high levels in middle and lower canopy
weather also tends to increase grasshopper populations and damage,
often along field borders. Grasshoppers are much easier to kill when
they are small, so timely treatment is helpful. A general defoliation
threshold can be used for grasshoppers and other leaf-feeding insects.
The other leaf feeding insects include Japanese beetles, green
cloverworms, and other caterpillars. If soybeans are in pre-bloom they
can tolerate up to 40 percent defoliation before treatment is advised,
and 15 percent defoliation from bloom to pod-fill. These percentages
refer to whole-plant defoliation, not just a few leaves. Most local
fields are in full bloom to early pod development.
A word to the
wise about treating for defoliators: A good reason to avoid spraying too
soon, apart from simple economics, is that many pyrethroid products
will actually make spider mite problems worse by killing off the natural
predators. We often see the most intense spider mite flare-ups in
fields that have been treated with a broad-spectrum pyrethroid.
will be checking their fields over the next several weeks for mites and
insects. Dry weather does not prevent their feeding. Farmers have to be
vigilant and control these pests even in drought years. Just another
challenge and cost they face besides the weather to bring food to our
For additional mite and insect information, the Ohio State
University Extension Field Guide for Corn, Soybean, Wheat and Alfalfa
may be purchased at the Putnam County Extension Office.
Dr. Kelley Tilmon, and Dr. Andy Michel, OSU Extension Field Crop
Entomologists and Ed Lentz recently shared the above information.