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Handling Mite
Pressure | 2
Injury to foliage reduces yield only if it is extensive
enough to reduce the plant’s ability to conduct photosynthesis
and transfer energy to fruit. The foliage pests most damaging
in the Northern San Joaquin Valley are spider mites. All
spider mites cause essentially the same injury, leaves or
parts of leaves turn yellow or red and may drop. Mites can
cause heavily infested fields to defoliate.
Types Mites in Cotton. Spider mites in cotton
include strawberry spider mite, twospotted spider mite,
and the Pacific spider mite. Females of the strawberry,
twospotted, and Pacific spider mites are green or straw
yellow with dark blotches at the sides of the body.
Populations
Spider mites are present all year on perennial
hosts such as alfalfa and almonds. Mite populations thrive
on annual crops and weeds as these become available during
the year. A generation of mites can mature in as little
as 5 days in hot weather. Cotton generally hosts about 10
generations of mites each season. Mite populations are regulated
by temperature, condition of host plants, and the activity
of predators, especially flower thrips, minute pirate bugs,
bigeyed bugs, lace wings, predatory mites, and spider mite
destroyer lady beetles. Early infestations can arise from
mites that over winter on the soil or on weeds. Infestations
can also start with adult females carried by the wind from
perennial host crops. Mites are also associated with dusty
or dirty cotton along frequently traveled dirt roads. Mites
that are carried by the wind, often build up first along
the edge of the up wind side of the field, then spread in
a down wind direction. Infestations that arise from mites
overwintering on the soil can occur anywhere in the field.
Early Season
Strawberry spider mite is the most dominant species
on early season cotton. These mite colonies will be located
on the lower leaf surfaces of cotyledons and first true
leaves. Feeding injury appears as red patches on the upper
side of leaves directly opposite the location of the colony.
Early thrips often feed on these mites, therefore BASIC
field scout’s closely monitor this activity and encourage
growers to resist spraying if at all possible.
Mid-Late
Season
Twospotted spider mite and Pacific spider
mite are the most common mid to late season mite species
attacking cotton. Twospotted spider mite produce a diffuse
pattern of yellow spots over much of the leaf surface. Pacific
spider mite injury often spreads first along main leaf veins,
although it may later cover most of the leaf. Twospotted
spider mite and Pacific spider mite damage look very similar.
However, Pacific spider mites produce more webbing and are
the only mite commonly found on both surfaces of infested
leaves.
Natural
Controls
Insecticides often cause outbreaks of mites by destroying
their predators. There is generally no significant natural
control of mites for about a month following chemical treatment
for lygus bugs, aphids, or other insect pests. Managing
spider mites requires preserving natural controls as long
as possible each season. Natural enemy populations can be
increased or preserved in a variety of ways: establishment
of beneficial habitat, augmented releases of beneficials,
management of neighboring crops, and applications of beneficial
friendly insecticides like sulfur. BASIC growers are encouraged
to use these techniques in order to keep them off the pesticide
treadmill and reduce overall pesticides applied.
Ask
These Questions
Consider the following factors before making a decision
to treat for mites. What percentage of leaves is infested
with mites? How numerous are beneficials? How does the mite
population compare with that of previous weeks? What is
the previous history of mite outbreaks on the field? What
growth stage is the crop in?
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