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How to Manage Pests
UC Pest Management Guidelines
Corn
Spider Mites
Scientific names:
Twospotted spider mite: Tetranychus urticae
Banks grass mite: Oligonychus pratensis
Strawberry spider mite: Tetranychus turkestani
Pacific spider mite: Tetranychus pacificus
(Reviewed 1/06,
updated 8/08)
In this Guideline:
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Mite infestations on corn frequently include a mixture of spider
mite species, including twospotted
spider mite, Banks grass mite, Pacific spider mite,
and strawberry
spider mite.
Of these mite species, twospotted spider mite and Pacific spider mite are most
common. Adult mites are about 0.06 inch in length, have four pairs of legs, are
greenish to pink or cream colored, and have various sized black spots on the
body. Under warm conditions spider mites move rapidly within the colony area.
Spider mites have four stages of development: (1) the spherical, somewhat
translucent egg;
(2) a six-legged translucent larval stage; (3) an eight-legged nymphal
stage;
and (4) the eight-legged adult stage. A resting or quiescent stage occurs at
the end of the larval and nymphal stages. A generation may pass in as few as 5
to 7 days in midsummer, or in a month during cool periods.
All active stages of spider mites damage corn by removing juices
from infested leaves, causing premature drying that results in loss of leaf
tissue, stalk breakage, and kernel shrinking. Damaged leaves become somewhat
yellowish and stippled on the upper surface and grayish due to webbing on the
undersurface. Spider mites can be a serious problem on corn, particularly
silage and sweet corn.
Keep spider mite populations in check by reducing dust and weed
hosts and encouraging mite predators. If monitoring indicates a need, treatment
may be necessary on mid-size corn.
Biological Control
Spider mite populations may be held at very low levels by a number of
predatory insects and mites, particularly early in the season. Thrips are
effective early season predators, feeding primarily on spider mite eggs. Spider
mites provide an important food source for predators such as minute
pirate bugs and bigeyed
bugs.
Minimizing early season insecticide applications, which may reduce populations
of beneficials, will reduce spider mite outbreaks. Naturally occurring predatory
mites exert some level of control. In some areas, releases of predatory mites
have been used to manage spider mites in field corn. If predatory mites are to
be released, be sure to release the appropriate predatory mite species for the
area and time of year. Also use the correct release rate and the correct
timing. Definitive guidelines have not been developed, but make releases before
significant spider mite outbreaks occur.
Cultural Control
Reduce spider mite
problems by keeping fields, field margins, and irrigation ditches clean of weed
hosts. Spider mite populations may increase more rapidly in areas where dust
deposits are heavy on corn leaves. Thus, reducing dust may reduce the spider
mite problem.
Organically Acceptable Methods
Biological controls and cultural controls are acceptable to use in
organically grown crops.
Monitoring
Infestations usually begin on the lower portions of the plants and move
upward as mite numbers increase. Evaluating spider mite infestations is most
efficient if randomly selected, older, lower leaves are picked and inspected
for stippling on the upper surface and webbing, mites, and feeding scars on the
lower surface. Spider mite infestations that reach the ear leaf are most
damaging.
Treatment Decisions
If small colonies of spider mites are found on the lower leaves of
young plants throughout the field, control may be cost effective. Treat when
corn is 2 to 4 feet tall; applications made after the plants exceed 4 feet in
height usually result in poor control because good coverage is difficult to
obtain. Just treating a couple of swaths around the field can keep spider mites
from spreading into the remainder of the field.
| Common name |
Amount/Acre** |
R.E.I.+ |
P.H.I.+ |
| (trade name) |
|
(hours) |
(days) |
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| The following materials are listed in order of usefulness in an IPM program, taking into account efficacy,
information related to natural enemies and honey
bees and environmental
impact.
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| A. |
SPIROMESIFEN |
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(Oberon) 2SC |
5.7–8.5 f1 oz |
12 |
5 – green forage/silage |
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30 – grain/stover |
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MODE
OF ACTION GROUP NUMBER1: 23
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COMMENTS: For use on field corn. See label for plant intervals. Do not make more than 2 applications/crop |
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| B. |
PROPARGITE |
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(Comite) 6.55 lb/gal EC |
2–3 pt |
7 days |
30 |
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MODE
OF ACTION GROUP NUMBER1: 12C
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COMMENTS:
Apply to dry corn leaves. Apply before corn is 2–4 feet tall to ensure coverage. Tank mixing with oils and foliar fertilizers can result in injury. |
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UC IPM Pest Management Guidelines: Corn
UC ANR Publication 3443
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
S. D. Wright, UC Cooperative Extension, Tulare County
C. G. Summers, Kearney Agricultural Center, Parlier
C. A. Frate, UC Cooperative Extension, Tulare County
Acknowledgement for contributions to Insect and Mites:
M. J. Jimenez, UC Cooperative Extension, Tulare County
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