ID-48 SILK CLIPPING INSECTS ON CORN
by Lee Townsend, Extension Entomologist; Morris Bitzer, Extension Agronomist
Silk clipping insects like the corn
rootworm beetle and the Japanese beetle feed on the green silks of corn
plants and are most active during the plant's flowering stage. Damage from
these insects can cause reduced kernel set and yield if significant silk
clipping occurs during the critical pollination period. This publication
has been prepared to help you evaluate corn fields during this period and
to help you determine whether insecticide application is necessary to protect
the corn silks from these beetle populations during pollination.
Damage during pollination can drastically
affect kernel development and corn yield. An ear shoot which is not well-formed
and fully pollinated can never mature to a full-sized ear. In addition
to silk damage, poor soil fertility, delayed silking and hot, dry weather
can also affect the pollination process and thus reduce the number of kernels
per ear. Table 1 shows the sequence of events in the flowering and pollination
Each corn silk is a long, slender tube
leading to an ovule which, if fertilized, becomes a kernel of corn. Silks
from near the base of the ear emerge first; those from the tip appear last.
During pollination, pollen grains fall on the corn silks where they germinate
and new kernels begin to develop. The first visible sign of successful
pollination is that silks stop growing, wilt and turn brown.
During the first ten days to two weeks
after pollination, the cob develops to its full length and diameter. The
developing kernel does not reach the watery blister stage until one to
one and one-half weeks after fertilization
Table 1.- Events in the Pollination of a Medium Maturity Corn Hybrid
55 to 50 days after plant emergence
POLLEN SHED BEGINS
After tassel fully emerged and about 1 to 3 days before silk emergence
3 days after full tasseling
PEAK POLLEN SHED
By third day after tasseling
ALL SILKS EMERGED
Within 3 to 5 days after first silks appear
12 to 24 hours after pollen grain lands on silk
If corn is planted before May 10, most
of the pollination process will be completed by mid-July. This is generally
before peak silk feeding activity. Selecting an early planting date and
proper maturity group can help you avoid pollination when silk clipping
is likely to be most severe.
Silk Clipping Insects
Japanese beetles pose the greatest
potential for silk damage in the eastern and central portions of Kentucky,
while corn rootworm beetles are the most common silk feeders in the western
part of the state.
The Japanese beetle was accidentally
introduced into the United States about 1912. It has gradually spread westward
and now is established in the eastern half of Kentucky. This westward spread
across the state is expected to continue.
Adults have a metallic green body and
coppery brown wing covers. A row of five white spots can be seen along
each side of the body and a pair of spots on the top of the last segment.
The beetles are about 3/8 inch long and 1/4 inch wide.
Japanese beetles spend about ten months
of their year-long cycle as white grubs in the soil. Adults begin to emerge
from the soil in June and are active for about four to six weeks. The population
peaks about mid-July. They may be very numerous in corn fields, averaging
ten or more beetles per ear.
Corn Rootworm Beetles
Three species of corn rootworm beetles