5 Scouting For Insects
Seedling Insects
Scouting seedling corn on a weekly basis from emergence time until it
is about 18 inches tall provides information on insect abundance, stand
loss, and plant population. Equipment used for sampling includes a 5-foot
measuring stick, a record card, a pencil, and a hand trowel. Target pests
include armyworm, billbug, cutworm, stalk borers. and soil insects (wireworm,
seed corn maggot, rootworm, etc.)
Sampling Procedure
Walk a zig-zag pattern over the field looking for insect damaged plants
(Figure 10) (wilting, discolored, cut-off,
or foliage eaten) or poor stands. When insect damage or poor stands are
observed the insect (or other cause of the problem) is identified by examining
an affected plant and/or by digging up seed. If the problem involves insects,
99 more plants (33 per row for 3 adjacent rows) are examined in the same
area of the field for a total of 100 plants; record the number of damaged
plants (don't count plants with only minor feeding). This procedure is
repeated in 4 to 10 randomly selected places in the field, depending upon
severity of damage. If less than 4 percent of damaged plants are found
in the first two samples, only four total samples are recommended. If damage
in the first two samples exceeds 5 percent, take 10 samples per field unless,
after four samples, it is obvious that the field needs to be treated. Note
that if no insect damage is observed to initiate the first sample then
samples will not be taken.
For plant stand, count plants on 10 row feet; 2
5-foot sections of adjacent rows at a minimum of four sites (10 sites maximum).
Calculate the average plants per row foot (See Section 3).
Damage Symptoms
Armyworm. Eats the leaf tissue, usually from the edges inward, and plants will appear ragged (Figure 11).
Billbug. (Figure 12) Feeding slits will be on the lower part of the stem, unfurled leaves will often be cut within the plant, central leaves often will be wilted, and rows of holes may appear on the leaves; damage will be most severe near field edges, especially when bordering last year's corn. Cutworm. Small plants cut entirely off or larger plants with a large cavity eaten at or slightly below ground level; cut- off plants will frequently be pulled into the ground (Figure 13).
Stalk borers. Tunneled plants or leaf feeding from within the whorl (Figure 14).
Wireworm. Seed with rough holes eaten into the side or seedlings with holes eaten into the underground stem above the root crown and tunneling upward (Figure 15).
Corn rootworm (southern). Small hole eaten between main roots
at the root crown and tunneling upward (Figure
16). Seed corn maggot. Ungerminated seeds, hollowed out and with maggots
in them.
Second Generation European Corn Borer
Scouting for second generation European corn borer should begin after
the first generation moths begin to emerge and lay their eggs. The moth
flight can be determined with the use of a black light trap, pheromone
trap, or field edge surveys for moths (ECB moths often rest during the
day in grassy field edges). Typically, egg laying closely coincides with
pollination in normal planted corn.
Scouting for ECB egg masses
Examine the upperside and underside of the ear leaf and two leaves above
and below the ear (a total of five leaves) for ECB egg masses. Egg masses
are pearly white to yellowish in color; contain about 20 flattened, overlapping
fish scale like eggs; and are mostly found on the underside of leaves within
two-thirds of the distance from the corn stalk (Figure
17). Egg masses are difficult to see at first but with practice can
be easily detected by a person with good eyesight. Scout each field using
a sequential sampling system. With this system the number of samples taken
will depend on what is found in the first three or four samples. Each sample
consists of 10 consecutive plants in a row; examine the top and bottom
of five leaves in the ear zone of each plant. Record the number of ECB
egg masses found. After three samples, and each sample thereafter, evaluate
the need for additional samples by comparing the number of egg masses found
with the threshold chart (Table 5). If egg mass numbers are greater or
less than the thresholds for each sample number, then sampling is stopped,
but if egg mass numbers fall between the MAX and MIN numbers, take an additional
sample and reevaluate. A maximum of 10 samples are taken and if no decision
is made the field is scouted again in about five days. Avoid sampling field
edges or weak spots.
Scouting for ECB caterpillars
Scouting for caterpillars is recommended only in situations where the
opportunity for scouting egg masses has passed. After hatching, ECB caterpillars
are located in the leaf junctions, shucks, and ears for about a week. During
this time caterpillars can be detected and can be partially controlled.
Scouting involves examining the leaf axils of all the leaves from the bottom
of the plant to one leaf above the ear. Cater- pillars will be found in
the ligule area (base of leaf), usually among collected pollen, anthers,
and other debris. These sites should be examined carefully for caterpillars
around 1/4 inch or less. The same sampling pattern and number of samples
per field as is recommended for egg mass scouting is suggested.
Thresholds
Table 5. Egg Mass Scouting
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Scouting for ECB caterpillars. Treatment is suggested when an average of 75 caterpillars per 100 plants (or equivalent) is found in the leaf axils.
Thresholds
(These thresholds assume that there is a full stand; reduce by one-third
if thin stands are encountered.)
Armyworm. 20 percent of small seelings (to 18 inches) with significant
leaf feeding, 50 percent of larger plants, and army- worms present.
Billbugs. 5 percent seedling loss in infested areas. Cutworms.
5 percent cut seedlings with an active cutworm population.
Stalk borers. 5 percent seedlings infested.
Soil insects (wireworm, rootworm, seed corn maggot). No threshold
but record damage to evaluate soil insecticide program and for future reference.
Whorl-Feeding Insects
Corn earworm, fall armyworm, first generation European corn borer, and several minor pests can be evaluated using this procedure. Emphasis should be placed on scouting plants when caterpillars are small. This implies that only minor damage will be present although infestation may be high. Damage will be expressed as leaf surface feeding, windowpaning, and small holes eaten through the leaves. Caterpillars will typically be within the loosely furled leaves.
Sampling Procedure
Enter the field at least 50 paces and take the required samples (one per 2 acres of field size, minimum of four and maximum of 10 samples). Travel a "U" shaped or zig-zag pat- tern through the field. Avoid weak spots and take only one sample on a given row. Examine 10 consecutive plants per sample for damage in the whorl. On the last two damaged plants, pull the whorl from the plant and count the caterpillars while unrolling the leaves. Calculate the percentage of damaged plants and average number of caterpillars per damaged plant. Multiply these two figures to obtain a score. (For example, 90 percent damaged plants X 1.2 caterpillars per plant = 108.)
Which corn to scout?
Whorl feeders are not typically an economic problem and scouting time and effort are often not warranted. However, with late planted corn or in areas of high European corn borer incidence the procedure can be helpful. Also, when high whorl feeder damage is casually noted the procedure can be used to formally evaluate the infestation.
Thresholds
Corn earworm and fall armyworm. 50 percent of plants infested or 25 percent of plants if there are three or more fall armyworms per plant.
First generation European corn borer. A score of 100 or more.