| Arthropod pests are rarely distributed uniformly throughout
a corn crop or a corn producing region. For example, Banks grass mites are
more common along the edges and in drought-stressed portions of corn fields.
European corn borer adults tend to select earlier-planted fields during
the first generation flight and later-planted fields during the second generation
flight. Within a field, larger plants will be preferred during the first
flight and less mature plants during the second. Western corn rootworm larvae
are generally more abundant in those parts of the fields with heavier soils
and higher organic matter levels.
Management of corn insect and mite pests is generally accomplished
by (1) estimating the average population density or amount of damage of
an infestation in a given field; (2) comparing this estimate to an economic
threshold; and (3) application of an approved insecticide or miticide
to the entire field if the economic threshold is exceeded (see the Western
High Plains Integrated Pest Management Guide for Colorado, Western Nebraska,
Montana and Wyoming for economic thresholds and registered pesticide products).
Given that pest distributions are variable within a field and that treatment
decisions are made on a whole-field basis, it is likely that some portions
of the field will not be infested or damaged at a level above the economic
threshold. Site-specific scouting and application methods would allow
producers to avoid the unnecessary expense of treating these areas.
Sampling for corn insects generally involves counting insects,
usually eggs or larvae, or damaged plants on a set number of plants at
each of several locations per field. These counts can then be converted
to percent infested or damaged plants or to insects or damage per plant
for economic threshold comparisons. The amount of effort involved will
depend on pest species and plant growth stage, but treatment decisions
are usually based on information gathered in one hour or less per week
per field. Determining the treatment need for small field subsections
using this approach would be exceedingly expensive. Two possible, more
cost-effective, alternatives are
(1) more efficient scouting methods or
(2) a management zone approach.
|
|
| Figure 1. Pheromone trap. |
We have investigated the use of pheromone traps for more intensive sampling
of Western bean cutworm and European corn borer, as part of the precision
farming project. Pheromones are chemicals produced by one sex (the female
in both of our target species) of an insect species to attract the opposite
sex. Pheromone sampling is accurate in that the only insects attracted to
the trap are of the target species. This can save time in training and in
scouting. Also, pheromone sampling is relatively cheap. The trap we use
(Figure 1) is modified from a one gallon plastic milk jug and the pheromone
lures usually cost $2 - 3 each if purchased in quantity. We collected moths
from about one trap per acre in each of two corn fields for two years and
were able to map spatial variability for both species (Figures 2 - 3). We
were also able to use our data to calculate that 25 traps per field would
be adequate for our study site. This works out to about $1.00 per acre to
sample both Western bean cutworm and the European corn borer flights, excluding
labor (trap-to-trap travel time and time to sieve and count moths) and assuming
bulk purchases.
We now have an efficient method of sampling the adult stage
of these two important corn pests. However, corn is damaged by the larval
stage of these insects. The relationship between pheromone trap catches
and egg-laying or larval activity is not well known under our growing
conditions. We currently are trying to measure this relationship. If there
is a close relationship, then pheromone trapping could be a viable tool
for site-specific corn pest management decisions.
 |
| |
0.0 - 0.2
|
| |
>0.2 - 0.4
|
| |
>0.4 - 0.9
|
| |
>0.9 - 1.1
|
|
Figure 2. Daily European corn borer per
trap.
Field 1. 1998. |
A compromise between whole-field decisions and those made for acre or smaller
units is the management zone approach. This strategy recognizes that, while
sampling for small units can be prohibitively expensive, the fact remains
that most fields are sufficiently variable to justify management as several
subsections. Fields are broken up into several smaller units (management
zones), based on a few simple criteria such as bare soil color and texture,
that are managed individually. It might be possible to make pest management
decisions at the management zone level rather than the field level and realize
some cost savings. Pest management benefits relative to whole-field decisions
might include (1) identification of zones in need of treatment, contrary
to the whole-field decision; and (2) detecting zones that don’t need treatment,
contrary to the whole-field decision. The downside risk, of course, is having
similar outcomes at both levels, meaning the extra pest scouting effort
required by the management zone approach was not a good investment.
Regardless of their size, once the treatment need has been
determined for field subsections we then need the technology to treat
some subsections and leave others untreated. There is substantial concern
about the accelerated development of insecticide and miticide resistance
by the use of reduced rates. Therefore, site-specific management will
likely involve either treatment with a full rate or no treatment.
 |
|
| Figure 3. Total western bean cutworm per trap. Field
1. 1998 |
Treatment difficulty will vary with the pest and crop
growth stage. Western corn rootworm larvae are commonly treated with granules
applied with planter or cultivator-mounted equipment. Site-specific application
software and hardware to control the granule meters should be easy to adapt
or develop. First generation European corn borer larvae are best controlled
with granules applied with aerial or ground equipment, or with chemigated
liquids. Site-specific granule application technology is not readily available
for aircraft. Chemigation of field subsections (sectors) is currently feasible.
Treating smaller sections will be possible as span-by-span and nozzle-by-nozzle
chemigation controls are commercialized. Western bean cutworm, second generation
European corn borer, Western corn rootworm adults and Banks grass mite are
controlled by treating fully-grown corn with liquid insecticides applied
with aircraft, high clearance ground sprayers, or chemigation. Additional
site-specific application software and hardware may have to be developed
for these situations as well.
Site-specific management of corn insects has the potential
to make corn production more efficient through more targeted insecticide
and miticide applications. Improvements in pest sampling methods and application
technology will be necessary to realize this potential.
by Frank Peairs
Professor
Dept. Bioagricultural Sciences and Pest Management
Colorado State University
|
