How to Manage Pests
UC Pest Management Guidelines
Measures To Minimize Water Quality Impairments To Surface And Ground Water
In this Guideline:
Because lettuce yield is reduced by mild
deficits in soil moisture, the crop is often excessively irrigated, which may
cause off-site transport of pesticides and nutrients into surface and
groundwater. For example, the nitrate form of nitrogen is very mobile in soil
and moves with water percolating below the root zone into tile drains or into
ground water supplies. Nitrate and orthophosphate (water soluble phosphate) are
also transported in surface run-off. Non-soluble forms of nitrogen and
phosphorus can be transported with sediments suspended in surface run-off.
Pyrethroid pesticides, although unlikely to leach, adhere to sediments carried
in surface run-off. Organophosphate pesticides, such as diazinon and
chlorpyrifos, are soluble and are transported in surface run-off. Chlorpyrifos
also adheres to soil colloids transported in run-off.
The following practices have been shown to
minimize impacts of irrigation on the quality of surface and groundwater.
Conduct regular audits of the distribution uniformity of the irrigation system.
Less water needs to be applied using an irrigation system with a high
distribution uniformity compared to a system with low distribution uniformity.
An audit of the irrigation system during operation by a mobile irrigation lab
will determine the distribution uniformity and suggest methods that the
distribution uniformity may be improved (increased).
CIMIS evapotranspiration data and soil moisture monitoring to schedule
irrigations. Irrigating too much can result in excessive tail water, and/or
leaching of nutrients beyond the root zone of the crop.
Improve maintenance of irrigation equipment. Leaks from pipes and drip lines
contribute to run-off and limit the distribution uniformity of the irrigation
Train irrigators and foremen so that scheduling errors are minimized. Over and
under application of water by personnel operating the irrigation system can
interfere with attempts to improve irrigation scheduling. Irrigators and their
immediate supervisors should be trained to operate the sprinkler and drip
systems at optimal pressures. They should be trained on how to use pressure
gauges and flow meters to assure that the irrigation system is operating
for Control of Soluble Nutrients and Pesticides
The main water-soluble nutrients of concern
for water quality in lettuce are nitrate-nitrogen and orthophosphate.
Water-soluble pesticides of concern are organophosphate pesticides such as
diazinon and chlorpyrifos, which are usually applied to soil to kill insect
pests of seedlings.
Avoid applying nitrogen fertilizer through overhead sprinkler water. Tail water
leaving the field when fertigating with sprinklers can carry high
concentrations of nitrogen into surface water supplies.
soil tests to determine if preplant fertilizer (N and P) can be reduced, and
use the quick nitrate test to determine if side-dress applications of nitrogen
can be reduced or skipped.
drip irrigation is used, fertigate nitrogen at rates that match the nutrient
uptake pattern of the crop.
side-dress applications of nitrogen, fertilizer rates should be appropriate for
the nitrogen uptake rate of the crop during different stages of development.
The lowest nitrogen uptake rate is during the first 4 weeks of the crop (less
than 1 lb of N per acre per day) and the highest nitrogen uptake rate is during
the last 3 weeks of the crop (3 to 5 lb of N per acre per day).
Capture surface run-off in retention basins. Reuse captured water for dust
abatement, preplant irrigations or non-leafy green vegetable crops. (Note:
presently the food safety metrics of the California leafy green marketing order
do not allow reuse of tail water for in-season irrigation of lettuce)
Establishing lettuce using drip irrigation or using drip early in the crop
cycle can eliminate runoff that carries soluble soil applied pesticides.
Operate overhead sprinklers in a manner to minimize run-off. Practices such as
shutting off the sprinklers when the soil is obviously saturated, and not
operating the sprinklers at excessively high pressures (greater than 60 psi)
can reduce tail water runoff.
When shanking pesticides into the soil, avoid leaving product on the top of the
beds or on edge of the field where it is susceptible to being carried in
run-off from overhead sprinklers.
Apply enzymes to tail water to breakdown water-soluble organophosphate
pesticides. Manufactured enzymes such as Landguard hasten breakdown of
organophosphate pesticides including diazinon and chlorpyrifos, and can greatly
reduce the toxicity of these pesticides to water organisms. The enzymes can be
metered directly into tail water using a dosing unit, or applied periodically
to water captured in a retention pond before the water is released.
for Control of Sediment-Bound Nutrients and Pesticides
The main nutrients of concern in lettuce
production that move with suspended sediments in irrigation run-off are total
nitrogen and total phosphorus, which includes the organic and inorganic
fractions of nitrogen and phosphorus that are bound to sediments. Pyrethroid
pesticides and, to a lesser extent, chlorpyrifos also adsorb to sediments and
therefore can move with suspended sediments in run-off. Strategies that retain
sediments in the field can minimize water quality impairments caused by these nutrient
and pesticides in surface water.
drip irrigation as early as possible in the crop cycle to avoid surface run-off
that often occurs from overhead sprinklers and furrow systems.
Inject anionic polyacrylamide (PAM) polymer at low concentrations (about 5 ppm)
in irrigation water to minimize sediment transport in surface run-off. Research
studies conducted by UC have shown as much as a 95% reduction in suspended
sediments and a 60-70% reduction in total P and N in irrigation run-off. Liquid
emulsified formulations of PAM can be directly injected into pressurized
irrigation systems using specialized pumps. Dry formulations can be added to
head ditches of flood irrigation systems or to the water catchment ponds to
settle out sediments before water is released off the farm.
Planting vegetation in permanent and seasonal ditches to slow flows of
irrigation run-off and drops out suspended sediments and adsorbed nutrients and
pesticides. The effectiveness of this practice to remove suspended sediments and
associated nutrients and pesticides depends on the amount of run-off, length of
vegetated ditch, vegetation coverage, and load of suspended sediments.
Retention basins and sediment traps can drop out silt and sand-sized particles
carried in tail water. These practices can be effective in removing large-sized
suspended particles but are less effective for small-sized particles, such as
clays, which may need a residence time of several days to a week to settle out.
Water chemistry, dimensions of the retention basin, volume of tail water, and
soil texture, all factor into the effectiveness of retention basins to remove
sediment from tail water.
UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
M. D. Cahn, UC Cooperative Extension, Monterey County
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