|Traditional Field Crops (Peace Corps, 1981, 283 p.)|
|Pest and disease control|
When is Treatment Necessary?
Farmers should apply insecticides in response to actual insect problems rather than on a routine and indiscriminate basis. Ideally, insecticides should be used only when damage has reached the economic threshold. This level varies with the insect species, the crop, and the type and extent of damage.
General guidelines (see also the unit on major reference crop insects):
· Soil insects. These pests should be treated preventatively by making pre-planting or atplanting insecticide applications if a known problem exists. Treatments after planting are generally not effective except in the case of cutworm baits.
· Leaf-eating insects (beetles, caterpillars): Crops can tolerate considerable defoliation as long as new leaves are being continually produced. Loss of leaf area becomes more serious as the vegetative stage nears its end, although defoliation in the very late stages of grain development will not have a big effect on yield. Stem-borers usually cause more serious damage at much lower populations than most leafeating insects. The sorghum shoot fly, sorghum midge, and one species of bean leafhopper (Empoasca kraemeri) are other examples of insects that reach the economic threshold of damage at relatively low populations.
· Sucking insects: Not all species of aphids and leafhoppers spread virus diseases. For example, CIAT found that bean yields were reduced about 6 percent for each Empoasca kraemeri leafhopper present per leaf, even though this species does not transmit any viruses. Bean plants can tolerate aphids well unless they are of a species capable of transmitting common bean mosaic virus.
Using a Sprayer Effectively
Achieving the Correct Coverage
The extent and uniformity of coverage needed depend on the insects' location and whether or not a systemic insecticide is being used. In some cases such as armyworms feeding in the maize leaf whorl, the insect is very localized, so general coverage is not needed. Other insects are more general feeders and require thorough spray coverage over the whole plant. Since they are translocated, systemic insecticides do not require the uniform coverage nonsystemics do.
The amount of water need for adequate coverage varies with plant size, density, type of product (systemics versus nonsystemic), and insect location, but there are some rough guidelines:
Water rates for insecticides: When covering the entire foliage of full size plants, at least 500-550 l of water per hectare will be needed when using conventional sprayers. When spraying is localized or plants are very small, water volume may be only one-quarter of this amount.
Too much spray is being applied if there is a visible amount of runoff from the leaves, although this can also be caused by not using enough wetting agent (spreader).
Using a Spreader And Sticker
A spreader (wetting agent) reduces the surface tension of spray droplets, allowing them to spread out rather than remaining as individual globules on the leaf surface. Spreaders markedly improve the uniformity of spray coverage and also help prevent droplets from rolling off the leaves.
A sticker (adherent) is a gluelike substance that helps the spray stick to the leaf surface and resist being washed off by rainfall or sprinkler irrigation.
Many commercial stickers and spreaders are available, including combination sticker-spreaders. The pesticide label will indicate if a spreader or a sticker is needed. If spraying the soil, neither a spreader nor a sticker is needed. When spraying the leaf whorl of maize, a spreader is not needed, though a sticker might be helpful. Use of a sticker and spreader is especially important when applying most foliar fungicides.
Commercial stickers and spreaders are relatively cheap. However, if not available commercially, they can be made at home. Egg white, cassava (yuca, manioc) flour, and corn starch can be used as stickers at about 15 cc per 15 liters. Liquid dishwashing detergent makes a satisfactory spreader at about the same rate.
Non-ionic spreaders: Paraquat and diquat post-emergence herbicides are unusual in that they require the use of special non-ionic spreaders in order to avoid deactivation (loss of effectiveness). Ortho-77 is one commonly available non-ionic spreader.
Choosing a Spray Nozzle
Spray nozzles are available in a wide variety differing in output, spray pattern angle, and type of spray pattern. Proper nozzle selection has an important influence on pesticide effectiveness.
Nozzle Output: Many backpack (knapsack) sprayers come equipped with adjustable nozzles which allow the farmer to vary the output by making the spray finer or coarser. This would seem to be an advantage, buth such nozzles usually do not maintain their setting well and output can change considerably during application. This is unsatisfactory where accurate dosages are necessary, and it makes sprayer calibration difficult. Fixed orifice nozzles are available in a wide range of outputs and should be used whenever possible.
Tractor boom spray nozzle
Illustration courtesy of Rohm & Hass Co., Philadelphia, Pennsylvania
Ideal tractor spray boom arrangement for applying insecticides and fungicides and achieving uniform coverage. Note that the drop nozzles are angled about 30° upward as well as 30° forward. Only one tier of "drop" nozzles may be needed on small- to medium-size crop plants.
Spray Pattern Angle: See flat spray
Type of Spray Pattern: Care should be taken to choose the right spray pattern for the job.
· Flat (Fan) Spray Nozzles are ideal for making broadcast (full coverage) applications of insecticides or herbicides over the soil surface (and small weeds). The application rate decreases at both edges, so the spray patterns of adjacent nozzles should be overlapped about three to four fingers width at the soil surface to achieve even distribution. Fan nozzles do not provide as good a coverage as cone nozzles when used to spray crop foliage. Fan nozzles are available in several different angles of spray width. Wider angles allow the spray boom to be carried closer to the ground and this lessens spray drift problems on windy days.
· Even Flat (Fan) Spray Nozzles should be used for making band applications of pesticides to the soil. Spray output does not decrease at the edges, so spray patterns should not be overlapped and used for broadcast applications.
· Solid Cone Spray Nozzles provide better coverage of plant foliage than fan nozzles but should not be used to apply herbicides and insecticides to the soil.
· Hollow Cone Spray Nozzles offer somewhat better foliar coverage than solid cone nozzles due to greater leaf agitation as the spray pattern passes over the plants.
· Whirlchamber (nonclog) Spray Nozzles are special wide angle hollow cone nozzles that can be used in place of fan nozzles. Their design reduces clogging, and drift is minimized because of the wide angle pattern (enabling lower boom height) and larger droplet size.
Nozzle Screens: Nozzles used on tractor boom sprayers usually have mesh or slotted strainers to help prevent clogging. Some backpack sprayers have strainers or can have them added on. Routine cleaning is required, especially when wettable powders are used.
Tips on Using Backpack Sprayers to Apply Insecticides
· Use good pressure and a fine spray. Pressure is too high if excessive spray drift (misting) occurs.
· Maintain a steady pace through the field. Avoid pausing at each plant unless the crop is very large.
· Rotate your wrist while spraying so that the spray hits the foliage from different angles.
· Keep the nozzle far enough away from the foliage so that the spray has a chance to spread out before hitting the leaves.
· If using a wettable powder, remember to periodically shake the sprayer to keep the pesticide in solution.
· Keep a piece of soft wire handy for cleaning out clogged nozzles, but use it gently to avoid damaging the nozzle opening.
· Do not spray plants when their leaves are wet or when rain is likely within a few hours afterwards.
· Do not add wettable powders or EC's directly to the sprayer tank. First mix them thoroughly in a bucket with several liters of water. Make sure wettable powders are completely dissolved.
Most pesticides are compatible with each other in the spray tank, but check the fable to make sure. In some crops like peanuts and vegetables, foliar insecticides and fungicides are often applied together. Spray compatibility charts are available from many pesticide companies.
Water with a pH of 8.0 or above (alkaline) causes a rapid breakdown of organic phosphate insecticides. Such high pH water is usually confined to limestone or low rainfall areas. Special buffering agents are available to lower the pH if necessary.
Certain insecticides are phytotoxic (injurious) to certain crops. Always check the label instructions. Wettable powder formulations tend to be less phytotoxic than emulsifiable concentrates, especially in temperatures over 32 C.
Trichlorfon causes severe injury. Azodrin and methyl parathion cause some injury.
Minor foliar injury which shows up as reddish brown spots on the earliest leaves is sometimes caused by soil applications of carbofuran, Thimet, and Disyston. The plants usually outgrow the damage with no yield reduction. Runner varieties on sandy soils are the most sensitive, and dosage should be reduced by 25 percent under these conditions.
Insecticide Recommendations For the Reference Crops
Particular pesticides are not recommended for the reference crops in this manual because of the potential misclassification of pest problems and misused pesticides. Rather than rely on this manual for pest diagnosis and pesticide selection, it is recommended that you rely on the insecticide recommendations of your country's extension service if they are known to be effective and if they do not involve the use of high-toxicity Class 1 chemicals (see Appendix K). Before using any insecticide, refer to the safety guidelines and toxicity data in Appendix K. Always know the relative toxicity and environmental hazards of the products you use or recommend.