Module 4: “He Who Hesitates is Lost” Taking Action against Pests
Despite our best efforts, preventative methods sometimes fail to prevent excessive insect damage in vegetable crops. If you have correctly diagnosed a pest problem and your scouting activities indicate that the pest has reached dangerous levels and is not likely to be taken care of by farmers’ friends it is definitely time to take action. The consequence of doing nothing at this stage is summed up by the title of this module. Particularly in vegetables, growers must take action to reduce damage caused by pests and diseases even if yield is not too badly affected, as damaged crops are less marketable than perfect-looking ones. The illustration below makes this point quite well.
At this stage the use of insecticides may be the only alternative left to save the crop or its value and using insecticides is not incompatible with the principles of IPM as long as these are used only when necessary and in a responsible manner. Key points to keep in mind are that the best approach at this stage involves using tactics that are effective against the pest, minimize disrupting natural controls (farmers’ friends), and are the least hazardous to human health and the environment. If insecticides are used, care should be taken to choose one with low toxicity to man and to other warm-blooded animals. Insecticides should be used only in strict accordance with label directions if commercially produced. A good understanding of insecticides allows these materials to be used effectively without posing undue risks.
In the next lessons we will discuss the range of interventions that may prove effective. The main ones are:
- Physical removal
- Botanical pesticides
- Biological Control Agents (Pathogens)
- Synthetic pesticides
Lesson 4.1: Physical Removal of Pests
Removal of pests by hand, or hand-picking, is a way of avoiding or postponing pesticide sprays on small areas of crop. Each part of the plant needs to be inspected and any pests found are crushed or removed by hand. Although this takes time and can be hard work, it is practical and effective in small plots, provided the farmers can distinguish between pests and their natural enemies. It will not be possible to find and destroy all of the pests, but since the technique does not kill the natural enemies, these can keep any hidden or overlooked pests at a low level. Sometimes only a few of the plants in a large field are affected by pests (for example, when only a few cabbage plants are infested with aphids) and pests can be removed or squashed quite easily. In extreme cases the whole plant can be completely removed. Pests can also be physically washed off the plants with sprays of water and if soap is added (liquid, powder or bar soap) the soapwater sprays are more effective against sucking pests such as spider mites, aphids and whitefly.
We talked earlier about the use of traps to monitor insect populations but many growers are experimenting with using traps as a control tool and in some cases these have had limited success. The most common traps used in vegetable production include yellow sticky traps, light traps, and pheromone traps. Points to keep in mind when using traps is that traps must be placed over a wide area to be effective. Also, trapping is most likely to succeed when the pest density is low initially and immigration into the trapped area is minimal.
Farmers and researchers are busy working on ways to increase the effectiveness of traps by combining them with other tools. One way is to use traps baited with pheromones and feeding attractants to lure the target pest to a pesticide-laced meal. Another combines trapping with farmers’ friends although this is still in the experimental stage. This method involves attracting adults to a pheromone trap where they are infected with a pathogen before exiting. Researchers in England have developed special traps that allow diamondback moths to enter the trap, pick up the fungal pathogen Zoophthora radicans and then exit the trap. The moth then carries the pathogen to the crop where it can infect both moth larvae and other adults.
Lesson 4.2: Botanical and Natural Pesticides
Although most pesticides are man-made chemicals, they can also be derived from plants (botanical products) or other sources. Even such common substances as wood ash and laundry soap are effective in many situations. Such products have gained in popularity in recent years but it must be recognized that they are pesticides and, as such, must be used with the same care as synthetic pesticides. Botanical pesticides, for example, also leave residues, can be disruptive to natural enemies, and may be toxic to humans.
Many plant products with pesticidal properties are used by farmers to make their own pesticides, for example, Datura, Tephrosia, Wormwood and Eucalyptus leaves. They are natural products and most of them break down quickly on the leaves or in the soil. However, there is very little information on their effective dose rates, their impact on beneficial organisms or their toxicity to humans. In fact one or two are known to be quite hazardous to humans, for example, tobacco extracts. As a result, no firm guidance can be given on their safe and effective use.
Lesson 4.3: Biological Control Agents
Like humans, insects also suffer from diseases which can weaken or kill them, and we can sometimes use these as pesticides. Several types of fungal, bacterial and viral pathogens have been identified which only affect insects and are safe for humans and animals. They suppress pests by:
- Producing a toxin specific to the pest;
- Causing a disease;
- Preventing establishment of other microorganisms through competition; or
- Other modes of action.
A few of these are commercially produced as biopesticides and some others can be prepared on the farm. This involves collecting diseased or dead insects and then incorporating them into sprays applied to plants.
The sick pests are, crushed, mixed with water, then the home made liquid is sprayed on to the crop using equipment normally used to apply chemical pesticides. Examples of this occur in India and Thailand. The diseases are caused by pathogenic fungi, bacteria or viruses, and the collected pests mixed into the spray tank then infect other pests in the crop and kill them.
Unfortunately only a narrow range of products based on entomopathogens are produced by the big chemical companies (with the exception of Bt (Bacillus thuringiensis) a bacterium which kills larvae (caterpillars) of moths and butterflies. One type of pathogenic pesticide under development is based on insect viruses. An example is called Plutella xylostella granulovirus(PlxyGV) which kills diamondback moth caterpillars – a serious pest of brassicas.
Expert opinion differs on how important pathogenic sprays will be in the future. There are some good and bad points about using these pathogens for pest control. On the good side is the fact that the pathogen is safe to consumers, so the crop can be harvested, sold and even eaten soon after spraying. They also have the important advantage of being highly selective orspecific. In other words they do not harm other arthropods such as natural enemies. They work together with the natural processes which limit pest numbers, instead of against the natural processes which reduce pest numbers. This contrasts with most pesticides which also kill natural enemies. On the other hand, pathogens do not usually kill the pests immediately as the disease takes time to develop within the insect. And because they are living organisms, their shelf life (storage period) is not very long. Some may even have to be kept in a refrigerator.
Lesson 4.4: Synthetic Pesticides
Synthetic (man made) pesticides are chemicals compounds that do not naturally exist in nature. They are generally less expensive, more effective, faster acting, and may be longer lasting than botanical or biological control products. It is these characteristics that make them so attractive to farmers. While these chemicals can be a valuable tool in your IPM toolbox there are risks associated with their use and care should be taken to use them responsibly.
Some of the most important points to consider when using synthetic pesticides include:
- Choose a product appropriate to the pest or disease – consult literature, extension services and agricultural retail outlets.
- Choose a product which poses the least risk possible to humans. As a general rule, avoid the most toxic products.
- Choose a product which is as selective as possible.
- Read the safety and application instructions on the pesticide label and calibrate application carefully.
- Apply the pesticide in a responsible and effective way.
For additional information on pesticide toxicity:
- agLearn resource on Information on how the toxicity of pesticides is described
- The WHO recommended classification of pesticides by hazard and Guidelines to classification 2000-2002 –
For information on pesticide selectivity:
- Koppert Online Service Side Effects Database: Information about the side effects of pesticides on natural enemies and bumblebees – http://side-effects.koppert.nl/
Lesson 4.5: Advice on Choosing an Intervention
As you have seen in the previous lessons, pesticides come in a range of ‘flavors’. They can be naturally occurring compounds derived from plants or other sources, various types of pathogens or synthetic chemicals. These are all types of interventions that farmers may need to resort to when preventative practices fail. A key point to remember about pesticides is that their use in IPM differs from the approach used in conventional pest control. When possible, IPM relies on pesticides that target specific pests, can be applied at lower rates, and are less toxic to beneficial organisms. Application rates, timing, and frequency are chosen to minimize effects on beneficials. Pesticides that can be substituted for each other are interchanged to slow the development of pest resistance to pesticides.
Another increasingly important consideration is residues on produce and many countries have strict regulations on Maximum Residue Levels (MRLs) for synthetic and botanical pesticides. Growers should therefore consider Pre-harvest interval (PHI, i.e. the number of days a farmer should wait after spraying before harvesting the crop) in selecting an intervention. A short PHI is more practical for the vegetable farmer and indicates that the product is either not very toxic to humans or has low persistence on the crop. Of course, the perfect pesticide may not be available (or affordable) at the time the farmer wants it, but where possible, an effective product which is safest to humans, to natural enemies and to the environment should be chosen.
Most of the pesticides used by vegetable growers are either fungicides applied to control or prevent fungal diseases, or insecticides to kill pests. Herbicides, which control weeds, are less widely used by small-scale vegetable farmers, but if labour for weeding becomes more scarce or expensive, herbicides may become more popular. If a pesticide is needed, the choice of product is important. Extension staff, chemical companies, pesticide retailers or online sources should be consulted to decide what product is used and how it should be applied.
Some of the important factors to consider in pesticide choice are listed below:
- efficacy – the more toxic the active ingredient (the poisonous part of the insecticide) is to the pests, the smaller the amount of active ingredient needed.
- safety – the product should ideally have a low toxicity to mammals (humans, livestock) and to other animals such as birds and fish.
- specificity – ideally the product should be toxic to the pest but not to other types of arthropod. If it is toxic to many other types of arthropod it is called a broad spectrum compound, and may do more harm than good by killing natural enemies (farmers’ friends).
- persistence – the longer the product remains biologically active in the field, the more effective it is because it can kill pests later as they emerge from eggs or arrive in the area. However, there may be a more serious effect on other organisms, i.e. greater environmental impact, from a persistent product, or problems of wanting to harvest produce before the pre-harvest interval.
- route of entry – whether it is a contact or stomach action product and whether it is systemic or translaminar will determine its suitability for different targets,.
- speed of action – the faster the product works, the less crop damage will be caused and the better the feedback the control team has on the effectiveness of operations. However, sometimes speed of action is not important, e.g.for locust hopper bands far from crops.
- bioavailability – will the pesticide be available to the pest on the surface of the plant, or is it systemic/translaminar
- resistance status – some pests develop resistance quickly so, to prevent this happening, it is important to ‘rotate’ among a range of appropriate pesticides
- compatibility with equipment – does it fit in with current equipment available
- labeling – good clear labels in local language are likely to encourage proper use.
- availability – pesticides must be available in suitable formulations at short notice when needed.
- cost – pesticides are one of the most expensive elements in many intensive production systems, so cheaper products will greatly reduce control costs.
Before going on you might want to try a simple self test of your knowledge on pesticides and choosing appropriate interventions and control products. Click here to pop up the test. Close the popup window to return to the course site.
Lesson 4.6: Reasons for Control Failure when Using Interventions
Even though interventions are considered farmers’ last line of defense against crop loss or damage it is not uncommon for them to fail. This can be devastating to small farmers as their vegetable crop may be their only source of cash income. It is therefore very important to know why interventions can fail and how to avoid wasting valuable time and money needlessly. The most common reasons for control failure when using pesticides are:
- Applying the wrong product
- Applying the wrong dosage
- Spraying a pesticide to which pests have developed resistance
- Failure to reach the pests with the pesticide
We will talk briefly about each of each on the following pages.
Lesson 4.6.1: Wrong Product
The most extreme case of using the wrong product occurs when farmers apply a fungicide to kill insects or an insecticide to control a disease. This occurs more often than people imagine. Perhaps the farmer feels that any chemical is better than no chemical. Another common problem is when farmers spray an insecticide when the crop is being damaged by feeding birds or other larger animals. Of course, a better approach in such cases is to employ bird scarers or construct physical barriers such as netting or fencing.
There can be control failures even if an insecticide is applied against an insect pest. Although many synthetic insecticides are non specific (they generally kill a wide range of insects) the more recent ones may be recommended only for a particular type of pest. In other words they are more specific. The most specific of all sprays are the entomopathogenic viruses (insect virus diseases which kill the insect). Some only affect a single species of pests such as theSpodoptera littoralis nuclear polyhedrosis virus which kills the caterpillars of Spodoptera, but does not even harm most other caterpillars. Medium specificity insecticides such as insect growth regulators may only kill younger stages of insects, leaving the adults alive. Some insecticides are targeted at a narrow group of pests such as sucking pests, and leave leaf-chewing insects unharmed. An example is pymetrozine, which stops sucking pests feeding, but which does not affect leaf-eating pests such as caterpillars.
Lesson 4.6.2: Incorrect Dosage
It is important to apply the correct dose, that is, the correct amount of the active ingredient in a pesticide formulation for a particular area of crop. For commercially sold pesticides of all kinds, the correct dose is usually recommended on the label. Underdosing might not kill the pests effectively and overdosing will be an unnecessary cost and may lead to high pesticide residues in the produce or build-up of pest resistance to the pesticide. If non-selective products are used, overdosing will also have an unnecessarily large impact on natural enemies in the crop.
These considerations are just as important for applying botanical or biological pesticides as they are for synthetics. In particular, botanical products are generally not specific and many plant extracts will also kill or repel beneficial insects and overdosing may lead to such problems as pest resurgence or pest replacement.
Application of the wrong dose can happen in several ways:
- Farmers do not correctly follow the ‘recipe’ when preparing botanical or biopesticides
- Commercially bought products may be adulterated and not contain the amount of active ingredient stated on the label
- The farmer puts too little or too much of the concentratedpesticideinto the sprayer tank each time he/she refills it, or
- he/she applies too little or too much volume of spray liquid to the crop.
How to get it right
When making your own botanical mixtures follow recommended instructions and measure carefully. When buying a commercial product be sure to purchase from a reputable dealer. Also, read the label. If the application instructions are not clear, ask the supplier to explain and put it in writing. If it is still not clear, see if there is an alternative product which has better instructions.
See our guide on Calibration of Knapsack sprayers
Lesson 4.6.3: Resistant Pests
Repeated application of the same insecticide or related insecticide compounds tends to result in the pests becoming harder to kill, or resistant, to the insecticide. The resistance builds up because even the best insecticide and the best application leaves one or two surviving insects which are, by the variation in nature, able to tolerate the chemical better than the other individuals. When the survivors meet up and breed, the next generation will probably contain a few more individual insects which can survive being sprayed with that product. If these surviving insects breed a further generation, the numbers capable of withstanding the poison will be even higher. And so it goes from generation to generation. In each stage the farmer sprays and probably kills the majority of pests, until one day the insects are mostly the ones which can survive the pesticide. They have become a resistant population.
Lesson 4.6.4: Control Product Not Reaching Pests
Sometimes the spray applied to a crop, synthetic or natural, does not reach the part of the plant where the pests are hiding. Mostly farmers spray plants from above, so spray wets the top surface of leaves nicely, and kills any pests on the top surface. However any pests sheltering from the sun under leaves, or ones that prefer to live on the lower surface of the leaves might not get enough spray to kill them. One example of such a pest is red spider mites. The miticide (pesticide for mite control) will kill most of the spider mites, but those underneath leaves may survive and very quickly re-colonise the whole of the plant, including the upper leaf surfaces. Another example of such a pest is aphids on brassicas – they often prefer underleaf surfaces, which need to be targeted with upwardly directed spray (or a systemic insecticide) in order to achieve good control.
A simple but useful tool for getting your control product on the hard to reach undersides of leaves is the V lance pictured on the right below. We have also prepared some supplementary information on the V lance. Check it out by clicking on the link below.
- supplement on The V Lance
Onions are a particularly difficult crop to spray. Crawling pests such as thrips are mostly right down at the base of the plant where the leaves are pressed closely together – the spray must be directed strongly downwards into these sheltering places. An onion crop is a fairly upright one so there is a danger that most of the pesticide will simply fall on the soil – clearly spraying by hand will be more efficient and environmentally sound than tractor spraying since a knapsack sprayer can be switched off between plants. Onions also have quite waxy leaves which means that spray will tend to roll off. Much better coverage and retention will be achieved if a small amount of soap, detergent or other wetting agent is added to the spray liquid.
The farmer may also have nozzle problems. The best type of nozzle for most small scale vegetable spraying is a small or medium sized hollow cone nozzle – see picture below . This produces quite small droplets traveling in many different directions and gives good coverage on complex vegetation shapes found in vegetable crops. A flat fan nozzle is better suited to tractor-mounted boom sprayers and if used on a knapsack sprayer can give a stripy and uneven deposit. The same poor deposit can even be caused by a hollow cone nozzle if it is worn or damaged from being cleaned with a hard metal object such as a nail or knife.