Vegetable IPM: Module 3

Module 3: “Look Before You Leap”

Module Overview

We certainly covered a lot of ground in the last module on pest prevention. Following these practices can definitely mean not having to implement expensive ‘cures’ during the growing season. But even if a farmer does everything discussed there is no guarantee that pests will not find a way to threaten his or her crops and livelihood. When this happens something has to done. The question is what. Doing the wrong thing might be worse than not doing anything at all.

That’s why we gave this module the title we did. We want to highlight the importance of first looking and only then taking action based on what is observed – Look before you leap. Doing so can mean considerable savings in terms of time and money and make sure that your actions deliver the results you want.

This module will give you some tips on how to determine what problems are affecting vegetable crops (diagnosis) and also on ways to find out if it is really necessary to do anything about them (scouting and sampling).

Lesson 3.1: Problem Diagnostic Guide

Correct diagnosis is key before taking or recommending action. If you can accurately determine the cause of a problem it is much easier to come up with a cure that works. But diagnosis is difficult work. Pests and diseases are not always obvious. Sometimes pests or pest symptoms are below ground so plants need to be lifted to see the problem. Some pests are so small that they are difficult to see properly with the eye. A diagnosis either has to be done based on the symptoms or a magnifying lens is required – even for quite sharp eyes!

We also talked earlier about the importance and difficulty of proper identification. If there are organisms or disease symptoms which cannot be identified, samples should be taken to show to neighbours or extension staff. If it is not clear if an arthropod is a pest or a beneficial organism the previously discussed ‘insect zoo’ can be a great tool.

There are many systematic approaches for diagnosing pest and disease problems in vegetables. Here is a simple one that we have found quite useful and easy to follow. It consists of 6 steps that help to determine the most probable causes.

Step 1. Check whether there is a serious leaf or fruit pest. If they are present make your identification and corresponding recommendations. If there are no signs of such pests the problems is probably a disease or an under-soil pest and the most likely cause is a soil-based problem. Go to step 2.

Step 2. Check whether the farmer has been growing the crop in the same ground every year – if so go to step 4.

Step 3. The climate may have been poor in recent years, but it is more likely that the soil nutrition has changed. Try incorporating manure and fertiliser before the next crop. End of diagnosis.

Step 4. Some pest or disease is probably accumulating (building up) in the soil. Go to step 5.

Step 5. If the plants are wilting badly it is probably a bacterial or fungal wilt disease. More information can be found in the reference section. End of diagnosis

Step 6. Pull up a plant and look at the roots. If there are parts with small bulbous growths (called galls or knots) the problem is probably caused by presence of nematodes or eelworms (root knot because they create the typical swellings called knots), hence the name root knot nematodes or RKN for short, (check out the agLearn network resource on RKN).

An alternative to pulling up plants for examination is to dig a hole carefully next to the plant and washing the roots with water from a sprayer or bucket to check for root knots. After examination, the roots can be covered again with soil. End of diagnosis

Lesson 3.2: Scouting

Diagnoses like we talked about in the last lesson are certainly important and can provide solid information on probable causes and potential actions. But, for IPM, something more is needed to help a farmer make good decisions about how to respond to pest and disease problems. This goes back to our previous discussion on the role and importance of farmers’ friends.

To ensure that actions are not taken needlessly it is important to evaluate pest and natural enemy numbers over a period of time – a ‘snapshot’ can be very misleading. This is because of the interdependence and continuous fluctuations of pest and natural enemy populations. The graph below may help to explain this concept.

pests vs natural enemies over time

It shows that as a pest population builds up, so does the population of its natural enemies. The greater number of enemies then start to control the pest and both populations fall until another cycle begins. Given time the two populations can arrive at an equilibrium with pest numbers which are below the economic injury level. Click here to play with our interactive EIL simulator to gain a better understanding of this concept. It’s easy to see from the graph above that if only a single assessment was undertaken at time 1 (t1), a very different picture of natural enemy abundance would be observed compared with a later assessment at time 2 (t2).

Management practices, which may include applications of synthetic or biological pesticides or cultural manipulations, will also tend to contribute to fluctuations of pest and natural enemy abundance, so the role of these must be taken into account during evaluations. Farmers, and agricultural professionals, must understand and become familiar with the natural processes going on in the crop in order to make good judgments on whether to take action, when and how.

Periodic scouting (looking carefully at the crop) gives farmers the kind of information needed on whether action is necessary. Even if pests are present, there may be no need to spray. If the number of pests has not increased since the previous scouting it means that natural regulatory processes such as natural enemies or host plant resistance are preventing them multiplying to damaging numbers. In this case, do not use pesticide since it will kill the natural enemies and disturb the natural balance and could make pest and disease problems worse. Scouting should be carried out each week or more frequently. It means walking through the crop and stopping 10 – 20 times to examine the leaves/flowers/fruit for pests, diseases and natural enemies. The numbers and types of organism and disease should be noted.

Sample scouting form

Click to view a sample scouting form for use in vegetables. This is NOT a rigid guideline since the scouting needs of every individual will be different. A reduced amount of information may be sufficient for some farmers, and the form itself may not be necessary if the farmer can keep track of the information gathered in his/her head or on a piece of paper. Nevertheless it demonstrates most of the useful bits of information which a farmer might need before making spray decisions on interventions and emphasizes the importance of looking for natural enemies such as ladybirds, lacewings and syrphids. If there are a plenty of natural enemies present, the crop may not need spraying, particularly early in the season. This is because the natural enemies are able to reduce the number of pests, and also because some crops can recover from early season damage and compensate by growing more quickly. Even if there are only low numbers of natural enemies present, spraying may not be required as one natural enemy can eat many of its prey or hosts.


A key consideration when scouting is that proper sampling procedures should be followed. Sampling is collecting information upon which decisions can be based and involves determining population densities by calculating the density in a few sample areas and then extrapolating this to represent the entire field. Sampling is important in IPM and provides a fast estimate of the population densities of pests, potential pests, and beneficials. Any feature of a crop field and its inhabitants can be sampled. By using a proper sampling technique, your estimates will be reliable, comparable, and statistically valid

Using traps to monitor insects

While scouting and sampling are a key component of IPM many farmers, particularly if they are growing on large areas, may find these practices quite time consuming. Quite often they can rely on trapping to make their lives a bit easier. Detection devices such as sticky traps or bands and pheromone traps can alert growers to pest populations migrating into new fields from adjacent crops.

For more information on using traps to monitor pest presence and population here are some good references.