Various scouting methods are used to identify and estimate counts of insect pests in and on plants and soil as part of an integrated pest management (IPM) program.
Integrated pest management (IPM) involves optimally using chemical, biological, cultural, and biotechnological pest management strategies to provide an environmentally friendly and profitable approach to pest management. Insect pest monitoring involves using one or more scouting techniques to reliably estimate pest populations, migration, and suppression following management efforts. Scouting is the first step in IPM, which is accurate pest identification.
The accuracy of scouting techniques depends on the sampling procedure, consistency of method, and weather conditions. Scouting involves visually identifying insect specimens or plant injury symptoms. The term injury refers to the effect of a pest on the plant or plant part; thus, injury is a biological concept. The term damage refers to the economic impact of the pest on host plants.
Scouting Objectives and Practices
The first objective of scouting is to obtain an absolute estimate of insect numbers. Finding the exact number of insects living in any given area is challenging; however, it is important to know how many insects inhabit a plant, shoot, or leaf lamina in order to decipher pest population dynamics, egg-laying preferences, pest habitats, and mortality rates. Absolute estimates may also provide valuable information about how the pest infestation levels affect crop injury. Absolute estimates are time-consuming processes with rigid protocols and meticulous record keeping. These estimates may be difficult to extend to large areas.
Scouting to obtain a relative estimate of insect numbers is a more feasible method of population estimation because of its flexibility and speed. With a relative estimate, the actual insect numbers are not as important as the timely diagnosis of infestation levels to make a management decision. A relative estimate is based on the principle that if a large population of an insect pest is present in an area then it must be causing a measurable effect on the plants. A sequential sampling plan (SSP) is a special form of relative population estimate in which the total number of samples examined depends on the population density and distribution pattern of the insect. Thus, SSP allows rapid assessment of insect population density. A major drawback of relative estimates is that the accuracy of the measure depends on the resources invested into scouting and sampling—as the intensity of scouting increases so does the reliability of the population estimate.
Scouting Techniques for Foliar Insect Pests
Random walking is the simplest but the least accurate method of insect scouting. The scout randomly walks across the sampling area without sophisticated sampling equipment and visually records pest populations or crop injury. Transect walking is following a specific route through the area and is useful for large areas that may be divided into quadrants of any convenient shape and size.
Another method of scouting is using an insect sweep net, which is a specific net that is 15 inches in diameter and 3 feet long. It is one of the most used scouting procedures due to its simplicity; however, its accuracy depends on the methodology. To sample using a sweep net, swing the net from side to side in a 180-degree angle to your body. Try to take one full stroke in each step and hit the top of plants at an angle to dislodge insects in quick moves. Certain weather conditions such as high wind or rain will drive insects deep into vegetation, making this technique less effective.
Swing nets are specifically used to scout for grasshoppers, leaf beetles, foliage-feeding caterpillars in short crops, leafhoppers, and other medium-size plant bugs.
A drop cloth or beat cloth is a scouting procedure suitable for quickly assessing insect population levels below the crop canopy. The cloth should be a heavy white or black cloth that is large enough to contain the insects until their numbers can be recorded and to be laid flat in between crop rows. The technique is to lay the cloth between rows and shake the plants vigorously at an angle over the cloth to dislodge the insects. The accuracy of this procedure greatly depends on the scout’s identification skills, the weather conditions, and adequate random scouting throughout the field. Drop or beat cloths are used to scout for various plant bugs and caterpillars of moderate size in their slow-moving life stages. This method is not suitable for some fast-moving insects and small insects such as grasshoppers and flying insects.
Scouting Techniques for Soil Insect Pests
Using injury to assess insect populations is technically called a population index. Although injury on plant parts can be assessed rapidly, it is often difficult to correlate the actual pest prevalence level with crop damage. Scout for indirect evidence such as exuviae from aphids, frass from cutworms, chopped or lodged seedlings from grasshoppers or cutworms, webbing from caterpillars and mites, and nests from leaf-cutter ants.
Another technique is above-ground insect traps that use natural insect behavior to lure pests. A variety of insect traps are commercially available to assess the population level and migration pattern of extremely mobile insect species. Traps allow easy visibility of small insects as they are attracted in large numbers to these traps. The cost of traps depends on the technology involved. Color paper traps may cost a few dollars, but some automatic pheromone traps are expensive. In general, traps are very effective monitoring tools, but they will neither provide information about crop injury nor control insect populations over large areas.
The germinating seed bait technique for soil pests used to be a popular technique for sampling soil insect pests, but the appeal has fallen in recent years. Shallow placement of germinating seedlings can be used to detect wireworm infestations. Crop residue may attract insects such as cutworms and squash bugs that hide in stubble during the day and actively feed at night. Simple shovel sampling is another commonly used technique because it is a quick method and accuracy of the scouting is not a concern. Use any of these methods consistently and keep records for comparison of pest numbers within or across seasons.
General Tips for Scouting Specialty Crops
There is no one method suitable for all insect monitoring. Choose the most appropriate scouting technique depending on the crop and suspected pest, remembering to stay timely and consistent. Combining multiple scouting techniques will increase the accuracy of insect and beneficial population counts. Keep a record of your scouting activities for future use.
Improve the accuracy of scouting by randomizing your search. It is a good idea to study the layout of the crop area and scout in a zigzag path. During critical crop stages, examine numerous plants to increase the accuracy of your count or observation.
Carry some basic sampling equipment while scouting. To accurately identify insect species, collect specimens of insects and their host plants. Plastic bags, hand- held magnifying lenses, forceps, sweep nets or beat sheets, and killing jars or vials are all important tools for scouting. Some common online vendors for purchasing professional insect collection and storage kits include BioQuip Products and Carolina.
Remember to count beneficial insect populations when looking for pests. Beneficial insects such as lady beetles and lacewings can be easily seen and counted. This information can help in environmental conservation efforts.
Correctly identify insects. For expert identification, submit insect specimens in a sealed box or in a vial to your nearest county Extension office, research station, or plant diagnostic laboratory. Contact a regional Extension agent to assist you with sample preparation for mailing. With the availability of smartphones, sending texts of good-quality insect images can be useful to producers who need rapid assessment and IPM recommendations.
Other helpful resources include the following IPM scouting videos available on the Alabama Extension YouTube channel at https://www.aces.edu/go/1927:
- Cowpea Curculio Management Part
- Yellowmargined Leaf Beetle
- Greenhouse Vegetable IPM 101
- Don’t Let Spider Mites Show Their Might!
Also visit the Alabama Extension Beginning Farmer Program videos at https://www.aces.edu/go/1913.