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Scouting for Insect Pests in Brassica Crops

Scouting is the foundation of integrated pest management programs. This article describes how to scout brassica crops effectively and outlines two commonly used methods for making responsible treatment decisions: individual treatment thresholds and the presence/absence method.

Introduction

Regular scouting is the first step of a successful integrated pest management (IPM) program. An overview of an IPM program’s basic concepts is provided in the Land-Grant Press publication, Integrated Pest Management: Concepts and Strategies.

In brassica crops, insect populations, especially caterpillars, may develop quickly, causing rapid and significant damage to yields and/or marketability. To facilitate timely treatment decisions aimed at limiting damage, fields should be scouted a minimum of once per week. Weekly scouting (or more often depending on weather conditions), beginning just after planting, allows growers to closely monitor pest populations, in addition to other potential issues.

Use the following recommendations to help scout effectively:

  • Take the following materials to the field: A jeweler’s loupe or hand lens (10X magnification is sufficient for most applications), vials or specimen bottles (to collect insects that cannot be identified in the field), and scouting forms (paper or electronic). Sample forms are provided at the end of this publication.
  • Walk a zig-zag pattern (figure 1) through the field to evaluate a representative sample. Pest populations are not always evenly distributed throughout the field. Change the path each time to avoid continually scouting the same areas. Select an area to examine randomly and without bias towards noticeably damaged plants (figure 2). Human nature is to focus on damaged areas; however, doing so may cause population levels to seem higher than they really are.
  • Examine groups of 5 consecutive plants in the same row. This strategy is significantly faster than examining single plants throughout the field (e.g., 10 groups of 5 plants vs. 50 single plants).1
  • Examine the underside of each leaf carefully. Caterpillars often position themselves adjacent to leaf veins, making them easy to miss if not examined thoroughly.
  • Record the number of plants scouted and all pest species and life stages found. Retain records in paper or digital format so pest activity can be compared to previous years.

Diagram of a zig-zag pattern

Figure 1. Walk a zig-zag pattern through the field while scouting. This will help evaluate a representative sample rather than focusing on one section or one side of the field. Image credit: Bek Diamond, Clemson University.

Collards in a field with a small area marked.

Figure 2. Select an area to examine while far enough away that the degree of insect damage cannot be easily seen. From where this photo was taken, the damage within the marked area cannot be easily seen. Image credit: Justin Ballew, Clemson Extension.

Treatment Thresholds

Most crops tolerate a certain amount of feeding damage before yield and marketability are affected; therefore, the goal of a responsible insect management program is to maintain pest populations below an economic threshold. This is the population level at which pests begin to cause significant damage to yield or marketability. Once a pest population reaches the economic threshold, an insecticide should be applied to return the population below the threshold. If a pest population level is below the economic threshold, the cost of making an application will be greater than the returns.2

The following treatment thresholds may be used to help make treatment decisions.3,4 For heading type brassicas (cabbage, cauliflower, broccoli, etc.), use table 1. For leafy type brassicas (collards, kale, mustard, turnip, etc.), use table 2. For help identifying pests, see the Land-Grant Press publication, Identification of Common Insect Pests of Brassica Crops.

Table 1. Treatment thresholds for heading type brassicas (cabbage, cauliflower, broccoli) based on pest species and crop growth stage.

Pest 0-4 True Leaves 4 Leaves to Head Fill Mature Head
Diamondback Moth 5 per 10 plants 5 per 10 plants 5 per 20 plants
Cabbage Looper 1 per 10 plants 1 per 10 plants 1 per 20 plants
Other Caterpillars 1.5 per 10 plants 1.5 per 10 plants 1.5 per 20 plants
Harlequin Bugs Bugs present on 5% of plants Bugs present on 10% of plants Bugs present on 5% of plants
Aphids Aphids present on 10% of plants Aphids present on 10% of plants Bugs present on 5% of plants
Flea Beetles Beetles and damage present on 50% of plants Beetles and damage present on 50% of plants Beetles and damage present on 50% of plants

Table 2. Treatment thresholds for leafy type brassicas (collards, kale, mustard, turnip) based on pest species and crop growth stage.

Pest 0-4 True Leaves 4 Leaves to Harvest
Diamondback Moth 5 per 10 plants 5 per 20 plants
Cabbage Looper 1 per 10 plants 1 per 20 plants
Other Caterpillars 1.5 per 10 plants 1.5 per 20 plants
Harlequin Bugs Bugs present on 10% of plants Bugs present on 5% of plants
Aphids Aphids present on 10% of plants Bugs present on 5% of plants
Yellowmargined Leaf Beetle 1 adult per plant 1 adult per plant
Flea Beetles Beetles and damage present on 50% of plants Beetles and damage present on 50% of plants

A sample scouting sheet is included at the end of this publication to copy and use in the field. Individual thresholds should be used when scouting small fields, when caterpillars are not the primary pest, or when only one or two species of caterpillar are expected. When multiple species of caterpillars are expected, use the scouting method outlined below.

Presence/Absence Sampling Method

It is not uncommon to find multiple species of caterpillars feeding within the same areas. This can make it difficult to determine which specific threshold to use when making treatment decisions. A simple solution is to use a scouting method that allows a treatment decision to be made without distinguishing the population levels of each individual caterpillar species. The presence/absence method is based on whether caterpillars (of any species) and feeding damage are present.1,2

Using the presence/absence method outlined below, a treatment decision could potentially be made after sampling a minimum of 20 plants. An example scouting form for using the presence/absence method is shown below, and a blank form is included at the end of this publication to copy and use in the field.

It should be noted that this method is only accurate for caterpillars and should not be used to make treatment decisions for aphids, harlequin bugs, or beetle pests.

Procedure and Example Form for Caterpillar Presence/Absence Scouting Method

  • Each row on the form represents a single plant scouted.
  • Put a mark in the “Damaged Plants with Larvae” column for each plant found with caterpillars and feeding damage present. If feeding damage is present but no caterpillars are found, do not count that plant.
  • Keep count of the cumulative number of damaged plants in the “Cumulative # of Damaged Plants” column.
  • Once a minimum of 20 plants has been scouted, it may be possible to make a treatment decision.

○  If the cumulative number of damaged plants is below the number in the “Low Limit” column, do not treat.

○  If the cumulative number is above the number in the “High Limit” column, treat.

○  If between the low and high limits, keep scouting until a decision can be made.

○  If a decision cannot be made after sampling 45 plants, scout again in three days.

  • Mark your treatment decision in the “Treat” column.
Plant No. Damaged Plants

with Larvae

Cumulative # of Damaged Plants Low Limit Continue Sampling High Limit Treat

Yes or No

1 X X X
2 X X X
3 X 1 X X X
4 X X X
5 X 2 X X X
6 X X X
7 X X X
8 X 3 X X X
9 X 4 X X X
10 X X X
11 X X X
12 X X X
13 X X X
14 X X X
15 X 5 X X X
16 X X X
17 X X X
18 X 6 X X X
19 X X X
20 0 1-5 >5 Yes
21 0 1-5 >5
22 0 1-5 >5
23 0 1-5 >5
24 0 1-5 >5
25 0 1-5 >5

In the example form above, after scouting 20 plants, the cumulative number of plants found with larvae and feeding damage present is 6. 6 is above the high limit (5 after 20 plants); therefore, this field should be treated.

Individual Threshold Scouting Form for Caterpillar Pests of Brassicas

Field:                                                                     Date:

Plant Number Pest Species Natural Enemies
Diamondback moth Cabbage Looper Imported Cabbageworm Other Caterpillars Other pests
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Total

Presence/Absence Method Scouting Form for Caterpillar Pests of Brassicas

Field:                                                                                      Date:

Plant No. Damaged Plants with Larvae Cumulative # of Damaged Plants Low Limit Continue Sampling High Limit Treat?

Yes or No

1 X X X
2 X X X
3 X X X
4 X X X
5 X X X
6 X X X
7 X X X
8 X X X
9 X X X
10 X X X
11 X X X
12 X X X
13 X X X
14 X X X
15 X X X
16 X X X
17 X X X
18 X X X
19 X X X
20 0 1-5 >5
21 0 1-5 >5
22 0 1-5 >5
23 0 1-5 >5
24 0 1-5 >5
25 0 1-5 >5
26 0 1-5 >5
27 0 1-5 >5
28 <2 2-6 >6
29 <2 2-6 >6
30 <2 2-6 >6
31 <2 2-6 >6
32 <2 2-7 >7
33 <2 2-7 >7
34 <2 2-7 >7
35 <3 3-7 >7
36 <3 3-7 >7
37 <3 3-7 >7
38 <3 3-7 >7
39 <3 3-8 >8
40 <3 3-8 >8
41 <3 3-8 >8
42 <4 4-8 >8
43 <4 4-8 >8
44 <4 4-8 >8
45 <4 4-8 >8

References Cited

  1. Smith JP, Shepard BM. A binomial sequential sampling plan using a composite threshold for caterpillar management in fresh market collard. Journal of Agricultural and Urban Entomology. 2004 Jul; 21(3):171–184. http://www.scentosociety.org/Volumes/JAUE/v21/171.pdf.
  2. Francis RL, Smith JP, Shepard BM. Integrated pest management for cabbage and collard: a grower’s guide. Clemson (SC): Clemson Extension; 2005 Sep. EB 156.
  3. Holmstrom K. Cole Crops IPM field guide for NJ. plant & pest advisory. New Brunswick (NJ): Rutgers University; 2015 May. https://plant-pest-advisory.rutgers.edu/integrated-pest-management/vegetable-ipm-holmstroms-crop-capsules/cole-crops-ipm-field-guide-for-nj/.
  4. Ayanava Majumdar A. Yellowmargined leaf beetle in crops. Normal (AL): Alabama A&M University, Alabama Cooperative Extension System; 2018 Sep. https://www.aces.edu/blog/topics/ipm-farming/yellowmargined-leaf-beetle-in-crops/.

Additional Resources

Bryant T, Reay-Jones FPF. Integrated Pest Management: Concepts and Strategies. Clemson (SC): Land Grant Press by Clemson Extension; 2020 May. LGP 1051. https://lgpress.clemson.edu/publication/integrated-pest-management-concepts-and-strategies/.

Ballew JB. Identification of common insect pests of brassica crops. Clemson (SC): Land Grant Press by Clemson Extension; 2021 Sept. LGP 1122. https://lgpress.clemson.edu/publication/identification-of-common-insect-pests-of-brassica-crops/.

Ballew JB. Common natural enemies of brassica insect pests. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2021. LGP 1130. http://lgpress.clemson.edu/publication/common-natural-enemies-of-brassica-insect-pests.

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