Dry Beans XIII-4

Defoliating Insects

Gary L. Hein & Frank B. Peairs

Flea beetles, grasshoppers, western corn rootworm, Mexican bean beetle

There are several defoliating insects that may affect dry beans through the season.  The most serious in the High Plains is Mexican bean beetle.  Grasshoppers can be a sporadic problem when beans are grown adjacent to hatching beds when grasshoppers are numerous.  Other defoliators are much less likely to cause significant damage, but in isolated situations may become serious.  Damage from all defoliating insects must be judged based on the amount of defoliation that has taken place, the number of the insects present, and the potential for continued defoliation (e.g. younger insects have greater potential for continued feeding). 

A number of randomly chosen plants should be examined and defoliation estimated to arrive at an estimate of the current defoliation.  Examples of several defoliation levels on dry bean leaves are shown in Figure 1.  Defoliation of plants or individual leaves is difficult to estimate and often is over-estimated.  These examples can be used as guides to estimate leaf defoliation.

The impact of defoliation from insects will depend on a number of factors.  Perhaps the most important are the amount of defoliation that takes place and the plant growth stage.  Plants in early vegetative stages of growth can withstand much more defoliation with little impact on yield; whereas, beans that are flowering or filling pods can tolerate much less damage before yield loss is impacted significantly.  Figure 2 shows the relationship between defoliation and yield loss for navy beans from manual defoliation (not insect induced).  The information in Figure 2 can be used as a guideline in determining the amount of defoliation that is tolerable.  For example, forty percent defoliation of vegetative beans will result in only about five percent yield loss.  However, 40 percent defoliation of flowering and pod filling beans will result in about 15 percent yield loss.  Flowering and pod filling beans will suffer 5 percent yield loss from only about 10 to 20 percent defoliation.

In addition to estimating the defoliation level of beans, it is important to determine the population level and status of the insect that is doing the damage.  For treatment to be warranted, the insects must be present in large enough numbers and in a young enough stage where damage is likely to continue.

Figure XIII_1. Six levels of bean leaf defoliation (adapted from original by J. Barrigossi).

Figure XIII_2. Relationship between percent defoliation and yield loss at different bean growth stages (Adapted from Schaafsma and Abbott, 1994, J. of Prod. Agr. 7:202_205).

Categories:  Dry beans, Defoliating insects, Flea beetles, Grasshoppers, Western corn rootworm, Mexican bean beetle

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