Damage and survival of western bean cutworm in dry beans

Investigator: Gary L. Hein, Panhandle Res. & Ext. Center, Scottsbluff

Collaborator: Ron Seymour, West-Central Research and Extension Center

Objectives:
Study 1: a. Determine the development of western bean cutworm damage through the season.
b. Determine the total impact of western bean cutworm damage on dry bean yield and quality.
c. Determine the survival of western bean cutworm larvae in dry beans and if there are major factors that modify larval populations.
Study 2: a. Determine the impacts of soil types on the ability of western bean cutworm larvae to survive through the winter.
Study 3: a. Determine the relationship between western bean cutworm moth activity and the differential oviposition in corn and dry beans.

Results and Discussion:
Study 1: a. Determine the development of western bean cutworm damage through the season.
b. Determine the total impact of western bean cutworm damage on dry bean yield and quality.
c. Determine the survival of western bean cutworm larvae in dry beans and if there are major factors that modify larval populations.
Small plot areas of dry beans were established at the Panhandle Research and Extension Center at Scottsbluff and at the West Central Research and Extension Center at North Platte. Western bean cutworm moths were collected, allowed to lay eggs on cages corn plants, and eggs were collected from these cages. Eggs were infested into the bean plots at different densities. Plots were monitored to determine the development of pod and seed damage and ultimately yield loss. Larval survival and development were also monitored in these plots by placing egg masses in small screen cages and later counting the number of larvae surviving through the season.
Results from the seasonal pod damage monitoring are shown in Figures 1 and 2. Damage began to build in the plots about mid August and peaked by about September 1. The overall levels of damage were much lower than expected for the egg load that was added to the plots. Also, at North Platte there was a significant background population of cutworms that resulted in significant damage in the uninfested plots. The subsequent seed damage from these plots is shown in Table 1. Again the low levels of damage were surprising as a very heavy egg load was added to these plots. It appears like levels of pod damage in the range of 6-9% will yield seed damage levels around approximately 1-1.5%.

Table 1. Western bean cutworm damaged seed found for varying levels of egg infestations, North Platte and Scottsbluff, 1999.
Egg Infestation Levels North Platte% Damaged Seeds Scottsbluff% Damaged Seeds
0 0.60 0.12
4 0.65 0.60
16 0.85 1.30
48 1.25 1.58

Figure 3 gives the levels of survival of the western bean cutworms that were infested as egg masses in the survival cages. Inspection of the infested egg masses indicated a very high egg and early first instar survival rate of about 96%. However, by the time that the next sample was taken about two weeks later, there were very few larvae left in the cages. For the remained of the season larval estimates of survival in the cages indicated survival in the range of 1-2.5%. It would appear that some factor shortly after egg hatch has a very significant impact on western bean cutworm populations and subsequent damage. This is an important finding in that this factor may relate to the poor predictability of cutworm damage in dry beans, and it may also help explain why the very high egg levels that were infested in this study resulted only in low to moderate pod and seed damage.

Study 2: a. Determine the impacts of soil types on the ability of western bean cutworm larvae to survive through the winter.
This study will be done in soil plots that have been established at the panhandle Research and Extension Center where three different soil types ranging from about 30 to 80% sand have been established. Western bean cutworm larvae were collected from corn and placed within cages located within each of these soil types. The larvae were allowed to move into the soil and pupate in the soil. In June we monitored the emergence of the moths from these cages areas. We found a very low rate of survival from all the soil types and could distinguish no differences between the soil types. The overall average survival was 4.2%. The low survival rates indicate poor potential survival of the over wintering larvae or perhaps a low tolerance to being collected and reinfested into our field plots. Techniques in this area need to be improved.
In these same soil plots larvae were allowed to feed on dry beans and when mature to burrow into the soil and establish over wintering chambers. In the fall, these areas were dug up and the level of the over wintering larvae was established. These data are shown in Figure 4. As the soil became sandier, there was an increase in the number of larvae that created chambers below the 4-inch depth, from about 12% for the loam soil to 40% of the larvae for the loamy sand. This indicates that in sandier soils the larvae can access deeper depths, and this helps explain why the western bean cutworm has typically been more of a problem in sandier soils.

Study 3: a. Determine the relationship between western bean cutworm moth activity and the differential oviposition in corn and dry beans.
Plots were established that included areas of dry beans and corn within a single caged plot during the period when western bean cutworm adults were active and laying eggs. Known numbers of western bean cutworm female moths were introduced into the cages and allowed to lay eggs for 3-5 days. Every 4-5 days these cages were moved to new plot locations and new females introduced into the cages. The density of egg masses in each of these areas was monitored in these cages.
The data from this study are summarized in Figure 5. During the early part of the egg laying period, more eggs were laid on corn than on beans. At mid oviposition, there was a switch in preference from corn and beans, and this switch resulted in a greater level of variability in the egg sampling that appeared to be related to variances in plant maturity between the various caged plots. Later in the egg-laying period there was a definite preference for laying more eggs on dry beans than corn. The attraction of western bean cutworms to pretassel corn has been know for a long time; however, little has been known of the oviposition preferences in dry beans. It appears likely that cutworm eggs would most likely be laid in dry beans after corn is no longer attractive for oviposition. This would result in a slight lag in larval development in beans as opposed to corn. Continued studies to further develop this relationship will continue next year.