NDSU

Abstract
Lygus bug, Lygus spp., is an economically important insect pest to the field peas and lentils grown in the northern Great Plains, causing “chalk spot” damage. The financial losses from chalk spot damage are very significant. Seed lots with chalky spot on more than 1% of pea seeds and 2% of lentil seeds are downgraded by the USDA Federal Grain Inspection Service to a lower grade, animal feed market. Damaged seed cannot be separated from good seed. There are increasing concerns about Lygus bug damage on field pea and lentil grown in North Dakota. In 1997 and 2002, North Dakota producers experienced significant quality loss due to chalk spot. Two of the major local processors, Premier Pulses and Superior Grains, have also expressed concerns over amount of Lygus bug damage on field pea and lentil. Research objectives were to determine the most effective monitoring techniques, and to determine the insecticide efficacy and the most effective spray timings for control of the Lygus bug in field peas and lentils grown in North Dakota. Three different monitoring techniques were evaluated for the adult and nymph stage of Lygus bugs: sticky traps placed in the field and edge, sweep net samples, and visual field observations. Sticky traps placed in the field or field edge were the most effective monitoring technique for adult Lygus bugs in peas. For lentils, the sweep net or sticky traps placed in the field or field edge were effective for monitoring the adult Lygus bugs. Visual field observations were not effective for monitoring the adult or nymph stage of Lygus bugs in either peas or lentils. Since very few nymphs were captured, it is difficult to make any conclusions on their most effective monitoring technique. The absence of nymphs in fields suggests that Lygus bug populations were not actively reproducing in pea or lentil fields, and that adult Lygus bugs were immigrating into the field from other crops, like alfalfa. Movement of adult Lygus bugs also occurred late in the crop development (late seed fill and maturity). These later stages are less susceptible to chalk spot injury than earlier crop development stages. For the insecticide study, the Lygus bug populations were so low that no significant differences occurred between different timing of insecticides or insecticides in pea or lentil. Although there were no significance differences, the percent with chalk spot or percent damage was generally lower in the treatments where insecticides were applied at both the early (late flowering) and late (early pod) applications, than the early or late application alone. Both Asana 0.66 EC at 5.8 fl oz./A and Warrior 1 EC at 2.56 fl oz./A appeared to have similar efficacy against Lygus bug regardless of crop. Additional research is needed to validate these results during higher Lygus bug pressure.

Objective 1: To determine the most effective monitoring technique for Lygus bug in field peas and lentils.

Materials and Methods
Three pea fields and two lentil fields were monitored weekly throughout crop development to determine when Lygus bug movement into fields. One field of field peas and lentils was located at the NCREC in Minot, and the other three grower fields in North central / northwest regions of ND: two pea fields - Bowbells and Keene and one lentil field - Keene. In the Pacific Northwest, Lygus populations are generally assessed in lentils using sweep net samples. However, yellow sticky traps and visual counts were also assessed as a possible monitoring technique for Lygus bugs.

Three separate methods of estimating Lygus population densities were conducted and compared:

1. Sticky traps - Commercially available sticky yellow traps (4x6 inches) were placed in the field before flowering. Traps were replicated four times at two different locations, parallel to the field edge and near the field center. Each trap was spaced at 20 ft. intervals. The number of Lygus bugs captured per trap was identified to species, counted, and recorded.
2. Visual counts - Fields were scouted using an X pattern and a total of 50 plants or five plants per 10 sites were inspected for Lygus bugs. The number of nymphs/adults per plant was recorded during each field observation.
3. Sweep net – Four sets of 25 sweeps each was taken with a 15" sweep net. Sweeps were taken along transects following an adapted “Z” pattern to assure uniform representation of the field. The number of Lygus bug nymphs/adults per 100 sweeps was counted during field observations.

Results and Discussion
For field peas (Table 1), over 44% and 45% of the adult Lygus bugs were captured by sticky traps placed in the field and edge, respectively. Sweep net captured only about 10% of the total number of adult Lygus bugs, and visual field observations only 1%. No nymphs were captured or detected in field peas. For lentils (Table 2), sticky traps in the fields, sticky traps on the edge, and sweep net samples had comparable numbers and percentages, about 30% of the total number of adult Lygus bugs per season. Visual field observations recorded no adult or nymph Lygus bugs in lentils. Overall, very few nymphs were captured (5 total): 60% with the sweep net and 40% with the sticky trap in edge. In summary, sticky trap placed in the field or field edge was the most effective monitoring technique for adult Lygus bugs in peas. For lentils, the sweep net or sticky traps placed in the field or field edge was the most effective monitoring technique for adult Lygus bugs. Visual field observations were not effective for monitoring the adult or nymph stage of Lygus bugs in either peas or lentils. Since very few nymphs were captured, it is difficult to make conclusion on their most effective monitoring technique. The absence of nymphs in field suggests that Lygus bug populations were not actively reproducing in the pea or lentil fields, and that adult Lygus bugs were immigrating into the field from other crops, like alfalfa. Movement of adult Lygus bugs (Figure 1 to 5) also occurred late in the crop development (late seed fill and maturity). These later stages are less susceptible to chalk spot injury than earlier crop development (early seed fill). Additional research is needed to validate these results and to develop a more useful friendly, reliable monitoring technique, such as, use of a degree day model. Movements of Lygus bug from other crops also need to be further studied in North Dakota cropping sequences.

Table 1. Total number of adult and nymph Lygus bugs captured per season in pea fields.

Table 2. Total number of adult and nymph Lygus bugs captured per season in lentil fields.
 

Crop	Technique	Total # Captured Adult	Percent	Total # Captured Nymphs	Percent
Lentil	Trap In-field	64	35%	0	0%
	Trap Edge	60	32%	2	40%
	Sweep net	61	33%	3	60%
	Visual	0	0%	0	0%
	Total	185	100%	5	100%

Materials and Methods
Insecticide yield trials for peas and lentils were conducted at the North Central Research and Extension Center in Minot, ND. In addition, insecticide efficacy trials were conducted in three grower fields: two pea fields - Bowbells and Keene and one lentil field - Keene. A RCB experimental design with 4 replications was used with plots approximately 4.5' wide by 22' long at the center and 10’ wide by 30’ long in the growers’ field. The pea variety, Nitouche, and lentil variety, CDC Richlea, were planted using recommended agronomic practices at the center.

The following treatments were tested using 8001 nozzles, 10 gpa with 40 psi at all sites:

1. Asana 0.66 EC (rate of 5.8 fl oz/A) – Dupont;
2. Warrior 1 EC (rate of 2.56 fl oz/A) – Syngenta; and
3. Untreated check.

Field plots were sprayed when the following crop phenology stage was reached: 1. Treatment at late flower = Early; 2. Treatment at early pod = Late, and 3. Treatment at late flower + early pod = Early + Late. Best management practices were used regarding fertility, weed control and harvest operations. Plots at center were harvested at physiological maturity and yield measured. A sub-sample of seeds was taken from each plot (500+ seeds per plot) and the percent of seed with chalk spot damage determined. A seed sample of 1000 gm was also sent to the USDA Federal Grain Inspection Service for rating on total percent damage and percent insect stung for samples from center and grower fields located near Keene.

Results and Discussion
Due to the light Lygus bug pressures at the NCREC in Minot, there were no significant differences between treatments in the lentil or pea study (Table 1 and 2). Figures 1 to 5 depict the Lygus bug populations at Minot and grower fields in McKenzie and Burke Counties. Most fields were below economic threshold levels of 7-10 bugs per 25 sweeps for lentils. (Note: There is no economic threshold that has been developed for Lygus bug in peas.) In Figure 2, Lygus bug populations were above economic threshold levels, but the lentil field was near full maturity. The incidence of chalk spot damage is lower near seed maturity than early seed development. Thus, low levels of Lygus bug feeding injury also occurred in grower fields located in McKenzie and Burke Counties. As a result, no significant differences occurred between different timing of insecticides or insecticides in the lentil or pea studies (Tables 3, 4, and 5). Although there were no significance differences, the percent with chalk spot or percent damage was generally lower in the treatments where insecticides were applied at both the early and late applications, than the early or late application alone. Both Asana 0.66 EC at 5.8 fl oz./A and Warrior 1 EC at 2.56 fl oz./A appeared to provide similar efficacy against Lygus bug regardless of the crop. Additional research is needed to validate these results during higher Lygus bug pressures.

Acknowledgements
The authors would like to thank the following growers for allowing us to work in their fields during the summer of 2003: Larry Singelness and Jay Jellsted of Keene in McKenzie County and Ron St. Croix of Bowbells in Burke County. We would also like to acknowledge the help and support of Superior Grains for donating the pea seed; NCREC seed increase program for donating the lentil seed; Lynn Brudevold of North Central Crop Consulting; Greg Johnson of Premier Pulses; and Kent McKay and Mark Halvorson of the NCREC. This research was supported by the Alternative Crops CSREES, North Dakota Pea and Lentil Association, Syngenta Crop Protection and Dupont.