Findlay, J.R. , Extension Educator, University of Idaho
Hogge, J., Area Cereals Educator, University of Idaho
Leslie, M., Field Agronomist, Bingham Cooperative
Reitz, S., Professor and Director, Oregon State University
Sagers, J., Extension Educator, University of Idaho
Thomas, J., Extension Educator, University of Idaho

ABSTRACT

Thrips insects are currently damaging Idaho alfalfa crops through their feeding. University of Idaho Extension Educators evaluated thrips population growth and associated damage to alfalfa crops during the 2019 growing season. Initially, small populations of both onion thrips and western flower thrips were found in alfalfa fields. Later in the season, large populations of western flower thrips were found in alfalfa. These later populations were associated with damage to the alfalfa crop. Adjacent small grains crops and rabbitbrush stands do not seem to be a source of thrips population build up prior to their movement into alfalfa fields.

Introduction

Thrips are small insects that feed on various crops using piercing mouth parts (Figure 1). Their feeding can cause damage to leaf surfaces. This feeding results in leaf cupping and yellowing (Figure 2). Damage to alfalfa can result in visible chlorotic strips through the field. These strips of damage occur following harvest and are caused when thrips seek cover under the windrow.  Excessive thrips feeding can occur under the windrow prior to baling operations (Figure 3).

 

Figure 1. Thrips insect.

 

 

Figure 2. Leaf cupping and chlorosis of alfalfa due to thrips feeding.

 

 

Figure 3. Thrips feeding damage on alfalfa.

 

 

 

Thrips have not been considered a major insect pest of alfalfa in Idaho in the past. Thrips are not listed as a pest of alfalfa in the Pacific Northwest Pest Management Handbook (Hollingsworth et al., 2019). Thrips are also not listed as an alfalfa pest in the Alfalfa Management Guide (Undersander et al., 2011). Thrips are commonly found in alfalfa crops, but have not been shown to cause economic damage to the crop in past years (Godfrey et al., 2017; Tickes et al., 2003). We have found thrips to be an emerging insect pest of alfalfa in Southern Idaho since 2018.

In 2018, Southern Idaho field men and growers informed Extension educators of significant damage to area alfalfa crops from thrips feeding. Alfalfa growers asked Extension educators to assist in determining where the initial populations of thrips originate. It was hypothesized that thrips populations might build on small grain crops or rabbitbrush and then migrate to adjacent alfalfa fields. This input from growers resulted in a cooperative evaluation study between University of Idaho Extension educators and crop consultants of the Bingham Cooperative. Thrips populations and the damage associated with their feeding were evaluated during the 2019 cropping season. The study area was located west of Blackfoot, Idaho.

 

Methods

The research group identified an area west of Blackfoot, Idaho to monitor thrips populations. Area growers were contacted for permission to enter fields and take samples. Only Bingham Cooperative crop consultants entered the fields. six alfalfa, two small grain, and two rabbitbrush fields were identified and monitored in 2019 (Figure 4). These locations were monitored every 2 weeks beginning April 29^th, and ending October 7^th. The final sampling followed the first fall frost. Our intent in monitoring the thrips populations was to determine the timing of population increases. We also wanted to determine what species of thrips were damaging the alfalfa. Additionally, we were interested in whether the thrips populations originated in small grain crops, grass crops, or adjacent rabbitbrush fields. We did not attempt to calculate an economic threshold for the thrips in alfalfa. We did link the increasing thrips populations with increasing injury levels to the alfalfa leaves. 

 

 

Figure 4. Thrips study area. Red stars represent alfalfa fields, yellow stars represent rabbitbrush, and blue stars represent small grain fields.

 

 

Thrips collecting and counting were accomplished as follows.

Thrips collecting protocol:

• Six stems were collected randomly (zig-zag pattern) from each field.
• The top six inches of each stem were placed into a jar containing one cup of alcohol.
• The jar was immediately closed and shaken to kill and dislodge thrips.
• All sampling jars were marked with the field number and date. 

Thrips counting protocol:

• The thrips filtration system consisted of filters and catch basins.
• All stems and leaves were washed with water into the filtering system.
• The collection bottles along with their lids were rinsed into the filtration system to ensure all insects were collected.
• All thrips on each sampling filter were counted.
• 10% of the collected thrips were identified by species.

Alfalfa damage assessments:

• Each alfalfa site was evaluated for thrips damage at the time of insect sampling.
• Damage was assessed on a scale of 0 to 5. (0 = no crop damage, 1= detectable levels of damage, 2 = leaf cupping and yellowing with some loss in yield, 3 = significant levels of crop damage with definite yield losses, 4 = the majority of the field exhibiting significant crop damage and yield loss, 5 = total crop failure.)

All statistics in this study made use of the Multivariate General Linear Hypothesis for calculations. Thrips population counts and alfalfa damage assessments were completed on the same date in order to link thrips populations to observed crop damage. Linear regression was used to calculate the relationship of thrips population counts to alfalfa damage.

 

Results

The lowest thrips counts of the study were found in the small grain fields (Table 1). Thrips population build up in small grains lagged behind thrips population build up in alfalfa fields. It was concluded that small grain crops do not seem to be the source of thrips population build up in alfalfa.

Low populations of thrips were found in the rabbitbrush stands through August (Table 2). Western flower thrips populations spiked in September and then dropped after a hard frost at the end of that month. Thrips population build up in rabbitbrush lagged behind thrips population build up in alfalfa. Rabbitbrush does not seem to be a source of thrips population build up in alfalfa.

Table 1. Average thrips counts in small grain - Bingham County 2019.

 

 

Table 2. Average thrips counts in rabbitbrush - Bingham County 2019.

 

 

We initially found very low populations of both onion and western flower thrips species in the monitored alfalfa fields (Table 3). These low thrips populations were associated with low crop damage assessments. Beginning in the month of July, low populations of onion thrips were counted. Onion thrips populations never reached high densities the rest of the growing season. Western flower thrips populations began to rise at the end of July and peaked at the end of September. Western flower thrips populations were much higher than onion thrips populations. High populations of western flower thrips coincided with increased damage to the alfalfa crop in the months of July, August, and September. All thrips species populations were low after a hard frost that occurred at the end of September.

 

 

Table 3. Thrips counts and alfalfa damage - Bingham County 2019. Thrips counts are measured using the right axis and damage assessments are measured using the left axis.

 

 

The linear regression model that was calculated had a statistically significant p value of 0.001. The R² value of 0.61 represents the amount of variability in the observed alfalfa damage assessments that were explained by the regression equation (Table 4). There is a significant link between the observed damage to alfalfa crops and the increasing thrips populations. The regression model calculated was y = -0.01 + 0.0135 x. The interpretation of this equation is as follows: starting with a base damage of zero, every additional thrips counted increased the damage to alfalfa by 0.0135 points (on a scale of 0 to 5). When the thrips population reaches the count of 75 or more you will begin to see damage to the alfalfa crop. When the thrips population reaches counts of 150 or more you will see significant crop damage.

 

 

Table 4. Thrips damage on alfalfa- Bingham County 2019.

 

 

 

Discussion

Alfalfa growers in Eastern Idaho are interested in having a field monitoring tool that can be used to evaluate thrips populations. Growers that follow the thrips collection protocol described above will be in a position to calculate projected damage to alfalfa crops. This projection will assist growers in making management decisions related to thrips insect damage on their crops.

There is a need for further research into this emerging insect pest of alfalfa in Idaho. This study did not calculate an economic threshold for thrips in Idaho Alfalfa. An economic threshold can be studied and determined using the techniques Dominic Reisig used to determine an economic threshold for thrips in timothy (Reisig et al., 2008). The thrips scouting method described in this study could also be augmented with unmanned aerial insect scouting methods (Neufeld et al., 2019).

 

 

Literature Cited

 

Godfrey, L., Natwick, E., Goodell, P., Long, R., and Barlow, V. (2017). Alfalfa Thrips. UC Pest Management Guidelines. University of California Agriculture and Natural Resources. Retrieved from: https://www.ipm.ucanr.edu/PMG/r1302011.html

Hollingsworth, C. (2019). Pacific Northwest Insect Management Handbook [online]. Corvallis, OR: Oregon State University. Retrieved from: http://pnwhandbooks.org/insect

Neufeld, J., Ryu, J., and Barbour, J. (2019). Development of a UAS-Based Insect Scouting Method. Journal of the NACAA. 12(2). Retrieved from https://www.nacaa.com/journal/index.php?jid=1008

Reisig, D., Godfrrey, L., and Marcum, D. (2008). An Economic Threshold for Thrips in California Timothy. Proceedings of the 38^th California Alfalfa and Forage Symposium and Western Alfalfa Conference. Retrieved from: https://alfalfa.ucdavis.edu/+symposium/proceedings/2008/08-77.pdf

Tickes, B., Ottman, M., and Natwick, E. (2003). Alfalfa Report Yuma County, Arizona, September 8, 2003. University of Arizona, Cooperative Extension. Retrieved from: https://www.cals.arizona.edu/crops/counties/yuma/alfalfareports/2003/alfalfarpt090803.html

Undersander, D., Cosgrove, D., Cullen, E., Grau, C., Rice, M., Renz, M., Sheaffer, C., Shewmaker, G., and Sulc, M. (2011). Alfalfa Management Guide. American Society of Agronomy. ISBN: 978-0-89118-179-8. Retrieved from: https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf

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