Update on Tulelake Grasshopper Outbreak

Winged grasshoppers are starting to show up in several locations (Image: Rob Wilson)

The following is from Intermountain Insights, University of California Agriculture and Natural Resources, Intermountain Research & Extension Center, Rob Wilson.

Expect large numbers this summer!

I was able to get out Friday and today to scout for grasshopper nymphs around Tulelake.  My findings were quite alarming as it appears the grasshopper populations are very widespread with many locations having 20+ grasshopper nymphs per square yard.  

Almost every survey location on Hill Rd, Dorris Brownell Rd, Stateline Rd, Sheepy Ridge, Sheepy Creek Rd, Lower Klamath Refuge, and Tulelake Refuge had grasshopper populations over 8 per square yard.  

As this route spanned over 50,000 acres that is a lot of grasshoppers.  

My survey locations were primarily non-tilled sites including roadsides, fallow fields, rangeland, and non-crop areas.  I also scouted several irrigated crop fields. Most irrigated fields (except for permanent pasture and perennial grass hay) had low grasshopper numbers.  

This suggests tillage, irrigation, and flooding are effective non-chemical approaches for preventing overwintering grasshopper populations.  

Unfortunately, the irrigated crop fields may still be inundated with swarming winged grasshoppers later this summer.

High grasshopper nymph population in a field of prickly lettuce (Image: Rob Wilson)

I was surprised to see a wide range in grasshopper nymph development.  

Most locations had grasshoppers from the 2nd instar to winged stage with most being in the 2-4 instar stage.  This suggests an extended egg hatching period (1+ months) which the literature suggests is favored by cool, dry conditions.  

I was also very surprised to see thriving grasshopper populations at locations that lacked grass vegetation.  

Fields of perennial pepperweed and prickly lettuce were loaded with grasshoppers.  I found high grasshopper populations on hillsides dominated by annual grasses with green mustard and forbs.

The annual grasses have dried down, but it appears the grasshoppers switched their diet to mustards and other forbs.  Really dry sites (where I found grasshopper earlier) were void of grasshoppers suggesting the grasshoppers ran out of food and died.

Grasshoppers are readily feeding on perennial pepperweed and other broadleaf vegetation not just grasses (Image: Rob Wilson)

Time will tell how bad the grasshopper swarms will be this year, but all signs are pointing to a bad year.  

One possible outcome of the extended egg hatching period is grasshopper swarms may be less dense but extend over a longer time period compared to last year.  

2023 spring temperatures were much warmer and consistent compared to 2024.  

Local PCAs and land managers have indicated thousands of acres have already been treated for grasshoppers including Lower Klamath Refuge.  

My scouting suggests there is still time to treat grasshopper nymph infestations, but the timeframe is quickly closing.  The extended hatching period this year will limit the effectiveness of a single insecticide treatment as most insecticides provide less than 30 days of control.  

It would be wise to scout fields treated more than 20 days ago to make sure young nymphs are not reinvading the site.  Below is a short video showing grasshoppers feeding in a wide range of vegetation types around Tulelake.