Research Protocol

Sweetpotato: Measuring Root Damage with the Wireworm-Diabrotica-Systena (WDS) Severity Index

Authors: Jessica Weaver, Laura Schulz, Jordan White, Carol Miles 
Affiliation: Washington State University Northwestern Washington Research and Extension Center
_{https://vegetables.wsu.edu/sweetpotato/
December 2025} 

Introduction 

Wireworms are the larval stage of click beetles (Agriotes lineatus, A. obscurus, and Limonius canus) and are a primary pest in western Washington that can cause extensive damage to sweetpotato roots. The Wireworm-Diabrotica-Systena (WDS) severity index counts damage from wireworms and damage from striped cucumber beetle (Diabrotica spp.) and flea beetle (Systena spp.) larvae. Evaluating cultivars for this damage will help farmers select the right cultivar to grow on their farm and produce marketable sweetpotatoes even when these insects are present in the field. The WDS severity index provides a simple, accessible measurement that can be used to compare and determine which cultivars would be the most suitable for production based on insect pressure. This method does not distinguish between damage caused by different insect pests, as the damage from these insect pests is virtually indistinguishable from each other and all damage can cause roots to be unmarketable. 

Supplies 

• Shovel – to dig up sweetpotato roots for sampling 
• Hose – to wash roots 
• Crates – to sort roots, 1 crate per damage category, 4 total 
• Scale – to weigh roots in each damage category 

Experimental set up 

Wireworm-Diabrotica-Systena severity index (Schalk et al. 1993):  

• 0 = no insect scars/holes 
• 1 = between 1 and 5 insect scars/holes 
• 2 = between 6 and 10 insect scars/holes 
• 4 = more than 10 scars/holes 

There is no number 3 rating on this scale. This is to provide a weighted rating as a root with 10 or more holes is considered unmarketable.

Assessing insect damage 

1. Carefully hand harvest roots from the plants for data collection. Harvest 5 plants per location in a field and sample 4 locations per field. If an experiment, measure damage immediately after roots in each plot have been measured for yield.
2. Wash roots with water by spraying with a hose. Removing dirt makes it easier to see WDS scars.  
3. Set up crates, labeled: 0 scars, 1–5 scars, 6–10 scars, >10 scars.  
4. For each sweetpotato in a field sample or plot: 
   • Observe the sweetpotato skin fully by rotating the root 360 degrees. 
   • Count the number of insect scars. 
   • Place the sweetpotato into the corresponding crate by number of insect scars. 
5. Count the number of sweetpotatoes in each category and record the value for each sample or plot. 
6. Weigh the sweetpotatoes in each category and record the value for each sample or plot (Figure 3). 

7. Repeat steps 4–6 for each sample. 
8. Calculate WDS Index for each sample using the following equation:

Where: N₁₋₅=number of roots with 1−5 scars
N₆₋₁₀=number of roots with 6−10 scars
N_>10=number of roots with >10 scars
N_total=total number of roots