Iowa scale
Assessment of the amount of damage by larval feeding of D. virgifera virgifera on maize roots can be done by digging out the root system taking a diameter of approximately 25 cm to a depth of 15 cm. As much soil as possible is removed from the roots by gentle shaking and/or beating, care being taken not to break off any of the crown roots. The root systems are then washed so that all crown roots are fairly clean.
Damage on the root systems can be assessed by using a scale such as the following (Iowa State University 1 to 6 scale)
1 = No visible damage or only few minor feeding scars (not illustrated, but similar to a rating of two); 2 = Visible feeding scars present but no roots eaten off to within 4 cm of the plant, or with one or two shortened roots if the rest of the system was relatively free of damage; 3 = Several roots eaten off to within 4 cm of the plant, but never the equivalent of an entire node of roots destroyed 4 = One node of roots destroyed or the equivalent completely destroyed 5 = Two nodes of roots destroyed completely 6 = Three or more nodes completely destroyed (not illustrated, but similar to a rating of five) |
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('destroyed' means 'of no functional value to the plant')
These drawings show damage at approximately 67 days after planting. They represent the appearance of a given root system for each of 4 damage categories (2-5). Root-bearing nodes are numbered from one to seven, with roots of node eight just beginning to appear. The darkened areas represent significant damage from feeding and tunneling.
A new Iowa scale (1 to 3) can be downloaded from:
http://www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html