J.J. TOLLEFSON
Iowa State University, Department of Entomology, Ames, Iowa, U.S.A.,  50011-3140

The mechanisms of host resistance to insects have been classified as non-preference, antibiosis, and tolerance.  Non-preference describes plants that have lower populations of a pest because they are relatively less preferred than others.  Antibiosis refers to the harmful effects of a host on a pest, resulting in suppression of pest densities.  Tolerance is the ability of a host to produce higher yields while sustaining similar densities of a pest.  Because resistance due to non-preference and antibiosis results in the presence of fewer pests, pest density is commonly used to evaluate these forms of resistance.  With tolerance, however, pest densities do not differ measurably and comparisons must be made using differences in host growth and yield.

In the USA, an extensive search has been made for corn germplasm that is resistant to western corn rootworm (Diabrotica virgifera virgifera LeConte (WCR)) larval feeding.  The WCR larvae are subterranean, making density estimates difficult to obtain.  Consequently plant indices of larval density have been used to screen for non-preference and antibiosis.  The most common index has been the root-injury rating.  Plant characteristics that have been used to compare corn germplasm for tolerance to CRW larval feeding include: root size, root re-growth, plant lodging, and the amount of vertical force required to pull plants from the soil.

Attempts have been made to contrast root size using volumetric displacement of water and visual ratings.  The volumetric displacement of water provides a quantitative measurement of size, but the measurement error has been too large for it to be useful.  While visual ratings subjective, statistical differences in the ratings have been found and the ratings have been useful in ranking tolerance.

Root re-growth is the measure of a plant’s ability to recover from injury caused by WCR larvae.  Quantitative measurements of re-growth have been made by weighing root masses during and after larval feeding and calculating the difference.  Visual ratings of re-growth have also been used to evaluate for tolerance using a 1 – 6 scale of fibrous root growth.  While root weights are more precise than the subjective visual ratings, they are more difficult to obtain because the plots must be evaluated twice and the root masses dissected, dried, and weighed.

Plant lodging and the force required to pull plants from the soil are general estimates of root mass and health.  Because lodging only results under certain environmental conditions, it has not been a reliable indicator of corn tolerance to WCR feeding.  The force required to pull plants from the soil is less subject to environmental variability and has been more useful.  The measurements of pulling force are quite variable, however, requiring sample sizes that are larger than visual estimates of root size and re-growth.