Resistance Bioassay from Southern Chinch Bug (Heteroptera: Lygaeidae) Excreta

Abstract

Antibiosis is considered to be the mechanism of resistance to the southern chinch bug, Blissus insularis Barber, because of high mortality and reduced oviposition rate of southern chinch bugs confined on ‘Floratam’ St. Augustinegrass, Stenotaphrum secundatum (Walter) Kuntze, compared with susceptible cultivars. To evaluate an alternative resistance bioassay, we determined the feeding response as excreta weight and grass residence frequency, based on the frequency of confined bugs on the grass. Within 24 h after presentation, excreta dry weight and grass residence frequency were reduced when bugs were confined on the resistant ‘Floratam’, compared with the susceptible ‘FX-313’ St. Augustinegrass. Excreta weight differed among hosts in several experiments involving 1 to 20 bugs per sprig. Excreta dry weight on resistant St. Augustinegrass genotypes such as ‘Floratam’ was always low (1-3 μg per bug per day) compared with susceptible hosts (range mostly 10-20 μg per bug per day). Excreta weight was highly correlated with oviposition rate (r2 = 0.42) on a genotype-mean basis. Because visual evaluation of the amount of excreta was more efficient than weighing in detecting resistance differences, we conclude that this technique is a rapid and powerful tool for detecting resistance. In two experiments, bugs reinstated within 2 and 5 d from ‘Floratam’ to a susceptible host showed no continuing effect from the prior host. Therefore, we conclude that the basis of resistance by ‘Floratam’ must be a feeding resistance.