Over the last decade, lygus has taken center stage as the No. 1 pest in Arizona cotton. It's responsible for about half the insect-caused yield losses, and is mostly Lygus Hesperus.
University of Arizona Entomologist Peter Ellsworth, based at the Maricopa Agricultural Center, shared important information about lygus in Arizona cotton during the Second International Lygus Bug Symposium at Pacific Grove, Calif.
The research co-author, Steven Naranjo, is a research entomologist with the USDA's Agricultural Research Service at the Arid Land Agricultural Research Center at Maricopa.
“To control lygus in Arizona cotton, the average grower will spray from zero to 2 times,” Ellsworth told the group of 50 lygus researchers from 12 states and seven countries.
“Lygus has become a higher management focus for growers following advances in pink bollworm control with Bt transgenic cottons and control of whiteflies (Bemisia tabaci) with insect growth regulators.”
According to Ellsworth and Naranjo, decade-long Arizona cotton studies have focused on the development and refinement of action thresholds, termination rules for chemical control, and selective chemistry development to allow further gains in conservation biological control.
“We identified 15 total lygus per 100 sweeps as a density associated with economic loss; however, correlations with yield were weak.”
Nymphs, especially large ones (instars 3-5), were usually associated with yield loss over different conditions. Regression analyses showed a maximum yield at 15 total lygus, with at least 1.7 nymphs per 100 sweeps. Revenues were maximized over a range of economic conditions at 15 total lygus with at least 5.2 nymphs per sweep.
Extension recommendations shared with Arizona and Mexico growers suggested action thresholds of 15 lygus, with at least four nymphs per 100 sweeps (15:4). As the plant matures and fewer flowering/fruiting sites are produced, the need for and return on controls diminishes.
Termination rules for discontinuing lygus chemical controls over 12 different production scenarios — two planting dates times two irrigation timings times three different maturity groups — showed a dynamic relationship between yield/revenue and insect density.
Shorter season cotton varieties benefit less from extended lygus protection, the entomologists noted. Longer season varieties appear to be especially vulnerable to lygus damage and more responsive to lygus controls.
Broad spectrum chemistry
Cotton producers have historically depended on older, broad-spectrum chemistry to control lygus. This has tended to place them at risk of pest resurgence and secondary pest outbreaks, Ellsworth and Naranjo said.
Exploratory studies with new chemistries over the last five years have yielded two promising control agents — metaflumizone and flonicamid — aimed at selectively controlling lygus.
Large plot studies confirmed that compound usage leaves the natural enemy complex intact compared to untreated controls, when compared to broad-spectrum standards like acephate that substantially reduces natural enemies, they said.