Population dynamics of onion thrips (Thrips tobaci) and western flower thrips (Frankliniella occidentalis) in New Mexico onions

Onion thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis) are the major arthropod pests of bulb onions in southern New Mexico. Insecticides are currently the sole control method to maintain thrips populations below economic threshold levels. Binary insecticide tank mixes are currently the most used with up to six applications made to fields seasonally. Previous research has shown that F. occidentalis is the dominant species in immature onions with T. tabaci dominating the later growth stages. Understanding the interactions between thrips species and impact of specific insecticides to those interactions is important for their management. Research was conducted in the Mesilla Valley to determine if the change in thrips species dominance in western onions is due to: changes in migrating thrips species population densities; influence from specific insecticide active ingredients; or from natural species intra-plant competition (i.e. fecundity, feeding sites). Survey results of selected weeds and crops determined F. occidentalis as the dominant species throughout the season. Alfalfa, grown on approximately 8,000 hectares regionally, is believed to be the primary migrant source of thrips to onions. A small plot replicated study was conducted to determine the impact of registered and candidate insecticides on changes in thrips species composition and density. Field plots consisted of onions exposed to migrant thrips populations or onions grown in cages excluding migrant thrips populations. Plot-caging and insecticide applications were initiated after thrips densities exceeded regional economic threshold levels. Thrips densities and species varied between insecticide treatments with T.tobaci dominating as the season progressed.