Monitoring onion crops for onion thrips <i>Thrips tabaci</i>

Abstract

Onion thrips Thrips tabaci (Thysanoptera Thripidae) is the main insect pest on New Zealand onions The onion industry recommends monitoring 50 or 100 plants in 10 or 20 groups of 5 plants and applying a cluster of insecticides when populations reach 01 thrips/plant The robustness of these guidelines was tested by monitoring six crops before and after the first cluster of insecticides Two people each recorded the numbers of adult and larval thrips in each crop on each occasion using the following strategies (1) 100 randomly stratified plants (2) 20 groups of five plants randomly selected from 1 m² and (3) 20 groups of five adjacent plants giving a total of 300 plants sampled When sampling 100 plants estimates of thrips/plant were similar for each of the three sampling strategies However to obtain a particular level of precision fewer plants could be sampled using stratified randomly selected plants than groups of five plants