Abstract
Onion thrips (Thrips tabaci) is the main insect pest on New Zealand onions (Allium cepa). Insecticide applications are triggered by thrips numbers exceeding action thresholds. A new crop monitoring protocol that involves sampling a minimum of 100 plants (up to 4 ha) or 25 plants/ ha, was tested in 10 young or mature onion crops. Two people independently sampled each crop by zigzagging up and down groups of onion beds defined by pairs of sprayer wheel tracks and selecting a plant at random at predetermined intervals along the crop beds. The number of thrips adults and larvae per plant was recorded. A strong relationship was found between total numbers of thrips and the percentage of plants infested. If the onion industry raised its spray action threshold from 0.1 thrips/plant to 2 or 3 thrips/plant (c. 50% plants infested), it could adopt presence‐absence sampling that would be faster than counting all thrips. The time to monitor a crop was affected by field size, and only increased slightly with higher levels of infestation. The new guidelines could also be used for monitoring disease and weeds.
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