While thrips are occasionally found in cucurbit crops, it is unusual to see significant amounts of damage. With the warm, dry conditions, high populations have been observed in several fields in Southwestern Ontario. The occurrence appears to have coincided with the drying down of neighbouring wheat crops.
Symptoms can take on several different forms, depending on the severity of the infestation. Plants may appear “off-colour”, yellowish or even bronze. On some plants superficial chewing may be apparent on the lower leaf surface, or even a window-pane like lesion.
Damage is often on the older growth, while the new leaves are vibrant green and healthy. The thrips are not always easy to spot with the naked eye. A hand lens or even a microscope may be required to confirm suspicions.
If the plant is actively growing, damage to the older leaves is not necessarily a concern. However watch the field closely to monitor the overall crop growth and health. Thrips may exacerbate stressful growing conditions such as dry soils or high temperatures.
Controls may be required if the thrips start to cause stunted leaves and shoots, or scarred, deformed fruits. Avoid unnecessary insecticide treatments as they may also kill any beneficial insects, causing the thrips populations to quickly rebound to damaging levels. Overhead irrigation or a timely rain may help to knock back the thrips and also promote beneficial insects.