Thrips are a common insect pest that most growers have had difficulty controlling due to their small size and ability to hide in plant parts. Although this is not an insect associated with growing media, most species of thrips spend part of their lifecycle in the growing medium or soil. Thus, the question arises whether they can come from unused, packaged growing media. The simple answer is that it is very unlikely. To understand why, we will address the lifecycle of thrips and the common issues associated with them.
There are about 5,000 different thrips species in the world and those that have wings are the smallest winged insects. Most feed on fungi while living in leaf litter or decomposing wood, but some of these species feed on live plants. Although there are some variations among each species, there are 6 stages in their lifecycle, as seen in Figure 1. The length of the lifecycle is greatly influenced by temperature: the warmer the temperature, the faster the lifecycle. Common thrips species that attack greenhouse crops are tiny, typically 1-2 mm in length, and are active in a temperature range of 50-90°F (10-32°C), but their optimal range is 80-85°F (27-30°C). Thrips do not develop at temperatures below 50°F (10°C).
Female adults lay eggs in live plant tissue through their ovipositor. The eggs hatch within 2-4 days, thus beginning the first of two larval stages (see Table 1). The first instar stage lasts 1-2 days, then after molting begins the second instar stage, which lasts 2-4 days. While in these two stages, thrips are voracious eaters by using a mandible to punch a hole into a plant cell and insert a stylet which draws out the contents of the cell. Depending on the species, they feed on the base of flower petals, pollen, young fruit and/or new tender leaves in the terminal ends of stems. They are very hard to see as they are well hidden and smaller than adults.
At the end of the second instar stage, thrips relocate to the growing medium. While there, thrips go through a prepupal stage where they start to develop wing pads. This stage lasts 1-2 days. The next stage is the pupal stage in which the wings develop, consisting of long sparse hairs giving a feather-like appearance. The pupal stage lasts 1-3 days until the wings are fully developed, and then they emerge from the growing medium as adults. During the prepupal and pupal stages in the growing medium, thrips are inactive and do not feed or cause plant damage.
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