Answer: The thrips that damage your roses are in there because mama thrips made a small slit in the bud or new growth tissue, then laid her eggs. After a few days, the eggs hatch and go about sucking plant fluids until they get fat enough to pupate, commonly in the soil. One species, the Western Flower Thrips, sometimes pupates in plant litter or protected areas on the plant as well.
Eventually they emerge as a flying adult, mate, and the process renews. This whole life cycle, from egg to adult, takes only about two weeks during warm weather, and about a month in cooler weather.
Damage to the rose bud is most noticeable in light colored roses, although thrips may attack all roses. If the buds open at all, the petal edges may look brown or discolored. Sometimes the buds will only partially open. (Although this is not a single indicator, as high humidity will also keep roses from fully opening - called "balling".) Sometimes, the buds will simply wither and die.
My technique for checking for thrips is to take a blossom and, with your fingers, pull back the petals. If you see small slivers of creme, yellow, brown or black scurrying about headed for cover, these are thrips? well, actually the larvae. A hand lens will assist you in identifying these critters as they are only about 1 - 3 mm long.
As with the aphid controls, we rosarians have an arsenal of weapons with which to combat these pesky critters. These range from those high-pressure watering wands designed for insect control to natural predators like lacewings and predatory mites to botanical and chemical pesticides.
The problem with thrips is that they can transmit diseases, which can alter the growth of your roses. I wonder if your roses are developing a virus?
Q&A Library Searching Tips