The plague thrips (Thrips imaginis), which is native to Australia, may occur in plague numbers in spring and early summer, when the climatic conditions arc favourable for its development. This species is present every year in limited numbers, but serious spring outbreaks have been found to follow an autumn and winter in which the rainfall has been above average.

The adult thrips feeds in the blossoms of weeds, fruit trees, vegetables and ornamental garden plants. Apples, pears, peaches and plums may be heavily infested, and the injured blossoms may turn brown and fall prematurely, preventing the setting of fruit. Citrus' and grape blossoms may also be attacked. but the setting of the fruits is not affected to any extent. Garden flowers, particularly roses, may be seriously damaged. The thrips enter the opening blossoms, feed on the petals and cause them to turn brown, and the dark excrement on light-coloured blooms adds to the disfigurement.

The minute, transparent eggs are laid in all parts of the flowers, and also in the young leaves adjacent to the blossoms. While the immature forms may feed upon the young leaves, they usually cluster inside the blooms where they feed mainly on the pistil and stamens. There are two larval stages, during the first of which the insects become yellow, and in the second orange-yellow. When fully fed the larvae pass down into the soil. where they enter their prepupal and pupal stages. Later, they become adult, make their way to the surface and fly hack to
the blossoms.

The life-cycle, from egg to adult, may vary from about ten days to a month or more, depending largely on temperature.