These gladiolas are infested with an insect called thrips; they are the major insect problem for the blooms. Thrips are tiny, slender black insects, about 1/25 of an inch long, obviously, difficult to see with the naked eye. Thrips cause white streaking on the leaves and blooms. Flowers can also be misshapen, discolored and occasionally will fail to open.
Thrips feed by puncturing the plant tissue and sucking out the cell contents. Preferring to feed on rapidly growing tissue, thrips often cause stunting. These insects are the culprits which also cause silvery to brownish scarring on avocado and citrus fruit surface, but this cosmetic damage does not harm the internal fruit quality.
Horticultural oils, azadirachtin (Safer BioNeem), neem oil and insecticidal soaps can be somewhat effective for the temporary reduction of the number of thrips if applied when damage first appears. Sprays must be applied to thoroughly cover susceptible areas of the plant including new leaf growth and buds. Note: these insecticides only kill newly hatched thrips and recently emerged adults, thus repeat applications are required, usually 5 to 10 days apart depending on the temperature. In the future, moving the location of the gladiola bed each year will discourage the spread of thrips.
Avoid the use of insecticides such as malathion and carbaryl, because they are highly toxic to natural enemies and can cause dramatic increases in spider mite populations and are generally not effective against most thrips.