Further Studies of Damage to Safflower Plants by Thrips and Lygus Bugs

Abstract

Trials with western flower thrips, Frankliniella occidentalis (Pergande), confined on growing safflower buds for 34 days, demonstrated that the buds are able to tolerate fairly large numbers of nymphal thrips before the seed yield is reduced significantly. Adult thrips were introduced into small cages covering selected buds (¼ inch in diameter or less) on plants growing in the field. An indirect sampling method was used to estimate the subsequent buildup of the population of nymphs. The tests were set up with initial thrips densities adjusted to average 5, 10, 20, and 40 adults per head. The density of nymphs averaged as high as 75 per bud when 10 adults per bud were introduced into the cages, and there was no consequent drop in seed production. Approximately twice as many nymphs per bud developed when initial introductions averaged 20 adults per bud. Infestations started with 20 or 40 adults per bud decreased significantly the number of good seed heads produced, the number of seeds per head, and the total yield. Analogous trials with lygus bugs, Lygus hesperus Knight, indicated that the threshold of economic damage to the safflower crop was exceeded when the ratio of bugs to buds exceeded 1:8. Significant decreases in yield criteria were obtained when the bug to bud ratios were adjusted to 1:4 or higher. A new stripe, or thin-hulled, variety of safflower appeared to be more susceptible to injury by Iygus bugs than the variety U. S. 10. Seed yield of the variety U. S. 10 was not materially decreased when the plants were completely disbudded 13 days before the start of the blooming period. This operation reduced the number of seed heads which finally matured, but these bore larger and more numerous seeds than the seed heads of check plants. Complete disbudding or defoliation done at the onset of the blooming period caused very significant reductions in yield values. Attempts to improve seed yield with pesticides applied to control thrips and lygus bugs were unsuccessful, because the insect infestations and the plant damage incurred were not great enough to affect the yield.