SCIENCE TO THE GROWER: One thrips, two thrips, fed thrips, dead thrips
by Richard Evans
Western flower thrips (Frankliniella occidentalis), a widespread pest of many ornamental crops, is hard to control because both juveniles and adults live within flowers and buds, rarely loitering in the open where they would be vulnerable to chemical sprays. Not that insecticides are all that effective anyway. This insect has become resistant to almost everything except Lunchables, and nobody has come up with a suitable spreader-sticker for applying Lunchables to ornamental crops.
Speaking of Lunchables, one pest management approach worth considering is to feed plants in a way that makes them less appetizing to pests. For instance, several years ago researchers at Texas A&M (Chau and Heinz, 2006) examined the effect of liquid feed concentration on western flower thrips abundance on potted chrysanthemums. Plants received a 15-16-17 fertilizer at rates ranging from 75 to 750 ppm nitrogen. The number of thrips was highest on plants fertilized with 280 to 375 ppm N. Mums fertilized at a rate of 188 ppm N had about half the number of thrips found on mums fertilized at 375 ppm N, yet were the same height and produced a similar number of flowers. Unfortunately the authors didn’t report plant dry weight or leaf elemental composition, both good measures of fertilizer effects on growth.
The same research group (Chow and others, 2012) recently investigated the effect of liquid feed concentration on western flower thrips abundance and the ability of a natural enemy (Amblyseius swirskii) to suppress the thrips population on greenhouse cut roses. Roses received a 15-5-15 fertilizer at a rate of either 50 or 150 ppm nitrogen. Plants in the 50 ppm treatment had 30% fewer thrips than those in the 150 ppm group. Fertilizer rate did not affect the population of the natural enemy, and the combination of low fertilizer rate and exposure to the natural enemy was a double whammy for thrips, reducing their population to about 25% of what it was in the well-fed roses that weren’t protected by Amblyseius. In fact, the low fertilizer treatment alone was as effective at controlling thrips as the natural enemy alone. By the way, reducing the fertilizer rate lowered nutrient levels in the rose leaves, but not enough to affect yields or quality. However, I suspect that a longer experiment would show that 50 ppm N is too low to sustain high productivity in roses. Other research has shown that greenhouse cut roses require between 75 to 100 ppm N for high yields.
Suppressing insect pest populations by manipulating crop fertilizer additions is an old idea, but one that deserves more recognition and application. This may be particularly true for ornamental crops, which typically receive far more fertilizer than they require. Come to think of it, I wonder if eating Lunchables would cut down my mosquito bites.
Richard Evans is Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.
Chau A, Heinz KM. 2006. Manipulating fertilization: a management tactic against Frankliniella occidentalis on potted chrysanthemum. Entomologia Experimentalis et Applicata 120: 201-209.
Chow A, Chau A, Heinz KM. 2012. Reducing fertilization: a management tactic against western flower thrips on roses. Journal of Applied Entomology 136: 520-529.