Effects of Plant Growth Regulators On the Fitness of the Asian Citrus Psyllid

The Asian citrus psyllid (ACP) is an important pest of citrus because it is the vector of Huanglongbing (citrus greening disease), the most devastating disease known in citrus. This pest feeds and reproduces exclusively on young vegetative growth of citrus trees. We hypothesized that plant growth regulators (PGRs) could be used to reduce vegetative growth of citrus, thus reducing ACP populations. Seedlings of Volkamer lemon were treated with six commercially available PGRs (10 replications each): Embark, Sumagic, Atrimmec, Apogee, Profile and Cycocel. Untreated seedlings served as controls. Seedlings were approximately 30 cm in height at the beginning of the study and were grown in 2.3-l citrapots using a standard soilless potting medium. Ten days after PGR application, a pair of ACP was caged on five seedlings of each treatment using transparent clear acetate cylinder cages (34 x 6.5 cm). The five seedlings from each treatment without ACP were used for vegetative growth measurements. All seedlings were maintained in a greenhouse at 28 C and 70% relative humidity. Over the course of fifteen days, the number of eggs laid on each plant was counted daily. Survival of ACP nymphs to adults was assessed. Adults were collected and weighed to determine adult body weight. Due to severe phytotoxicity no data were collected from Atrimmec-treated plants. Only Sumagic and Apogee significantly reduced total shoot growth. The average number of eggs laid per plant was reduced by 85% on Embark and Sumagic-treated plants, and by 65% on Apogee and Profile-treated plants. In contrast, the number of eggs laid increased by almost 40% on Cycocel-treated plants. Survival of nymphs to adults was also significantly affected by PGR treatment. Survival was lowest for plants treated with Profile (7%), followed by Sumagic (36%), untreated control plants (45%) and Apogee (47%). The highest survival rates were observed on Cycocel (72%) and Embark (68%) treated plants. ACP adults that matured on seedlings treated with Sumagic weighed significantly less than those from Embark, Apogee, Profile and control plants, but were similar in weight to those from Cycocel-treated plants. These data indicate that the PGRs tested significantly effected the overall fitness of the ACP; however, contrary to our hypothesis, these effects were observed even in the absence of significant growth reduction. This indicates that the PGRs are affecting ACP fitness through plant biochemical changes, direct toxicity or some other undetermined mechanism.