Aster Leafhopper Dispersal

C. W. Hoy (State Representative), S. A. Miller, L. R. Nault, L. Beanland, J. Zhang, X. Zhou

Ohio Agricultural Research and Development Center
The Ohio State University
Wooster, OH

We have developed a mathematical model of aster yellows epidemiology to explore strategies for managing this important disease of vegetable crops. The model simulates yellows epidemics in fields of lettuce, celery, and carrots. Epidemics are started by immigrating aster leafhoppers, Macrosteles quadrilineatus Forbes, and are influenced by subsequent population dynamics and phytoplasma transmission. Leafhopper movement after arrival in the vegetable growing area influences two critical rates in the epidemiological model: the rate at which uninfected leafhoppers acquire the phytoplasma and the rate at which inoculative leafhoppers transmit the phytoplasma to uninfected plants. Both rates depend on two scales of movement, interplant and interfield. Spatial effects have been included in the simulation model, using transition probabilities for movement among plantings and considering movement within plantings in transmission rates.

Model predictions were compared with observed aster yellows in a large-scale field validation experiment. Treatments were season-long control strategies in one-acre blocks that were divided into 24 plots, 6 plots of each of the following crops: lettuce (susceptible), carrots (susceptible), collards (not susceptible), and sweet corn (not susceptible). A strategy that included cultural controls and reduced insecticide application was compared with the conventional schedule of insecticide applications for aster leafhopper and an untreated check. Model predictions that were consistent with observations were the ranking among strategies and disease incidence in the later plantings of lettuce. Observed disease incidence was higher than that predicted for the earlier plantings. Disease incidence at harvest decreased over the six plantings; it was predicted to increase. We suspect that higher than expected rates of immigration of inoculative leafhoppers resulted in the additional infection in the early plantings. Conclusions were that we need effective monitoring of arriving migrants. It is important to determine not only the proportion infected and inoculative, but also when immigration ends because an integrated control strategy appears feasible in the absence of immigrating inoculative leafhoppers.

To improve estimates of dispersal among plantings within the production area, we conducted mark, release, recapture experiments using the marking methodology developed by James R. Hagler (Environ. Entomol. 1997. 26(5): 1079-1086). Approximately 25,000 leafhoppers were collected by vacuum sampling, treated in a simple field transfer container, and released in a large screen field cage in the center of a lettuce planting. On the following morning the sides of the cage were rolled up so that the leafhoppers could disperse. Leafhoppers were collected on yellow sticky traps and by vacuum sampling at 24 and 48 hr after rolling up the sides of the cage. The experiment was conducted once in July and once in August. No marked leafhoppers were recaptured on the sticky traps. Subsequent studies verified that the traps did not interfere with detecting the rabbit protein marker. The figure shows the number of marked leafhoppers recovered in vacuum samples, August 14, 1998, 48 hr after release.

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