Plant Community Diversity, Herbivore Movement, and an Insect-Transmitted Disease of Maize

Field experiments were carried out in Nicaragua to examine the influence of plant community diversity, plant density, and host plant quality on the spread of an insect-transmitted plant pathogen. Population levels of the corn leafhopper Dalbulus maidis, which transmits the corn stunt spiroplasma to maize, were monitored in four experimental communities; low-density maize monoculture, high-density maize monoculture, two-species (maize/bean) polyculture, and multispecies (maize/weeds) polyculture. Leafhopper abundance per plant and the incidence of corn stunt were lower in high-density maize monocultures than in low-density monocultures. Increasing plant diversity by intercropping with nonhost species such as beans or weeds also led to lower leafhooper abundance and decreased disease incidence, but the effect was not enhanced as additional nonhost species were added to the community. Manipulating host plant quality by increasing nitrogen fertilization resulted in higher leafhopper densities at higher nitrogen levels. To explore the role of vector movement in disease spread, leafhopper movement rates and emigration were estimated by observing changes in the spatial gradients of leafhopper densities over time. This method of movement analysis requires neither marking the insects nor releasing them at a single point, and thus reduces the extreme disturbance caused by traditional mark-release techniques. The analysis indicated that leafhopper movement rates were lowest in the polycultures. In particular, across-row movement was strikingly inhibited in the bean polyculture. This polyculture also had the highest rates of emigration. These results indicate that plant quality, density, and diversity significantly affect the spread of corn stunt through their effects on the abundance and movement behavior of the corn leafhopper. These factors could be manipulated in a program of cultural control for corn stunt in maize fields in tropical America.