Evaluation of the control of beet armyworm, Spodoptera exigua, with baculoviruses in greenhouse using a process-based simulation model

Scenario studies were carried out with a process-based model for control of wild-type and genetically modified baculoviruses in populations of Spodoptera exigua in glasshouse chrysanthemum (BACSIM). These scenario studies were used to evaluate the effectiveness of different spraying regimes, concentrations, UV-protection agents, and mean time to kill insect. In simulations on concentration and timing of S. exigua multicapsid nucleopolyhedrovirus (SeMNPV), good control efficacy was obtained with SeMNPV concentrations of 1x107 polyhedra/m2 or higher. An early timing of virus applications, soon after egg deposition, is essential for effective control, especially at high temperatures, when the developmental rate of larvae is high and the window of opportunity for virus control is short. UV-protection agents contribute only marginally to effective biological control in the glasshouse because the decay rates of SeMNPV and Autographa californica multicapsid nucleopolyhedrovirus already ensure a period of exposure to active virus that is long enough for caterpillars to acquire a lethal concentration under practical conditions. The effect of genetic improvement towards shortening the survival time of infected larvae depends on the ability to time virus applications accurately and the mean time to kill the unmodified virus. Recombinant viruses with a lower mean time to kill are more effective than wild-type viruses when applied against second, third, and fourth instars because in these stages S. exigua infests a considerable number of plants and the rate of foliage consumption increases rapidly. When applied against first instars that infest only a small number of plants and have low consumption rates, recombinants will only be slightly more effective than wild-type viruses. In the case of an unmodified virus that already has a relatively high mean time to kill, a further reduction in the time to kill will not result in improved control. Thus, accurate monitoring followed by virus application is indispensable for adequate control and may even alleviate the need for fast killing viruses.