Descriptive models for the system of the natural spread of infection of rice dwarf virus (RDV) by the green rice leafhopper,Nephotettix cincticeps Uhler (Hemiptera: Deltocephalidae)

Summary

Seasonal changes in the densities of eggs and of 1st instar nymphs ofN. cincticeps, a vector of the rice dwarf virus (RDV) and those in the proportion of virus-infected rice hills were examined by regular field censuses during the period of 1964–69 at two localities in Kochi, Nangoku and Ino. The proportion of virus-infected insects in the vector population was measured by the serological test. Mean fecundity of adult females ofN. cincticeps in different generations was assessed by the method proposed byHokyo andKiritani (1968) in 1966 and 1967.

Using observed variables as mentioned above, two mathematical models were built up for the system of the natural spread of infection of RDV. Submodels describing generation-to-generation changes in densities of eggs, 1st instar nymphs and leafhopper adults were involved in bothModels 1 and2. InModel 1, the proportion (P) of infected insects in the vector population was assumed to be determined directly by the amount (I _v ) of infected rice plants in the field. InModel 2, instead of relatingP directly toI _v ,P was related to the acquisition rate (W) of RDV by the insects and to the effect of RDV (αr) on fecundity and survival of infected insects. Then,W was related toI _v .

The estimates ofP by these models fitted well to the observed values for 1st to 3rd generations but a great deviation was noted in 4th and 5th generations. For the percentage of infected rice hills, both models gave relatively good estimates in the early planted rice, though they gave overestimated values in the late planted rice.