THE SPATIAL DISTRIBUTION OF CHILO SUPPRESSALIS WALKER LARVAE IN PADDY FIELD
SUN JIAN-ZHONG DU ZHENG-WEN (Inssisute of Plant Protection,Jiangsu Academy of Agricultural Sciences,Nanjing 210014)
The spatial distribution patterns of the larvae of the striped stem borer Chilo suppresalis Walker in paddy field are systematically studied and analyzed with the concept of mean crowding (m).The larval aggregational intensity,aggregational grain and process of dispersal are described in detail and the formations of spatial distribution of larvae in different ins-tars are discussed.The conclusions are as follows:1.Analysis of the distribution patterns of larval population within the paddy field by m-m regression method indicates that the basic component (α) of larval distribution varies with larval instars,from an individual clump in early instars to a single individual in late instars.The distribution of the basic component (β) for different larval instars shows that all distributions are aggregational(β1,β is a measure of the spatial disposition of clumps).The patch ness size at late (fifth-sixth) instar stages is about one hill of rice.2.The dispersal of larvae in the paddy field is a non-random immvement aceording in c2 (Freeman,1977),and the average dispersal radius is 28.5 cm in the japonica rice yuan-fengzao and 41.2cm in the hybrid rice Shanyou 3.The maximal dispersal radius is less than 100 cm in different rice varieties.3.The larval dispersal rate varies with different developmental stages.Dispersal rates in the hybrid rice fields at early,middle and late stages (10 days per stage) are 2.2340,2.0535 and 2.0517cm/2 days respectively.When the rale of dispersal is decreased the number of dead-hearts would increase rapidly.4.A synthetic analytical method of 'mean aggregation(λ)' (Southwood,1978) in association with 't' index (Iwao Kuno,1971) is applied for revealing the aggregation and dispersal causes of the striped stem Borer.The results indicate that the 't' values vary with instars from 0.46 (first and second instars) to 0.97 (fifth and sixth instars).The mortality and dispersal process of the larvae are restricted by the larval density when t1.The density-dependent effect on larval population would gradually decrease as the larval instar increases.According to the 'mean aggregation (λ)' calculated for the fifth and sixth instars,e.g.*λ2,the aggregation of mature larvae is probably affected by some environmental factors and not by active process.The value of 't' for mature larvae is about a unity.It can be concluded that the processes of mortality and dispersal of mature larvae are restricted by random factors.
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