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Life table analysis of WCR (Western Corn Rootworm) in European areas of introduction

For the first time a complete life - table of WCR (Western Corn Rootworm) populations were calculated by overcoming main methodological difficulties, i.e. a) by avoiding clumped populations by using artificial infestations and b) by the ensurance of compability between experiments by using absolute counts and by using same or proportional units for all life stages of WCR. However, its comparison to other studies is limited due to very few investigations of life - tables on insects with soil inhabiting stages.

Total mortalities were successfully measured for most age intervals within an WCR generation, and only estimated for the pupal stage in two cases. Reasons for mortalities within an age interval were mainly unknown. No indigenous natural enemies were found to have importance for the generational mortality. Thus, the usually important dependency between efficacy of natural enemies and clusters and density of the host population can be neglected. Also intra-specific competition is suggested to be low in this study, since infestations with 100 up to 2400 eggs per plant did not affect the total generational mortality in D. virgifera until adult emergence, and since maize plants, in this study, were infested with 80 to 100 eggs. Then, emerging adults found nearly unlimited food resources in maize fields.

Finally, aggregation as a beneficial factor in survival was never reported from larvae, pupae or adults of WCR. Conclusively, mortality and survival in the life stages of WCR in this study, was mainly a result of abiotic factors, such as of climatic conditions. The major factors behind the commonly high mortality in first instar larvae, such as of about 94 % in this study, have never been clarified. The role of predators or bacterial infection is unknown for this life stage.

In the life - table of WCR in Europe, a total generational mortality (K Total) of 2.48 was measured in average. It represents a total mortality of about 99 % from the egg stage to the emergence of adult females.

The highest reduction of D. virgifera numbers resulted from the generational mortality in 1 st instar larvae (ca. 50%), as well as in second instar larvae and in over wintering eggs (both ca 11%). accounting for 72% of the total generational mortality. However, only the variation in mortalities between years can influence the generational mortality and thus population size. This means, the very high mortality in first instars larvae had a limited potential in causing changes total generational mortalities, since its value deviated only slightly between the three studied cases (s.d. = 1, n = 67). In contrast, a high variation of marginal death rate between fields and years was found in second instar larvae (s.d. = 31.9), in 3rd instar larvae (s.d. = 32.2) and in over wintering eggs (s.d. = 14.5). Those mortalities seem to be crucial in causing changes of the generational mortality, although, due to the limitations to two years the found correlation coefficients between the kx of the above-mentioned age intervals and the total generational mortality K Total were not significant.

Finally, the life - table showed that a high fecundity could recompense high generational mortalities and lead to increasing populations. WCR was performing a high mean lifetime fecundity of 353 eggs per female in the laboratory, which can increase up to a potential fecundity of 956 eggs per female. This high reproductive potential made WCR a typical r - strategist. A failure of the population to realise their potential fecundity is the main factor that the populations remained stable from 2001 to 2002 (R0 = 0.9), or even declined from 2002 to 2003 (R0 = 0.21), and is reflected by the correlation between the k - values of the loss fecundity rate to the total generational mortality (K Total) in each field and year (R2 = 0.99, P= < 0.05).

The realisation of fecundity can be often suppressed in WCR by the dryness in soil, what is a typical situation in southern Hungary due to the lack of rainfalls in July and August, and as it was the case in the study sites in 2001 and 2002. Another reason for the lack of population increase in 2001 was the relatively high mortality in diapausing eggs suggested to be a result of low quality of eggs due to the exceptionally warm autumn in 2000. For 2002, in addition to the low realised lifetime fecundity, a comparatively high mortality in second and 3 rd instar larvae due to soil dryness, lead to an over-estimated declining of the population (R0 = 0.21).

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