Factors responsible for stage-specific mortality and sex ratio adjustment in Zygogramma bicolorata Pallister on parthenium in field conditions

Abstract

Stage specific mortality and sex ratio adjustment in Zygogramma bicolorata Pallister (Coleoptera: chrysomelidae), were evaluated monthly for two consecutive years, 2010 and 2011, in field conditions on Parthenium hysterophorus L. Development of immature stages was considerably affected during the cold seasons of 2010 and 2011, and it was completely arrested in January. Similarly, the mortality survival ratio and indispensable mortality were higher in December, as compared to other months. Generation mortality (K-value) was decreased in May and then further increased from September to December, in both years. A non-significant difference (P < NS) in K-values were evident in July and August 2010, as well as in June and July 2011. Different abiotic and biotic factors may have influenced the mortality of the various immature stages. Greater mortality of eggs was due to their infertility in December. Similarly, mortality of 1st and 2nd instar was significantly high in Feburary, March, October, November and December of 2010 and 2011 due to infection of entomopathogenic fungi Beauveria bassiana. One of the other causes of mortality of 2nd, 3rd and 4th instar was their predation by a pentatomid bug, Perillus bioculatus from 1st week of March to 2nd week of December 2010 and 2011. Pre-pupal mortality was due to lack ofpenetration in soil and pupal mortality was due to malformation. In addition to known factors, some unknown factors may have been responsible for mortality. The least value of sex ratio (proportion of male) showed the greatest number of female emergence. It was also noted that sex ratio was considerably higher in 2010 than in 2011, and that that female population increased gradually year by year. Therefore, the results suggested that Z. bicolorata are continuously adapting to the agro climatic conditions of Aligarh, India. Moreover, the information on the factors responsible for key mortality, can be helpful in devising strategies for its conservation.