Cannibalistic behavior during early larval development of the noctuid moth Hadena ectypa

Background/Question/Methods

Hadena ectypa is a noctuid moth that occurs throughout the eastern part of the United States. H. ectypa is the obligate pollinating seed predator of its host plant, Silene stellata (Caryophyllaceae), as H. ectypa females both pollinate and oviposit within flowers of S. stellata. The Silene-Hadena interaction has recently emerged as a model system for studying plant-insect interaction dynamics and the evolution of mutualism. Here we studied the frequency of cannibalism between larvae of H. ectypa during the first three instars and examined how synchrony with the host plant S. stellata affects the oviposition behavior of female H. ectypa and whether cannibalism between siblings occurs at equal frequencies as between non-siblings. In summer of 2011 and 2012, we recorded oviposition activity of H. ectypa as well as flowering phenology of S. stellata. We carried out a total of 42 cannibalistic trials. Each trial was consisted of one S. stellata flower and two H. ectypa eggs that were siblings or nonsiblings. Frequencies of cannibalism between the two treatment levels were tested using Pearson’s Chi-square test.  

Results/Conclusions

Frequency of cannibalism was 70.83 percent between nonsiblings and 77.78 percent between siblings (total frequency=73.81 percent). Chi-square test showed no significant difference between the two treatment levels (Chi-square=0.023, p-value=0.88). In both years, we observed highest adult activity of H. ectypa in early flowering season of the host plant when flower density was relatively low. The low density of flowers and high density of adult moths resulted in the clustering of eggs oviposited by different females within the same flowers. Furthermore, female moths were more likely to oviposit in egg masses in early flowering season, therefore increasing the chance of cannibalism between siblings. The high frequency of cannibalism is most likely due to severe competition for limited food resource, since all larvae must complete the first three instars within the flower where they hatched. Hadena larvae usually cause strong damage to host plant through fruigivory. Therefore, cannibalism between young larvae of H. ectypa provides a mechanism for regulating population density and could potentially contribute to the stability of the Silene-Hadena interaction.