Abstract
Populations of the native ninespotted lady beetle, Coccinella novemnotata Herbst, have undergone precipitous declines in North America following the establishment of the exotic sevenspotted lady beetle, Coccinella septempunctata L. Recent volunteer efforts have made it possible to establish colonies of the now-rare C. novemnotata and test mechanisms contributing to its decline. We evaluated the relative frequencies of intraguild predation and cannibalism of eggs between these two species. A single C. novemnotata or C. septempunctata adult was exposed to conspecific and heterospecific eggs in either the presence or absence of pea aphids. The study revealed two expected results: 1) eggs of C. novemnotata were consumed more frequently than eggs of C. septempunctata by both species, and 2) egg consumption was higher when aphids were absent, independent of predator and egg species. There were also two unexpected results from the study: 1) the asymmetry between egg predation rates was higher when aphids were present, and 2) higher predation rates on C. novemnotata eggs in the absence of alternate prey was due to a relatively higher rate of intraspecific cannibalism. This implies that C. novemnotata would have suffered higher egg mortality rates before the invasion of C. septempunctata, but even though the aggregate rate of egg predation on C. novemnotata eggs is lower post-invasion, it is still significantly higher than the aggregate rate of predation of C. septempunctata eggs. This differential pattern of asymmetric predation could contribute to habitat compression and the overall decline of C. novemnotata.
Highlights
Before the mid 1980s, Coccinella novemnotata Herbst was the most commonly encountered native lady beetle species in the northeastern region of the United States and among the most common nationally [1]
Across predator species and aphid densities an average of 0.36 (12%) more C. novemnotata eggs were consumed versus C. septempunctata eggs (p = 0.02)
The asymmetry in consumption between egg species was positive across both aphid densities indicating more C. novemnotata eggs were consumed than C. septempunctata eggs
Summary
Before the mid 1980s, Coccinella novemnotata Herbst was the most commonly encountered native lady beetle species in the northeastern region of the United States and among the most common nationally [1]. It has been shown that coccinellid eggs are vulnerable to intraguild predation and that they can provide a better source of nutrition to the consumer than aphids in terms of larval development [20, 13]. It has been demonstrated in a laboratory study [20] that H. axyridis larvae can complete development on a diet of the eggs of two native coccinellid species, Coleomegilla maculata De Geer and Olla v-nigrum Mulsant, the same two native species were not able to complete development when provided only H. axyridis eggs. It has been demonstrated that aphid density and egg consumption by fourthinstar coccinellid larvae were inversely proportional [14]
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