Egg Predation by the Introduced Lady Beetle, Coccinella septempunctata (Coleoptera: Coccinellidae), Lowers Mortality but Raises Relative Risk for the Native Lady Beetle, Coccinella novemnotata
June
Egg Predation by the Introduced Lady Beetle, Coccinella septempunctata (Coleoptera: Coccinellidae), Lowers Mortality but Raises Relative Risk for the Native Lady Beetle, Coccinella novemnotata
Rakim Turnipseed 0 1
Todd A. Ugine 0 1
John E. Losey 0 1
0 1 Department of Environmental Science , Policy , & Management, University of California , Berkeley, CA, 94720 , United States of America, 2 Department of Entomology, Cornell University , Ithaca, NY, 14853 , United States of America
1 Academic Editor: Olle Terenius, Swedish University of Agricultural Sciences , SWEDEN
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.
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Competing Interests: The authors have declared
that no competing interests exist.
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]. Since then, C. novemnotata and other native coccinellids, such as
Coccinella transversoguttata richardsoni Brown and Adalia bipunctata L., have undergone
widespread population declines throughout North America [1, 2] following the introduction of
exotic coccinellids for pest control, including Coccinella septempunctata L. [3]. Because of the
close timing between the extirpation of native and establishment of exotic coccinellids, it has
been suggested that C. septempunctata may have contributed to the decline of native
coccinellids, including C. novemnotata [4, 5, 6, 7, 3, 8]. Among several proposed mechanisms behind
the widespread decline of native coccinellids are competition with and intraguild predation by
invasive coccinellids [9, 10, 11, 12, 13].
Several invasive coccinellid species, such as C. septempunctata and Harmonia axyridis
Pallas, are known to be intraguild predators of native North American coccinellids when aphid
populations become scarce [14, 15, 16, 17, 18, 6, 19]. Additionally, it has been shown that
coccinellid eggs are particularly 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]. For
example, 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, however the same two native species were not able to
complete development when provided only H. axyridis eggs. The presence of alternative prey
has been shown to impact predation rates among coccinellids and other animals [21, 22, 23, 24,
25]. For example, it has been demonstrated that aphid density and egg consumption by
fourthinstar coccinellid larvae were inversely proportional [14].
Not all egg predation is due to intraguild interspecific predation; cannibalism is also a
welldocumented phenomenon within Coccinellidae. C. septempunctata have been reported to
cannibalize their own eggs [26], and that they prefer to eat their own eggs versus those of A.
bipunctata. The introduced species H. axyridis have been reported to cannibalized their own
eggs [1], although they showed no preference for conspecific versus heterospecific eggs. Adults
of the native species Cycloneda sanguinea, have been shown to cannibalize their own eggs, and
did so more compared to heterospecific eggs [27]. It has been suggested that when some
coccinellid sp (...truncated)