Allopatric versus sympatric interspecific mating in coccinellids: Familiarity breeds contempt

The Palearctic lady beetle, Coccinella septempunctata (C7) was intentionally introduced to the U.S. for biological control and is known to have detrimental effects on native coccinellids that may have influenced the decline of several native lady beetle species. To date, research has focused exclusively on competitive interactions such as competition for food resources and intra-guild predation, but another interaction, reproductive interference, may be involved.  Reproductive interference occurs when species recognition is weak leading to individuals wasting reproductive efforts by attempting to mate with heterospecifics.  This phenomenon is more likely to occur in closely-related species. Through increased exposure, sympatric species may have become better at discriminating than allopatric species resulting in fewer mating attempts with heterospecifics compared to closely-related allopatric species. The Nearctric lady beetle, Coccinella novemnotata (C9) has shared a distribution with C7 for only 20 years and is considered to be allopatric whereas the Holarctic lady beetle, Coccinella transversoguttata (CT) is sympatric with both C9 and C7. If sympatric species have stronger species recognition than allopatric species then we should see fewer copulatory attempts between them. In this experiment, pairings were made between C9/CT (sympatric), CT/C7 (sympatric), and C9/C7 (allopatric) to determine the frequency of heterospecific copulations. Out of 139 pairings 18 copulations were observed. Even though heterospecific copulations were rare, sympatric species were less likely to copulate than allopatric species. Due to the rarity of heterospecific copulations it seems unlikely that reproductive interference is playing a significant role in native lady beetle decline. However, the frequency of wasted reproductive efforts may increase as a species density decreases. The effect of density on heterospecific copulations, especially in declining species, should be investigated.