While nearly 90% of all flowering plant species use animals – most often insects – for pollination, a number of insect groups including cockroaches, true bugs, net-winged insects, and crickets, have only rarely been reported as functioning in this capacity. Cockroaches are known to act as pollinators of only 11 plant species in the world, including a single Apocynaceae species. To date, there is no well-characterized floral syndrome related to cockroach pollination.
In a new article published in the American Journal of Botany, lead author Wujian Xiong and colleagues studied a possible additional case of cockroach pollination in the rare twining Apocynaceae species, Vincetoxicum hainanense, a native of south China. The authors assessed the effectiveness of cockroaches versus other observed visitors by quantifying the number of visits and the amount of pollen deposited per visit. They also studied the literature around this phenomenon to find out if there is a set of characteristics common to cockroach-pollinated flowers.
The results indicated that V. hainanense is pollinated by cockroaches, ants, and Carabidae, with the most significant contribution made by cockroaches. Many groups of insects were found to visit the flowers at night, though these were not all functional pollinators, some being too large to access the reproductive organs. The cockroach species found to visit the flower, Blattella bisignata, did so in both its nymph and adult form. The nymphs, along with ants and Carabidae, were more likely to contribute to self-pollination or geitonogamous pollination (pollination by another flower on the same plant), though these resulted in lower fruit set. The adult form of the cockroach, however, was more likely to contribute to cross-pollination.
As for a syndrome associated with cockroach pollination, a review of known cockroach-pollinated flowers showed that they are open at night, pale in colour, and when scented, have a strong odour, either sweet or unpleasant. “Cockroaches are an ancient insect group, which evolved at least 300 million years ago in the Carboniferous period and are now found globally in a wide range of habitats,” write the authors. “[M]ost are nocturnal, actively emerging at night to feed omnivorously. Being nocturnal, cockroaches mainly use the sense of smell to find food, though some of them are attracted to pale colors.”
Though rare, the use of cockroaches as pollinators may be quite old. “Recent discoveries from Myanmar amber suggest that attraction and reward of cockroaches by gymnosperms and angiosperms may be an ancient pollination system that dates to at least the Cretaceous period,” note the authors.