Influence of medicinal plants on Blaberus craniifer cockroaches and their parasites, gregarines and nematodes

Cockroaches, especially those living in forest litter and feeding on foliage, have been consuming plants for millions of years. Can secondary metabolites of plants affect the parasite-host system and successfully treat the cockroaches’ parasites gregarines and nematodes? In our experiment, 204 Blaberus craniifer (Blattodea, Blaberidae) cockroaches consumed the standard diet supplemented with medicinal plants in a dose of 10% of the general fodder mass, particularly, plants of the families Acoraceae (Acorus calamus), Papaveraceae (Chelidonium majus), Rosaceae (Potentilla erecta), Juglandaceae (Juglans regia), Fagaceae (Quercus robur), Brassicaceae (Capsella bursa-pastoris), Hypericaceae (Hypericum perforatum), Gentianaceae (Centaurium erythraea), Lamiaceae (Origanum vulgare, Salvia officinalis, Thymus pallasianus), and Asteraceae (Achillea millefolium, Arctium lappa, Artemisia absinthium, Matricaria chamomilla, and Tanacetum vulgare). With age (i.e. gaining body mass), the intensity of infestation of cockroaches with female and male nematodes Cranifera cranifera significantly increased. The multifactor dispersion analysis found no significant effect of the number of parasites (two gregarines, Blabericola cubensis and Protomagalhaensia granulosae, and one nematode, Cranifera cranifera) on the rates of changes in body mass of the cockroaches during the experiment. Also, there was observed no effect of the number of parasites on the rates of cockroaches’ food consumption. The rates of body-mass gain during the experiment were closely associated with the initial body mass of the cockroaches: the larger the cockroach was in the beginning of the experiment, the greater the increase in its body mass later in the experiment. None of the three species of B. craniifer parasites that we studied significantly changed their numbers when subject to 10% dry medicinal plants in the hosts’ fodder, compared to the control group of cockroaches that did not consume the medicinal raw material. Intake of none of the 16 species of medicinal plants we tested led to significant changes in the rates of body-mass gain in the cockroaches. Compared to the control group, the experimental groups of cockroaches had no significant changes in the fodder consumption rates. Our experiment demonstrated that the cockroaches, together with their parasites, are substantially adapted to the influences of secondary metabolites of the 16 medicinal plants that we studied, although, in their natural range they had most likely never encountered any of those plants. Therefore, on the one hand, the considered parasite-host system was observed to be very balanced, the host being minimally harmed, and on the other hand, secondary metabolites of the plants had no significant effect either on the parasites (two species of gregarines and one species of nematodes) or on their hosts even in the highest tested concentrations (10% of the fodder mass).