Damaged goods: How Cotesia glomerata responds to hosts parasitized by its competitor, C. rubecula

The reproductive success of parasitoids is dependent on selecting the right host.  Host quality can decrease if a host has been parasitized by conspecifics (superparasitism) or heterospecifics (multiparasitism).  Females should avoid hosts parasitized by a competitor whose larvae outcompete secondary broods.  In parts of North America, Cotesia rubecula (Hymenoptera: Braconidae) and C. glomerata are both parasitoids of Pieris rapae (Lepidoptera: Pieridae).  Cotesia rubecula is competitively superior to C. glomerata when both species parasitize the same host; therefore, avoiding hosts parasitized by C. rubecula would be advantageous for C. glomerata.  This study examined whether C. glomerata in Colorado would discriminate P. rapae hosts parasitized by C. rubecula, which is nonexistent in Colorado  To measure host discrimination, choice tests were performed in which naïve C. glomerata females were presented with an unparasitized P. rapae host and a P. rapae host parasitized by C. rubecula.  Parasitized P. rapae hosts were placed in four time treatments according to the time since oviposition by C. rubecula:  1-5hrs, 24hrs, 48hr or 72hrs.  Ten C. glomerata females and 20 P. rapae (10 unparasitized, 10 parasitized) were used per time treatment for a total of 40 females and 80 hosts.  There was no significant difference between Cotesia glomerata oviposition in unparasitized or parasitized hosts across time treatments (p=0.76, Fishers Exact Test).  It is unknown how long Cotesia glomerata has lived without C. rubecula in Colorado.  A relaxation of selection for the ability to avoid multiparasitism may explain why C. glomerata accepted hosts parasitized by C. rubecula.