INVASION RATE AS A MEASURE OF INFECTIVITY OF STEINERNEMATID AND HETERORHABDITID NEMATODES TO INSECTS

The invasion rate of three steinernematids and two heterorhabditids into three larval stages of Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae) and into fourth-instar larvae of four Lepidopteran species was recorded as a measure of nematode infectivity. The invasion rate was determined by recording the mortality levels of insect larvae exposed to infective juveniles of the various nematode strains for 2,4, and 6 hr. nematode infectivity was finally expressed as LT50, the length of exposure required to bring 50% mortality of the insects. In comparison, the LD50 values of all nematode-insect combinations were determined in a dose-response assay. Among the different larval sizes, the fastest invasion rate was recorded with the smallest larvae, for all nematode strains. The LT50 values increase in proportion to S. littoralis larval size, indicating reduction in susceptibility. Among the four insect species tested Galleria mellonella was found to be the most susceptible with LT50 values two times lower than obtained with the other insects, for each nematode separately. The namatodes Steinernema carpocapsae strains All and Mexican, S. glaseri, and the IS strain of Heterorhabditis sp. were found to be similarly infectious to the different larval sizes or the various species tested, with LT50 values of 1.0-3.7 hr, whereas the LT50 values obtained with Heterorhabditis bacteriophora HP88 are 6-30 times higher than those of the other nematode strains, indicating its poor infectivity. Insect susceptibility and nematode infectivity as measured by the invasion rate assay are compatible with the results obtained in the dose-response assay. The present study stresses the use of nematode invasion rate as a mean to evaluate steinernematids' and heterorhabditids' infectivity. © 1992.