Microplitis croceipes (Cresson) is an important parasitoid of Heliothis spp. in the U.S.; M. demolitor Wilkinson was imported into the U.S. from Australia (Shepard et al. 1983). The following study was conducted primarily to determine how many times Heliothis virescens (F.) larvae molt after being parasitized by Microplitis spp. Heliothis virescens larvae were reared in plastic cups (25-ml) on an artificial insect diet (King & Hartley, 1985). Adult parasitoids were held in wooden cages (0.14 cu m) with Plexiglas? tops and ventilated in the back with organdy fabric. Undiluted honey was streaked on the top of the cage as a food source, and a water source was provided by means of soaked cotton balls held in plastic cups (25-ml). Rearing methods for the parasitoid colony are detailed by Powell & Hartley (1987). Host larvae were parasitized by exposing them singly on a small brush to several (15-20) female parasitoids (1-5 d-old) held in a plastic container (500-ml). Use of two to four such containers was rotated during manual exposure of hosts to wasps. This method improved the likelihood that each larva was parasitized because oviposition was observed. Multiple ovipositions in the same host were not allowed. Determining the occurrence of molting was facilitated by lightly powdering each host's entire cuticle with Day-glo? fluorescent pigment (DAY-GLO Color Corp., 4515 St. Clair Ave., Cleveland, OH 44103); when larvae molted, the dye was cast with the exuviae. While in the first instar (ca. 24-h old), 36 host larvae were exposed for parasitization to M. croceipes and 36 were exposed to M. demolitor. The same number and procedure were used for three additional groups of larvae when they reached the early second, third, and fourth instars. In addition, 36 non-parasitized larvae for each instar group were held as controls and treated in the same manner as test larvae. Larvae were checked during morning hours to observe the occurrence of molting. When molting was observed, control and test larvae were re-powdered and developmental times (to the nearest day) between molts were recorded. Each larva was held individually in plastic cups containing diet (total of 36 control and 144 test larvae). Studies were conducted at 26 + 2?C, 60 + 10% RH, and a photoperiod of 15L:9D. Data were analyzed using a least squares analysis of variance procedure with an a priori significance level (ox) of 0.01. The percentage of H. virescens larvae developing to different instars after parasitization by M. croceipes and M. demolitor is illustrated in Fig. 1. When host larvae were in later instars at the time of parasitization by either Microplitis spp., they molted fewer times than larvae parasitized as earlier instars. For example, 94% of the hosts exposed to M. demolitor in the second instar molted twice more to develop to the fourth instar, while 93% of those exposed in the fourth instar molted only once more. The