1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that hostplant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer -lepidopteran [ Plutella xylostella (L.)] -parasitoid [ Cotesia plutellae (Kurdjumov)], system.2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors.3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence.4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized.5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid.6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host-plant effects on superparasitism.7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts. Entomology, 33, 45-52 * The single-oviposition experiments, with dissection of host larvae 3 (experiment 1) and 5 (experiment 2) days after parasitism, and without dissection (experiment 3). † L 1 and L 2 denote late fi rst instar and second instar parasitoid larvae, respectively. ‡ The abbreviations TT, WI, RD and EGG are Chinese cabbage cv. Tip Top, common cabbage cv. Wheelers Imperial, common cabbage cv. Red Drumhead and caulifl ower cv. Early Green Glazed 'Plant Introduction 234 -599', respectively. § Values marked with different letters are signifi cantly ( P < 0.05) different.Plant quality and herbivore immunity 49

The effects of 10 commercial canola, Brassica napus L., cultivars widely grown in Iran--'SLM(046),' 'Opera,' 'Okapi,' 'RGS(003),' 'Modena,' 'Sarigol,' 'Zarfam,' 'Licord,' 'Hayula(420),' and 'Talaye'--on the demographic parameters of diamondback moth, Plutella xylostella L. (Lepidoptera-Plutellidae), were determined. The experiments were conducted in a growth chamber at 25 +/- 1 degrees C, 65 +/- 2% RH, and a photoperiod of 16:8 (L:D) h. The comparison of intrinsic rate of natural increase (r(m)), net reproductive rate (R0), and the survival rate of adult stage of P. xylostella on 10 canola cultivars suggested that this pest performed best on SLM(046). The r(m) value of P. xylostella ranged between 0.241 on RGS(003) and 0.304 on SLM(046). The R0, finite rate of increase (lambda), mean generation time (T), and doubling time (DT) values of P. xylostella on SLM(046) were 52, 1.35, 13.4, and 2.35 and on RGS(003) were 31, 1.27, 14.4, and 2.94, respectively. The Weibull model adequately described the shape of the survivorship curve of adult P. xylostella from life-table data. A significant fit was obtained with the Weibull model for P. xylostella in all experimental canola cultivars. As a result, SLM(046), Opera, and Hayula(420) were the most suitable hosts and had least negative impact on life-history statistics of the pest.

1. The effects of host-plant resistance on the population dynamics of the Diamondback moth, Plutella xylostella L., and its solitary parasitoid, Cotesia plutellae (Kurdjumov), were studied in replicated time-series experiments.2. Host-plant resistance did not affect the equilibrial abundance of the Diamondback moth, but it affected the dynamics of Diamondback moth populations.3. The mean population size of Diamondback moth showed no significant difference between Brassica rapa (a susceptible host plant) and Brassica napus (a partially resistant host plant) either in the presence or absence of the parasitoid.4. Time-series analysis suggests that the dynamics of Diamondback moth on B. rapa were underpinned by delayed density-dependent processes. In contrast, the dynamics of the moth on B. napus were influenced by a direct density-dependent process.5. Although measures of short-term parasitism showed a significantly higher rate of parasitism by C. plutellae on Diamondback moth feeding on B. napus compared with B. rapa, this individual performance does not translate into differences in the population dynamics. Analysis shows no significant difference in the persistence time of the population-level interaction between the host and parasitoid on the two different host plants.

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The development and mortality of diamondback moth, Plutella xylostella (L.), were studied in laboratory at 25 ± 1°C, 65 ± 5% RH, and a photoperiod of 16:8 (L:D) h on 10 canola cultivars: ‘SLM046,’ ‘Opera,’ ‘Okapi,’ ‘RGS003,’ ‘Modena,’ ‘Sarigol,’ ‘Zarfam,’ ‘Licord,’ ‘Hayula420,’ and ‘Talaye.’ Larvae successfully survived on all host plants. The developmental time of immature stages ranged from 15.03 ± 0.15 d on Hayula420 to 16.65 ± 0.29 d on Opera, with a larval period ranging from 7.67 to 8.91 d on these cultivars. Adult female longevity was longest on Hayula420 without any supplemental food. Life table entropy values (H &lt; 0.5) indicated Deevey's type I survivorship curve; however, the value of 0.541 on Hayula420 (H &gt; 0.5) corresponded to type III. Major mortality parameters such as fraction of original cohort dying between successive days of age, death frequency (dx) of immature P. xylostella, average daily mortality (), and central death rate (mx) were evaluated on canola cultivars and indicated that the highest mx occurred on RGS003, with relatively low potential of population growth, on fifth day of life when the pest is in early larval stages (Ll and L2). The early instars are the most susceptible stages and suffer the highest cause-specific mortality under laboratory conditions.