Is fecundity correlated with resistance to viral disease in the western tent caterpillar?

Introduction from large egg masses (Myers & Kukan, 1995). This prediction was tested by treating individuals from a wide range of egg mass sizes with a NPV, and relating the percentage mortality Reduced fecundity following high larval densities is a common feature of lepidopteran populations and may contribute to per family (egg mass) to maternal fecundity (number of eggs per egg mass). population declines (Myers, 1988; Munster-Swendsen, 1991). Species exhibiting density-related reductions in fecundity include the pine looper,Bupalus piniarius (Gruys, 1970; Methods Barbour, 1988), the Douglas-fir tussock moth, Orgyia pseudotsugata(Mason et al., 1977), the larch budmoth, Malacosoma c. pluvialefemales lay eggs (100–300) in a single Zeiraphera diniana(Baltensweileret al., 1977), the eastern mass on twigs of the host plant [often red alder, Alnus rubra spruce budworm, Choristoneura fumiferana(Miller, 1963), the (Bong.)] after mating in mid-summer (July). First instars hatch gypsy moth,Lymantria dispar(Campbell, 1978) and the spruce from the eggs in April or May and pupate in June or July. For needleminer, Epinotia tedella (Munster-Swendsen, 1991). a more detailed description of tent caterpillar biology see Although most often attributed to changes in host food plant Myers (1993). quantity or quality, viral disease may play a role in these Tent caterpillar egg masses were collected from a high observed fecundity changes (Myers, 1988, 1993; Rothman & density population on Saturna Island, British Columbia, Canada Myers, 1996). in the early spring of 1995 and stored at 4 °C. On 3 April In the western tent caterpillar, Malacosoma californicum 1995, egg masses were rinsed in a bleach solution (3% sodium pluviale (Dyar), fecundity tends to decrease following peak hypochlorite) to remove any possible contamination with NPV larval densities, and remains low for several generations during and tied to host alder trees at a site on the University of British the population decline (Myers, 1990; Myers & Rothman, 1995; Columbia campus. Myers & Kukan, 1995). On average, mean fecundity of female After hatch, the number of eggs per egg mass was counted. moths is reduced by 35% during this period compared with Larvae from these masses were brought indoors within 24 h mean fecundity of moths prior to peak population densities of moulting to third instar (beginning 13 May) and reared (range5 18–65%, SE5 9%,n 5 5 populations) (see Rothman, individually in 60-mL plastic cups in a naturally lit room at 1995). Myers (1990) proposed the ‘disease defence hypothesis’ room temperature (17–28 °C) (photoperiod and humidity were to explain this pattern, as nuclear polyhedrosis virus (NPV) is not controlled). Larvae were starved for 24 h and then fed an a common feature of high-density tent caterpillar populations. alder leaf disc treated with NPV ( n 5 25 per family) or distilled If reduced fecundity were a correlated characteristic of water (n 5 15 per family). Controls were reared to account for resistance to disease, fecundity might be expected to decline any families that came into contact with NPV in the field in populations after a viral epizootic at high host densities before third instar, or NPV on egg masses not removed by (Myers & Kukan, 1995). Recovery of fecundity to pre-peak bleaching. For treated larvae, 5 μL of distilled water containing levels would be delayed during the population decline if disease NPV at a concentration of 1.6 3 105 PIBs/mL were applied to resistance and fecundity were heritable traits. the leaf disc. This dose was expected to kill approximately 30% A prediction of this hypothesis is that caterpillars from small of treated larvae at third instar. Larvae (control and treated) egg masses will be more resistant to disease than caterpillars not eating the entire leaf disc within 48 h were discarded. Treatments were applied at third instar because individuals prior to this stage will usually only feed in groups. Correspondence: Dr L. D. Rothman, Centre for Biodiversity NPV for this experiment was originally extracted from Research, Department of Zoology, University of British Columbia, 6270 University Blvd., Vancouver, British Columbia V6T 1Z4, Canada. diseasedM. c. pluviale larvae using a discontinuous sucrose