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PHYTOCHEMICAL PROTECTION AND NATURAL ENEMIES IN THE
REGULATION OF A ROOT AND STEM BORING INSECT
A.V. WHIPPLE and D.R. STRONG
Bodega Marine Laboratory, University of California, Box 247, Bodega Bay, California 94923, USA
INTRODUCTION
Much debate has centered on why herbivores in natural systems do not more often have a large negative effect on the
plants on which they feed. Except for some aquatic trophic cascades in which plants are algae, (Carpenter et aL 1987,
Carpenter and Kitchell 1988) and a few large mammalian grazers (McNaughton et al. 1988), herbivores have rarely been
shown to substantially depress plant populations. Two main ideas why this is are top-down suppression by predators and
bottom-up plant protection (Hunter and Price 1992). The natural enemy hypothesis suggests that populations are generally .
controlled by their predators and/or pathogens (Hairston et al. 1960, Oksanen et al. 1981, Hairston and Hairston 1993). Thus,
the plant feeders are kept too sparse to substantially depress plant populations. The endogenous plant protection hypothesis
proposes that, in terrestrial and some aquatic systems, plant biomass is protected by noxious secondary chemicals (Ehrlich
and Raven 1964, Hawkins 1992, Hay and Steinberg 1992). An adjunct to this is that higher plant biomass is also largely
unavailable to most herbivores (Strong 1992, White 1993). This is because most herbivores cannot digest cellulose or lignin
without the aid of microbial symbionts (Martin 1991). Because cellulose and lignin make up a large portion of the plant,
many essential nutrients for herbivores are present only in low concentrations, which may limit the growth of herbivore
populations so that they are not able to have substantial impact on plant populations.
Of course, these hypotheses are not mutually exclusive, and in the system discussed here both plant defense and
herbivore natural enemies may play a role in determining the level of herbivore effect on the plant population (Price 1992).
The system of the bush lupine, Lupinus arboreus, and its stem and root boring ghost moth or "swift," Hepialus caI(fornicus,
is ideal for examining the roles of these different factors because the level of herbivore attack varies over space with some
areas exhibiting extensive plant mortality and others with little mortality (Strong et al. in prep.).
Directly or indirectly, abiotic factors are probably involved in the differences between the sites in level of attack. The
effect of an abiotic factor on the level of herbivory may act directly on the herbivore or be mediated through the herbivore's
host plant, which would be bottom-up regulation, or through predators and pathogens, which would be top-down suppression.
Herbivory
and Resistance in the Bush Lupine
The bush lupine is a dominant woody shrub of some coastal headland habitats in central California. At Point Reyes
and on Bodega Head in Marin and Sonoma Counties, a major source of mortality to the bush lupine appears to be the swift,
Hepialus californicus (Opler 1968, Davidson and Barbour 1977, Wagner 1986). The mid to late instars of the swift caterpillar
bore in the roots and stem of the bush lupine. The early instars are thought to feed on litter and rootlets (Wagner 1986).
The caterpillars reach 40-50 mm, and as many as 60 have been found in one bush. While those numbers are exceptional, it is :
difficult to find a large bush with no evidence of swift caterpillar damage (pets. obs.).
Lupines produce large quantities of quinolizidine alkaloids, which are thought to function in nutrient storage and in
defense against herbivores and pathogens (Wink 1992). As little as 0.11%fresh weight of these alkaloids can be toxic to
insect herbivores (Wink 1992). Bush lupine leaves contain high levels of these quinolizidine alkaloids in their foliage _
(Bentley and Johnson 1991). The bush lupine probably also has substantial levels of alkaloids in the stems and roots where
the swift caterpillars feed (Wink and Witte 1984). The alkaloid is probably concentrated in the epidermis of the stem and root
where exceptionally high levels of alkaloid have been found in other lupines (Wink 1992).
Mattson, W.J., Niemel/i, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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