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450
TOTAL HERBIVORES
400 [] 1991-]
[] 1992r
350 ID993/
.......
300
250
03
F,, O.
I
IIEI 150
100
50 _,_,_
1/...-/,,, J
O. _.',. .....
ERIOCEPHALA HYBRID SERICEA
SPECIES
Figure 2.--Density (+ 1 SE) of total herbivore numbers among S. eriocephala, S. sericea and their interspecific hybrids and
among years. Table 2 indicates which hypotheses were supported by total herbivore numbers.
The Additive hypothesis received the greatest support in each year (7 of 11 species, Table 2). Herbivore densities oa
hybrids are often intermediate between the densities on the two parental species. This finding is consistent with an Additive
model of inheritance of resistance traits. Other studies also provide support for the Additive hypothesis. Soetens et al.
(1991) found for a species of Pontania on European willows and their hybrids that gall numbers were intermediate on
hybrids (supporting the Additive hypothesis). Studies on sawflies on birches (Hanhim/iki et al. 1994), beetles on elms (Hall
and Townsend 1987), and beetles on willows (Soetens et al. 1991, and Fritz unpublished), and various species on oaks
(Aguilar and Boecklen 1992) also indicate that hybrids are often intermediate in suitability for herbivore growth. Hanhimfiki
et al. (1994) have studied herbivore performance on hybrid and parental birches that grew in a common plantation in Finnish
Lal_land. Of the 14 herbivore species studied (l 3 sawflies and a moth), I I species had intermediate growth rates on hybrid
leaves compared to parental leaves.
Monogenic and polygenic resistance traits where allele effects on traits are additive will have half the dosage of
alleles, on average, in hybrids compared to parents, unless parents have the same defense mechanisms determined by the
same loci. There is evidence from willows supporting additive inheritance of phenolic glycosides, tannins, and morphologi-
cal defenses (Meier et al. 1989, Soetens et al. 1991, Nichols-Orians and Fritz, unpublished data). How the altered dosage of
defensive traits influences herbivores will depend on the sensitivity of the herbivores to the resistance trait and the presence
of other resistance traits inherited from species that might not normally be host plants of the herbivore. A consequence of
additive inheritance of resistance is that backcrossing should result in a reconstitution of defense, rather than further break-
down of resistance, as hypothesized by Whitham (1989) and Paige and Capman (1993). The resistances of backcross
individuals should more closely resemble that of the recurrent parent as more parental genes are reincorporated into the
genome. Whitham et al. (1994) found evidence supporting this hypothesis in their work on eucalypts. When F1 hybrids
were compared with backcross plants and parents, often backcross plants were intermediate between FI hybrids and parents.
The Dominance hypothesis received support from Aculops mites, Phyllocnistis sp. Phyllocolpa spp., and Pontania sp.
in at least one year (Table 1). In this hypothesis, herbivore densities on hybrids resemble those of one parent but differ
significantly from that on the other parental species. Hanhim_iki et al. (1994) found for three species, a moth and two
sawflies, on the Finnish birches that performance on hybrids did not differ from that of one parent but did differ from the
other parent, supporting the Dominance hypothesis. The Dominance hypothesis was proposed by Fritz et al. (1994), and it is
particularly plausible because of the inheritance of defensive chemicals in hybrids of several crop and wild plants. For Lotus
(O'Donoughue et al. 1990), Nicotiana (Huesing et al. 1989), and Papaver (Levy and Milo 1991), chemical defense mecha-
nisms have been shown to be inherited as dominant traits in hybrids. Zangerl and Berenbaum (pers. comm.), in a review of
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