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F1 HYBRID SPRUCES INHERIT THE PHYTOPHAGOUS INSECTS
OF THEIR PARENTS
WILLIAM J. MATTSON, ROBERT K. LAWRENCE, and BRUCE A° BIRR
USDA Forest Service, North Central Forest Experiment Station
B-7 Pesticide Research Center, Michigan State University, East Lansing, MI 48824, USA
INTRODUCTION
Interspecific hybridization in plants is a widespread and important natural phenomenon. In fact, Stace (1987)
estimates that at least 50% of the extant angiosperm species have been derived via hybridization. Natural hybrids are also
common among the gymnosperms (Wright 1955, Hanover and Wilkinson 1969, Schmidt-Vogt 1977, Zobel and Talbert 1984).
In spite of the ecological and evolutionary importance of plant hybrid zones (Remington 1968, Barton and Hewitt 1985),
little is yet known about how they affect the dynamics of plant/herbivore relationships. Whitham (1989) speculated that
hybrids may be particularly prone to herbivory because usual resistance mechanisms derived from the parents are poorly
expressed. For example, polygenic resistance traits might be intermediate in F1 hybrids, whereas monogenic and oligogenic
traits might be fully expressed or not at all depending on the mode of gene action (e.g., dominant or recessive, etc.), and on
whether there is genetic dysfunction preventing normal gene expression (Fritz et al. 1994, Whitham et al. 1994). Furthermore,
to compound these problems, hybrids may attract specialist phytophages from both parent species, thereby being
predisposed to a much larger pool of potential consumers than either of the pure parent types (Whitham et al. 1994).
Insect Loading in Relation to a Plant's A/R Vectors and S/D Vectors
To examine the issue of hybrid resistance to insects in a more mechanistic framework, we consider that there are at
least two sets of plant traits affecting insect loading on plants: (1) the mix of properties which elicit insect attraction/
arrestmenffrepulsion (e.g., morphology, electromagnetic and biochemical cues, abiotic requirements, etc.) and thus govern the
number of insect species that attempt to colonize it, and (2) the mix of properties that are sustaining/defensive for each of the
would be colonizers as they attempt to feed, oviposit, and reproduce. These two sets of properties, which together set the
potential for insect loading (total insect biomass or numbers), can be represented as two vectors: the species attraction/
repulsion (A/R) vector, [a_,a2,a3..an],and the suitability/defense (S/D) vector [Sl,S2,S3..s]. Each species of insect, Si, is
represented by an element (a)in the species A/R vector, having a numerical value (0-1) that is proportional to the plant's
average capacity to attract and arrest individuals of the particular insect species. Likewise, the S/D vector contains specific
elements (s) that correspond to and represent the plant's average suitability, ranging from zero, where an insect colonizes a
plant but cannot complete development on it (immunity), to 1 where the plant fulfills all of the insect's requirements, for each
species in the species A/R vector. The product of the two vectors, [ayL+a2s2+..as ], represents the plant's overall potential
"resistance expression", or potential insect loading per plant.
To give an extreme example for one insect: the white pine weevil, Pissodes strobi, is highly attracted to Picea glauca
shoots as evidenced by the great number of feeding scars it produces (i.e., a = 1), but it can only rarely reproduce in these
shoots (i.e., s = 0), with the consequence that P glauca is considered "resistant" to the weevil (Lanier 1983). Actual insect
loading depends, in addition, on the quantity and genetic diversity of insects (N) that continually attempt colonization. The
total number of non-zero elements (all those a_> 0) in an individual plant's A/R or species vector (its species richness or
species loading) is positively linked to the size and density of its population, the total areal distribution of the species, and
negatively to its biochemical uniqueness wherein it occurs (for a review, see Tahvanainen and Niemela 1987). For many
natural hybrids, their populations and geographical distributions are usually small (compared to their parents) thereby
exposing them to fewer potential colonists, but their low biochemical uniqueness or high similarity to the two nearby parent
species will likely have a counteracting influence making them highly susceptible to the reservoirs of consumers fiom two
plant species (Whitham et al. 1994).
Mattson, W.J., Niemel_, 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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