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THE BLACKMARGINED APHID AS A KEYSTONE SPECIES:
A PREDATOR ATTRACTOR REDRESSING NATURAL ENEMY
IMBALANCES IN PECAN SYSTEMS
M.K. HARRIS and T. LI
Department of Entomology, Texas A&M University, College Station, Texas 77843, USA
INTRODUCTION
Plants defend themselves against herbivores in many ways. Conceptualization of these defenses has resulted in many
terms and contexts to describe them (see Harris and Frederiksen 1984). Snelling (1941) listed 15 categories. Painter (1951,
Fig. 3) depicted 11 plant factors interacting with 8 insect factors mitigated by 8 environmental factors influencing 5 insectplant
interaction factors as possible causes of resistance to insects, and then consolidated these factors into three resistance
mechanisms: tolerance, antibiosis, and preference. These mechanisms were, by definition, heritable, effective in isolation,
and particularly apt for usage in agriculture where producing a crop of good quality in the presence of the insect was paramount.
Harris (1980) proposed a succinct and natural characterization of plant defense, listing escape in space and time,
accommodation, confrontation, and biological associations as the primary mechanisms to consider.
Biological Associations
Of these, biological associations are the most complex and difficult to demonstrate as natural defense mechanisms,
because both the genetics of the plant and the arthropod are involved. Perhaps the best known case of a natural biological
association providing plants with a defense against herbivores is the ant-acacias. Howe and Westley (1988) review this and
other lesser known ant-plant associations, as well as other biological associations including those important in dispersal,
pollination, etc. Biological associations mediated by the genetics of the plant clearly play an important role in nature, yet
they have seldom been deliberately exploited to a significant degree in agriculture. Exceptions could be corn leaf aphid,
Rhopalosiphum maidis (Fitch) (Homoptera: Aphidae) on corn and sorghum, and apple rust mite Aculus schlechtendali
(Nalepa) (Acarina: Eriophyidae) on apple, which are relatively innocuous at moderate densities and, if left alone, will often
support densities of natural enemies that also suppress more pestiferous species of aphids and mites, respectively (Flint and
van den Bosch 1977). The agricultural importance of corn leaf aphid and apple rust mite is clear, but the origin and role of
these biological associations in natural systems are presently unknown. From an agricultural perspective, this area combines
aspects of biological control and host plant resistancentwo subdisciplines of entomology that lack a common paradigm.
BLACKMARGINED APHID AND PECAN SYSTEMS
This paper proposes another example of a biological association, viz. how pecan, Carya illinoensis (Wang.) K. Koch
(Juglandaceae), interacts with the blackmargined aphid, Monellia caryella (Fitch) (Homoptera: Aphidae), to influence other
biological associations that may impact plant defense within the context of the matrices of interactions noted above.
Pecan is a deciduous, monoecious, wind-pollinated, woody perennial that occupies alluvial soils from western Texas
to the Mississippi Valley on the east and from southern Illinois into Mexico (Little 1971). Pecan trees can live more than 200
years, grow to heights exceeding 35 m, and can constitute as much as 50% of the tree canopy in their natural habitat (Maggio
et al. 1991). Reproduction is by seed (=200/kg) abundantly produced every 2-7 years (< 500 kg/ha) in natural populations,
and each tree in nature is genetically distinct (Harris 1988). Leaves are compound and occur at a density of about 3 million
leaves or roughly 50,000 m2/ha.
Mattson, W.J., Niemel/i, E, 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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