The nursery industry is interested in producing plants resistant to insects to reduce pesticide use and prevent economic loss. Programs range from traditional breeding (Painter, 1951) to mechanisms of induced defense (Zavala and Baldwin, 2006) to the roles of genes in plant resistance (Smith, 2005; Smith and Boyko, 2007). Studies on plant–insect coevolutionary theory may help explain what types of phytochemicals may be investigated in traditional breeding programs to develop plants resistant to specialist leaf-feeding beetles (Coleoptera) and moths (Lepidoptera). Coevolutionary theory argues that new phytochemicals in a plant may permit the plant to escape from herbivory by confusing the insect with the novel chemical. Specialist herbivores use chemicals as behavioral, feeding, and ovipositional cues (Ehrlich and Raven, 1969; Futuyma, 2000). The theory speculates that herbivores are deterred by qualitative compounds that are toxins (less than 2% dry weight) such as alkaloids, phenolic glycosides, and furanocoumarins (Rhoades, 1979). However, specialist insects become physiologically (cytochrome P-450 systems for detoxifying phytochemicals) and behaviorally (feeding, oviposition) adapted to these toxins (Berenbaum and Zangerl, 1993). In contrast, quantitative compounds (5% to 20% dry weight) such as anthocyanins, polyphenols, and tannins are more difficult for specialists to adapt to because they often bind to nitrogen, reduce nitrogen availability, and slow insect development (Feeny, 1970).
Data support the premise that specialist insects use qualitative compounds as behavioral, feeding, and ovipositional cues. Bronze birch borer (Agrilus anxius Gory) used rhododendrol in bark of birch (Betula sp.) as an attractant (Santamour, 1999). The willow leaf beetle (Chrysomela aeneicollis Schaeffer) was attracted to salicylate in Sierra willow (Salix orestera Schneid.) (Rank, 1992). Black cottonwood (Populus trichocarpa Torr. & Gray) contained salicin, which was sequestered and transformed into the larval defensive chemical salicylaldehyde in cottonwood leaf beetle (Chrysomela scripta F.) (Warren et al., 2002).
Quantitative compounds are considered in plant insect coevolutionary theory to be feeding deterrents, even to specialist insects (Rhodes and Cates, 1976). In quaking aspen (Populus tremuloides Michx.), higher tannin levels decreased cottonwood leaf beetle larval growth rate by 30% (Donaldson and Lindroth, 2004). In white oak (Quercus alba L.) and black oak (Q. velutina Lam.), fewer specialists such as weevils (Attelabus sp.) (Curculionidae), southern oak dagger moth [Acronicta increta Grote (Noctuidae)], and the moth Chionodes pereyra Clarke (Gelechiidae) were found on individual trees with higher tannin concentrations in the canopies (Forkner et al., 2004). In cotton (Gossypium hirsutum L.), chrysanthemum (anthocyanin), gossypol (polyphenol related to tannins), and other anthocyanins decreased feeding and survival of tobacco budworm [Heliothis virescens (F.)] (Hedin et al., 1983).
Anthocyanins, one class of flavonoids, produce red and purple pigments and were reported to reduce herbivory (Close and Beadle, 2003; Harborne and Williams, 2000; Kursar and Coley, 1992; Simmonds, 2003). Correlations were reported between anthocyanin and total phenols (Close et al., 2001; Lee and Lowry, 1980), low levels of chlorophyll (Dodd et al., 1998; Krause et al., 1995), and low levels of nitrogen (Skillman et al., 1996). Feeding by the pyralid moth eggplant borer (Leucinodes orbonalis Guenée) was lower on cultivars of eggplant (Solanum melongena L.) with higher levels of anthocyanins, phenolic compounds, and glycoalkaloids (Bajaj et al., 1989). In arabidopsis [Arabidopsis thaliana (L.) Heynh.], feeding by fall armyworm [Spodoptera frugiperda (JE Smith)] was lower on purple leaves [0.38 ± 0.03 absorbance units (AU) of anthocyanins] compared with green leaves with purple veins (0.29 ± 0.03 AU) or green leaves (0.023 ± 0.003 AU) (Johnson and Dowd, 2004). Bronze wild radish (Raphanus sativus L.) flowers containing anthocyanins were fed on less than white or yellow flowers by imported cabbageworm [Pieris rapae (L.)], beet armyworm [Spodoptera exigua (Hübner)], cabbage aphid [Brevicoryne brassicae (L.)], western flower thrips [Frankliniella occidentalis (Pergande)], and grey field slug [Agriolimax reticulatus (Müller)] (Irwin et al., 2003).
Breeding resistant plants to specialist insects should involve the development of cultivars with novel qualitative chemicals or increased levels of quantitative chemicals that bind with nitrogen and reduce its availability in food. Ninebark (Physocarpus opulifolius) (Rosales: Rosaceae) is a hardy shrub ranging from Quebec south to Tennessee that grows along riverbanks in sandy soil (Wheeler and Hoebeke, 1979, 1985). Nurseries bred ninebark for atypical leaf color: a purple-leaved cultivar called ‘Monlo’ and yellow-leaved cultivar called ‘Dart's Gold’. The purplish leaves of ‘Monlo’ may be higher in anthocyanins, which can be feeding deterrents (Harborne and Williams, 2000; Simmonds, 2003) to the specialist ninebark beetle (Calligrapha spiraeae) (Coleoptera: Chrysomelidae). The objectives were to evaluate among two cultivars and native: 1) herbivore leaf preference, 2) herbivore fecundity in long-term rearing, and 3) leaf chemistry.
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