Reading:  Economy of Nature, pp. 446-459.

Herbivore Responses

In ecological time: Selective feeding on poorly defended plant parts or at the time of year when defense if poor

In evolutionary time: Selection for detoxification enzymes in herbivore gut

Prediction: Co-evolution (joint-evolution) of plants and herbivores yielding some highly specialized but successful herbivores, and plants with complex chemical defenses that deter most herbivores.

Trends in the co-evolutionary interaction between herbivores and plants are influenced by plant apparency, the likelihood of encounter with herbivores.

Apparent plants are long-lived and relatively predictable to herbivores so these plants are likely to invest more in defense than are unapparent plants which are short-lived, ephemeral, and unpredictable to herbivores.

Unapparent plants are most likely to produce relatively inexpensive qualitative chemical defenses which are good against generalist herbivores. Apparent plants are most likely to produce more expensive quantitative chemical defenses which are effective against a wide range of herbivores but may be susceptible to co-evolution by a specialist.

Some plant species, particularly apparent plants, may produce more than one chemical defense and may switch from qualitative to quantitative defenses as the season progresses.

Plant Apparency

Examples of Plant Responses to Herbivory

Cardenolides (cardiac glycosides) in milkweeds (Asclepias) and monarch butterflies
        A cardiac glycoside in the milky sap of milkweeds makes these plants toxic to vertebrate and most invertebrate herbivores. However, this chemical defense is not effective against the larvae (caterpillar) of monarch butterflies. Monarch butterflies, Danaus plexippus, lay their eggs on milkweeds, the larvae are herbivores on milkweed leaves and they store the toxin in their body tissues. This provides chemical protection against carnivores for both the larvae and adult monarch butterflies.

Tannins in oak leaves (Quercus) and moth larvae
        Tannins are polyphenolic chemicals that bind to proteins and make them unavailable for digestion. Tannin concentrations in oak leaves increase from spring (April 0.66% dry weight) to autumn (September 5.5% dry weight). Leaf protein content decreases from spring to summer and them remains low. Leaf toughness (cellulose) increases from spring to summer.

Insect herbivores (moth larvae, Lepidoptera) respond to these defenses by feeding on spring oak leaves, or by modifying their life cycle or feeding habits:

	Start feeding on oak leaves, complete feeding on herbaceous plants
	Larvae overwinter and complete growth the following spring
	Larvae bore into leaf and feed on low tannin tissues

Feeding habits of 201 species of Lepidoptera larvae on oak leaves in Britain (after Krebs, 1994, p 301, Table 15.2, data from Feeney, 1970).

Experimental studies with winter moth Operophtera brumata caterpillars (Feeney, 1968 and 1970).

In whole leaves both leaf toughness and tannin concentration may be responsible for the inhibition of growth with the old leaf diet, grinding leaves eliminates leaf toughness as a variable. Minimal differences in larval growth on ground leaves suggests that leaf toughness is the most important variable. However, on prepared diets of casein, tannin has an obvious inhibitory effect.

Senita cactus and fruit flies

Senita cacti, Cereus schottii, produce an alkaloid (lophocereine) and only the fruit fly, Drosophila pachea, is able to detoxify this chemical. It is the only fruit fly species that uses this cactus as food.

Saguaro cactus and fruit flies

A similar situation is seen in the saguaro cactus, Cereus giganteus which produces a different alkaloid (carnegeine) in its tissues. Only one species of fruit fly, Drosophila nigrospiracula, a different species than that feeding on senita cactus has evolved detoxification adaptation for the saguaro cactus alkaloid.

Mustards and flea beetles

Plants in the family Cruciferae produce a glucosinolate, allyl isothiocyanate, known as mustard oil. This chemical deters many insect herbivores but flea beetles, Chrysomelidae, use the downwind plume of this volatile chemical to locate their preferred prey.

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