Foraging Strategies

Insect behavior can be viewed from the standpoint of efficiency of resource acquisition and allocation (Sterner and Elser 2002, see also Chapter 4). Foraging should focus on resources that provide the best return and minimum risk for the effort expended. Hence, bumble bees, Bombus spp., forage on low-energy resources only at high temperatures when the insects do not require large amounts of energy to maintain sufficiently high body temperature for flight (Bell 1990, Heinrich 1979,1993). Other host-seeking insects tend to focus their searching where the probability of host discovery is highest (i.e., where hosts are concentrated or most apparent) (Bell 1990, Kareiva 1983).

Foraging theory focuses on optimization of diet, risk, and foraging efficiency (Kamil et al. 1987, Schultz 1983, Stephens and Krebs 1986, Townsend and Hughes 1981). Profitable resources provide a gain to the consumer, but nonnu-tritive or toxic resources represent a cost in terms of time, energy, or nutrient resources expended in detoxification or continued search. Continued search also increases exposure to predators or other mortality agents. Sublethal doses of defensive chemicals reduce nutritional value of the resource, so they should be avoided when resources are abundant but they may be eaten when more profitable resources are unavailable or not apparent (Courtney 1985, 1986). Consumers should maximize foraging efficiency by focusing on patches with high profitability (and ignore low-profitability patches) until their resource value declines below the average for the landscape matrix. Orientation toward cues indicating suitable resources improves the efficiency of food acquisition. Furthermore, learning confers an ability to improve resource acquisition as a result of experience.

Most insects must search for suitable food at some spatial scale. Even within a particular plant, nutritional quality may vary considerably among individual leaves (e.g., between sun and shade leaves, young and old leaves) (Schultz 1983, Whitham 1983). Foraging strategy represents a tradeoff between costs (in terms of reduced growth and survival) of searching, costs of feeding on less suitable food, and costs of exposure to predators.

Schultz (1983) developed a tradeoff surface to illustrate four foraging strategies for arboreal caterpillars (Fig. 3.11). Foraging can be optimized by searching for more nutritive food and risking attention of predators, accepting less nutritive food, or defending against predation. Natural selection can favor a reduction in cost along any of the three axes, within constraints of the other two costs. Feeding selectively on the most suitable food during the day incurs a moderate cost of movement (energy expenditure at higher temperature) and high risk of predation. Selective feeding at night reduces predation risk but increases the energetic cost of movement under cooler temperatures, which then restricts the time spent feeding, especially at high latitudes. Crypsis (camouflage) reduces the risk of predation for day-feeders, but the cost of movement (in terms of attraction of predators) is substantial and limits ability to search for the most suitable food. Conversely, apo-somatic (warning) coloration can reduce the risk of predation and allow greater freedom of movement, but the energy expenditure of movement and cost of biochemical sequestration must be considered. Species that mimic, and live on or in, their food can avoid both movement and predation costs but have no choice in their food after initial colonization. Hence, feeding costs may be quite high.

Risk Risk

Risk Risk

H3Q0 Tradeoff planes of selected caterpillar foraging strategies. Costs of feeding (i.e., metabolic costs of digestion, reduced growth, etc.), movement (metabolic costs of reduced growth), and risks (e.g., probability of capture or reduced growth as a result of time spent hiding) increase in the direction of the arrows: a: selective diurnal feeder, b: selective nocturnal aposomatic feeder, c: diurnal cryptic feeder, and d: food mimic. From Schultz (1983). Please see extended permission list pg 569.

Predators also face tradeoffs between hunting, ambush, or intermediate strategies. Hunting requires considerable expenditure of energy searching for prey, but it has a high return, depending on ability to detect prey from a distance. Detection can be increased by orienting toward prey odors or plant odors indicative of prey. Accordingly, many predaceous species are attracted to mating pheromones of their prey (Stephen et al. 1993) or to volatile chemicals released by plants in response to herbivory (Turlings et al. 1993).Ambushers either sit and wait or use traps to capture prey. As examples, dragonfly larvae hide in the substrate of aquatic habitats and grasp prey coming within reach, antlion larvae excavate conical depressions in loose sandy soil that prevents escape of ants and other insects that wander into the pit, and webspinning spiders construct sticky orb or tangled webs that trap flying or crawling insects. Movement costs are minimal for these species, but prey encounter is uncertain. Frequency of prey encounter can be increased by selecting sites along prey foraging trails, near prey nest sites, etc.

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