Feeding II: Physiological Adaptations Physiological aspects of feeding in insects can be divided into two main processes. • Food selection. An insect is surrounded by a confusing variety of potential food items. The process of food selection allows the insect to chose among suitable and unsuitable food items and restrict its diet to the most suitable items for its present circumstances. • Food processing. Once food items are selected, they must be processed in ways that enhance their nutrient quality and reduce the negative effects of toxins or other compounds that make food items less suitable for insect growth and reproduction. General classification of feeding habits • Herbivores. Food of herbivores is highly variable in quality. Feeding problems faced by herbivores are locating food, extracting nutrients and disposing of toxins. • Predators. Food of predators is usually of uniformly high quality. Feeding problems faced by predators are detecting, capturing and subduing prey. • Scavengers. Food of scavengers is rapidly changing in quality. Feeding problems faced by scavengers are the rapid temporal change in food quality and its rapid consumption by microorganisms or large vertebrates. • Ectoparasites (live outside the body of their host). Food is uniformly high in quality. Feeding problems of ectoparasites are locating a suitable host and avoiding behavioral defenses of the host. • Endoparasites (live inside the body of their host). Food is uniformly high in quality. Feeding problems faced by endoparasites are locating a suitable host, avoiding the physiological and immunological defenses of the host, and avoiding competition from other endoparasites (superparasitism and multiparasitism). Food selection • Monophagy. Diet restricted to a single species. Many scale insects and sawflies feed on a single species of host plant. Some of these insects are so host-specific that they are specialized on local populations of host plants. Some parasitoids are also extremely host specific. Most of the phorid parasitoids in the genus Apocephalus are restricted to one species of ant host. • Oligophagy. Diet restricted to a few species. The sulfur butterfly Pieris rapae feeds on plants in the family Brassicaceae and other species that contain mustard oil glycosides. • Polyphagy. Diet includes many species of plants or animals. Most predators are polyphagous. Grass-feeding orthopterans such as the locust Schistocerca are also polyphagous. The painted lady butterfly Vanessa is also polyphagous but prefers a restricted set of plants in several families. Monophagy in Lycaenidae Great Purple Hairstreak (Atlides halesus). Mistletoe (Phoradendron tomentosum). Soapberry Hairstreak (Phaeostrymon alcestis). Western Soapberry (Sapindus saponaria var. drummondii). Oligophagy in Lycaenidae & Pieridae Banded Hairstreak (Satyrium calanus). 4 species of oaks (Fagaceae), 1 species of hickory (Julgandaceae), and 2 species of walnut (Julgandaceae). Cabbage White (Pieris rapae). 4 species in Brassica, 1 species in Descurainia, several species in Erysimum, 1 species in Lepidium, several species in Nasturtium, 1 species in Raphanus, several species in Rorippa, 1 species in Sinapis, 3 species in Sisymbrium, and several species in Cleome (all in the Brassicaceae with mustard oil glycosides). Polyphagy in Lycaenidae Gray Hairstreak (Strymon melinus). Euphorbiaceae (2 species), Fabaceae (19 species), Malvaceae (9 species), Polygonaceae (3 species), Rosaceae (1 species), Poaceae (1 species), Verbenaceae (1 species), Zygophyllaceae (1 species). Red-Banded Hairstreak (Calycopis cecrops). Leaf detritus of many different species of plants. Factors restricting food selection The range of food items taken by an insect is first restricted by evolved limitations of its structural and physiological features. Phytophagous insects that require the detection of mustard oil glycosides to identify a host plant will not feed on a plant without these compounds, regardless of its nutritional quality. Example: the cabbage white butterfly. Phytophagous insects that do not have the physiological machinery to neutralize secondary plant compounds such as mustard oil glycosides cannot consume a plant that contains them, regardless of its nutritionally quality. Example: the banded hairstreak butterfly. Food selection by predaceous insects restricted more by size and structure Predators obviously cannot consume prey items that are too large to subdue. In general larger predators can consume a wider range of prey items than smaller predators. Small prey is more abundant than large prey, but large prey is more nutritious than small prey. BodysizePrey of Diet may be restricted further by other more ecological considerations including the relative abundance and Body size of Predator spatial distribution of food items. Stimuli used in food selection Phagostimulants are nutritionally important substances that promote feeding activity. Sugars are important phagostimulants for many scavengers and herbivores. Detection of sugars by tarsal chemo-receptors is sufficient to cause proboscis extension in butterflies, houseflies and blowflies. Curiously, nutritionally important amino acids by themselves do not stimulate or only weakly stimulate feeding in insects. Stimuli used in food selection Token stimuli are nutritionally unimportant substances that promote feed activity. Mustard oil glycosides are not nutritionally important to sulfur butterflies in the genus Pieris, but the presence of these compounds nonetheless is required to elicit feeding behavior. These butterflies feed on plants in the family Cruciferae (mustard family) that contain mustard oil gylcosides. Stimuli used in food selection Feeding deterrents are nutritionally unimportant substances that deter feeding activity. Many secondary plant compounds are feeding deterrents for insect herbivores. Feeding behavior of the polyphagous aphid Myzus persicase is deterred by the presence of mustard oils. The presence of spines or sequestered toxins in potential prey items deter feeding behavior in predators. Interaction among feeding stimuli Synergistic effects. Various kinds of feeding stimuli may interact synergistically to promote or deter feeding behavior in insects. For example, weak concentrations of sugar and the amino acid L-proline individually only weakly stimulate feeding behavior in the spruce budworm (Choristoneura fumiferana) but together they are highly stimulating. L- Proline Interaction among feeding stimuli Hierarchy of Cues. Most insects use a hierarchy of cues to increasingly narrow the selection of appropriate food items. The beewolf wasp, Philanthus, is a specialized predator of bees. When hunting, a female orients to a variety of insects of the appropriate size. On approach, the wasp will attempt to capture an insect only if it smells like a bee. Wasps will capture insects that have been given the odor of bees experimentally. After capture the wasp uses tactile cues to determine if the insect actually is a bee. If so, the wasp stings the bee and carries it to the nest; if not, the wasp releases it and continues hunting. Interaction among feeding stimuli Motivation. The sensitivity of an insect to various stimuli that promote or deter feeding behavior depends of its physiological state. For example, the phagostimulant sugar will not elicit proboscis extension in a fruit fly with a full gut because this reflex action is inhibited by excitation of stretch receptors in the gut. Likewise, feeding deterrents are less effective against a hungry insect than they are against a satiated one. Within limits the diet range of an insect varies depending on its motivational state. Feeding behavior in Chargas bugs Food processing Herbivores, predators, scavengers and parasites all face unique challenges in processing food items. 1. Herbivores must contend with low quality food that may also contain toxins. 2. Predators have access to high quality food, but it is usually heavily defended structurally or behaviorally (and sometimes chemically). 3. Parasites also have access to high quality food, but must also contend with various host defense mechanisms, including behavioral, physiological and immunological. 4. Scavengers must deal with food of highly variable quality and competition from microbes and vertebrates. Problem food substances Secondary plant compounds (xenobiotics). Plants have evolved an astonishing variety of chemical compounds to deter feeding by herbivores. These compounds are divided into three broad categories. Toxins are low molecular weight compounds that act as metabolic or neurological poisons. Examples include alkaloids (cocaine, codeine, caffeine, quinine, novocain, nicotine), cyanogenic compounds (in clover, flax, apples), peptides (α-amanitin in mushrooms), glycosides (mustard oil glycosides in mustard plants, and Tannin phototoxins (furanocoumarins in parsnips). Digestibility reducers are compounds of high molecular weight that bind with plant or animal tissue to reduce its digestibility to insects. Tannins in oaks, tea and other plants are an example. Insect hormone analogues are mimics of ecdysone (the molting hormone) and juvenile hormone (the maturation hormone) which interfere with normal insect development. Hormone analogues are Ecdysone common in ferns. Insect solutions to xenobiotics Insects have four main strategies for dealing with xenobiotics • Excretion. Many insects adapted to feed on plants with specific xenobiotics do not metabolize these compounds, but instead excrete them directly. The tabacco