Welcome to Principles of Entomology! ENY 3005/5006 Course Packet and Study Guides
10: Insects & Plants Over 360,000 species of insects feed on Angiosperms (the flowering plants), and insects have fed on Angiosperms since these plants first appeared. But, plants were not passive in this process, and they display an arsenal of defenses against insect herbivores. PHYTOPHAGY = herbivory, the ability to consume plants or plant fluids. There are types of insect that feed on all parts of plants, including buds, leaves, roots, stems, seeds Mining Chewing Feeding Guilds How to exploit the plant? Sucking Boring MONOPHAGOUS insects specialize Galling on one food plant species, while POLYPHAGOUS insects consume a broad range of plants. POLYPHAGOUS insects are thus generalists. Most plants are fed on my many different insect species, and most insects themselves are generalists. For example, white oak is attacked by over 1,500 species of insects!! Plant defenses to herbivory Many of the morphological characteristics/structures of plants can be considered defense characters. Structural Plant Defense Against Insect Herbivores Defense Mechanisms Constitutive Insects Induced Tolerance Chemical Affect the behaviour and physiology Structural defenses are morphological characteristics/structures of plants that offer protection. These include trichomes, or hair-like outgrowths that keep eggs up and off of the plant. They are hard to chew, and some are spiny and puncture or tear the insects gut when ingested. Others are barbed and puncture or hook the caterpillar prolegs, causing them to drain hemolymph and die. Other morphological defenses include sticky sap, spines (like on cactus) and thorns
(like on rose bushes). Chemical defenses are prevalent and extremely effective. These include alkaloids, phenolics, terpenoids, protease inhibitors, and insect growth regulators. The last ones actually mimic insect hormones (like JH) and mess up larval development. Others, called Precocenes, actually wipe out the endocrine organ that produces JH (the corpora allata) Interesting side note: many of the plant defensive chemicals that are used to defend plants against insects and other herbivores do so by altering the physiology of the herbivores. Some of these are ingested with humans for these same effects like caffeine and cocaine (alkaloids) and marijuana (terpenoids). Many medicines that humans use are plant defensive compounds the trick is finding ones that help humans with particular medical conditions. Many indigenous peoples pass down a tradition of eating (or drinking in the form of tea) certain plants when sick. Modern western medicine has learned a lot from these traditional societies. Plants put most of their protection into new leaves and seeds. These are so potent that most herbivores cannot touch them. But, this creates an open, uncontested resource of high nutrient value Many insects became specialists by evolving the capacity to detoxify these defenses. Some insects can not only thrive on these new shoots and leaves, but they also can sequester the plant toxins and use the toxins as defense against predators. The catch: Conquering the chemical defenses of one plant usually does not help with the defenses of other plant species, so insects that can sequester toxic chemicals often feed only on one species of plants (specialists are MONOPHAGOUS). Generalist herbivores feed on the poor quality, but less chemically defended, older leaves and tissues. They get around the toxicity by not eating too much of any one toxin. They change plants so that they never accumulate too much of any one toxin. NOTE: Plant defenses are expensive to produce. Plants that are stressed often must sacrifice allocation to defensive compounds. This is why environmental extremes (e.g. drought) or human-induced disturbances (like pollution) are often associated with pest herbivore outbreaks the plant defenses are down!! Another plant defense is to attract natural enemies. Many plants produce extrafloral nectarines which are localized deposits of nutrients. These are often lipid and protein rich, and they attract wasps, ants, and flies often insects that are predatory or parasitic that will attack herbivore pests on the plant. Other plants release volatiles when cut or fed upon, which also may attract parasitoids.
Some Acacia trees attract ants with both extrafloral nectarines, and by offering refugia: swollen, hollow thorns, where the ants can live. These refugia are called domatia. These ants have a nasty sting and are very aggressive. They often help by keeping other insects and mammalian herbivores (even ELEPHANTS!), vines and other plants, away from their host tree. If you see a hollow lump on a plant, how can you figure out if it is a gall or a domatia? Well, keep in mind that galls are induced by insects. Insects can actually secrete chemicals that act like growth regulators in plants, so the insects manipulate the plant into growing a home for them. Domatia, on the other hand, are always parts of certain species of plants. If you grow Acacia drepanolobium in a greenhouse for example, it will grow domatia whether or not you give it some ants. Specialist pollinators Example: Figs and fig wasps The fig flower actually consists of lots of tiny imperfect flowers arranged along the inside of a pear-shaped receptacle. Fig trees produce three types of fig each year, and these coincide with the life cycle of the wasp. The figs formed in the winter have mostly neuter flowers with a few male flowers clustered near the exit hole of the fig. Female wasps enter the fig, lay eggs into the neuter flowers, and then die. Larvae develop inside the flower ovaries and mature into adults in the late spring. The wingless, nearly blind, males emerge first (protandry) and these males crawl around inside the fig and locate all of the cocoons of the females (their sisters!). Males chew into the cocoons and mate with the females before they even hatch. The males die, but the females emerge after mating, and leave the figs as they do, they brush against the male flowers and get covered with pollen. By that season, the figs have produced a second type of fig. These contain a mixture of neuter flowers and FEMALE flowers. Female wasps pollinate these as they crawl into the fig and move around looking for neuter flowers to lay in. These larvae develop, except that when they leave, there are no male flowers to brush against (this prevents selfing by the fig, since male and female flowers are not located within the same fig). By this time, the fig trees have produced a 3 rd type of fig These have only neuter flowers, and mated females lay eggs in the flowers as before. When this generation emerges, the first type of figs are back and the cycle repeats. *This is a completely obligate system. By alternating the types of figs the plants ensure outcrossing, but must skip one wasp generation each year to recycle. *Evolution in figs and these wasps is very tightly coupled, with speciation in figs leading to speciation in wasps. There is an almost perfect match between the phylogenies of fig wasps and figs. A STRICT association between pollinator and plant is VERY efficient very little pollen is wasted. But, this is risky what if one member of the pair goes extinct? Then the other might as well.
10: Insects & Plants Study Questions and Objectives List the five main feeding guilds of insects Explain the major ways that plants defend themselves against herbivores. Draw the diagram used in lecture to illustrate the types of plant defenses. Elaborate on each section on the diagram and provide examples. What is another way that plants defend themselves that is not in the diagram? When searching for new medicines for people, scientists often first look at plants that have defensive compounds. Why? List three different groups of plant defensive chemicals. Why is it adaptive for plants to provide domatia to ants? How do some ant species help plants? What do they do? How can you figure out if a structure is a gall or a domatia? How do plants talk to one another about the risk of herbivory? What is the chemical used, and how does this relate to the concept of induced defenses? Explain in essay format the complex mutualism between some species of figs and fig wasps. Based on the paper by Gilbert (1971) in the journal Science and also available on the course website: In 1971 very little work had been previously conduced on the importance of trichomes on defending plants. This paper was in the journal Science (a prominent journal) largely because this was a very important discovery at the time. What was the species of plant examined? What is its common name of plants in this genus (look this up yourselves)? What were the two insect herbivore species examined in the study? What happened when he placed a few larvae on the leaves of Passiflora adenopoda (page 586, 1 st paragraph)? What was the cause of death as determined with high magnification (middle section)? Briefly explain (4-6 sentences) how the co-evolutionary race may have proceeded between this genus of plants and Heliconius butterflies. Study Terms Coevolution, Monophagy, Polyphagy, Specialists, Generalists Gall, leaf mine Inducible defenses, constitutive defenses, tolerance Structural defenses, chemical defenses, Tannins, alkaloids, terpenoids, Myrmecochory, Elaiosome Domatia, Pollination, Mutualism