Lecture 5 Herbivorous Insects PDF

Herbivorous Insects Ent 104 Johnson Outline Life as a plant eater – Plant defenses Physical Chemical – How insects overcome defenses – Life on poor protein and vitamin sources – Major insect strategies for plant feeding taxonomy Plants as food Plants defend themselves – Physically – Chemically – Bio...

Herbivorous Insects Ent 104 Johnson Outline Life as a plant eater – Plant defenses Physical Chemical – How insects overcome defenses – Life on poor protein and vitamin sources – Major insect strategies for plant feeding taxonomy Plants as food Plants defend themselves – Physically – Chemically – Biologically Calling for help from predators and parasitoids Physical defenses Waxy and tough cuticle – Plant cell walls are tough and indigestible for most herbivores – Blocks access to plant tissue Trichomes – Tiny little thorns – They often secrete toxins Eisner 1998 Chemical defenses Plant secondary compounds are used as defense against herbivores Tobacco: source of nicotine – Toxins Source of inspiration for pesticides – Digestion inhibitors – Physical inhibitors of feeding Neonicitinoid: nicotine like pesticide Production of secondary compounds Secondary compounds can either be: – Constitutive Always present – Induced Only produced after the plant is attacked – Within the same plant » Plant hormones – Even between plants » Plant communication Chemicals that physically interfere with feeding Silica and lignins deposited into tissue – Tissue is gritty and tough to grind – Chewing wears out the mandibles Latex is copiously secreted from damaged tissues – Gums up the mouthparts and slows feeding – Is also usually toxic Compounds that interfere with digestion Reduce nutritional value of food – Tannins interfere with digestion in the gut in mammals Perhaps make ROS – Lectins are carbohydrate bonding proteins They resist digestion and bind to the walls of the gut This interferes with the normal functioning of the gut Some can also escape the gut and interfere with other organ specific processes in the gut Proteinase inhibitors These proteins interfere with insect digestive proteins – Affect digestion Insect digestive proteins can also deactivate many plant defensive proteins The proteinase inhibitors prevent this and allow plant defensive proteins to do their work Poisonous compounds Neurotoxins – Nicotine Neurotransmitter analog – Caffeine Affects cAMP levels in the central nervous system A little bit of these compounds will also affect our nervous systems – Sometimes in a pleasurable manner – Too much is fatal Insect would be feeding solely on this plant and eating huge amounts of nicotine or caffeine Many defensive compounds are toxic to plants, too Hydrogen Cyanide – Precursors are stored in an inactive form Often the poison is attached to some other compound that makes it inactive cyanogenic glycosides – Feeding damages the tissue and leads to the activation of the toxic substance Releases enzymes that break the bonds keeping the substance in an inactive form Often the conjugated poison is in the vacuole and the enzyme to release it is in the cytoplasm – Hydrogen cyanide is produced during chewing and poisons the feeder How do insects overcome secondary compounds? Behavioral resistance – Avoid or get around contacting the poisons Evolve genetic resistance – Receptors or enzymes associated with the compound change their sequences Behavioral resistance Insects can sense the poison and avoid the plants with too much – Once the plant starts to defend, the insect moves on – Applies to both feeding and ovipositing – Bitter taste is used to determine how toxic the plant is Trenching behavior prevents the addition of toxins to plant tissue – The insect cuts the canals that transport substances within the plant tissue – They then eat the part of the leaf cut off from the rest of the plant Dussourd and denno 1994 Metabolic resistance Detoxification genes – Come in many forms (gene families) P450s most well understood – These enzymes break down and degrade toxic substances – P450s are rapidly evolving and can evolve the ability to break down novel substances – Plants and insects are in an arms race Plant make a new chemical Insect evolves a gene to break it down Plant makes a new chemical… Insensitivity to toxic compounds Many insects that are resistant to pesticides evolve a mutation that makes the receptor for the neurotoxin analog (the neurotransmitter) insensitive to it Few examples are known for insect resistance to plant secondary compounds May be that this method of resistance is more costly than the others and is only a short term response Neuromuscular junction Specialists Herbivores Monarch’s sequester cardiac glycosides – Not only are they resistant to the toxins – Their biology is dependent on the toxic compound They prefer it in their food – They use the poisons to defend themselves against predators and parasites Who are not resistant Insect herbivore classifications Monophages – Gall producers are usually species specific Oligophages – Monarchs feed on milkweeds of many species Polyphages – Wax scale feeds on over 200 species of plants – Most Orthoptera are generalists, but most other insect herbivores are relatively specialized Insect feeding strategies – Chewing – Sucking – Leaf mining – Gall producers – Other Farming symbioses Leaf chewers Damage easy to see and quantify and insect tend to be large – Most research on insect herbivory on these groups Major lineages – – – – – Orthopterans Lepidopterans Phasmids Beatles Hymenoptera sawflies Sucking insects Insect sucks the phloem or xylem (sometimes cell contents) – Usually phloem In all cases damage is difficult to quantify without careful studies – Unless the sucker is associated with a diseases – The herbivore can be a vector of diseases Most are hemipterans or thysanopterans – Mouthparts are highly specialized Canals for outflow of saliva and inflow of food for the insect – Aphids, whiteflies, western flower thrips Specialized Nutrition Zylem and Phloem are carbohydrate rich and protein poor – Many fluid feeders have symbiotic bacteria that provide them with essential amino acids Insects that feed on these fluids excrete excess sugar as honeydew Bees collect the honeydew Ants tend aphids, mealybugs, scales, and other homopterans – Ants get food – Aphids get protection – In some cases, ants move their “cattle” from plant to plant and even move them into their nests at night Leaf miners and plant borers Leaf miners live in between the epidermal layers of leaves – Larva feeds on the plant then pupates and leaves – Dipterans, lepidopterans, coleopterans, hymenoptera – Most are flies or moths Stalk borers, fruit borers, root borers, and wood borers – Eat living tissue from inside the plant where they live – Many are pests European corn borers, med fly Gall producers Galls are pathologically developed tissue in plants – Can be caused by pathogens, nematodes or mites, but insect galls are most common The gall provides food and protection for the insect at the expense of the plant Only about 2% of insects make galls – Hemipteans, dipterans, hymenoptera (major), thysanoptera, coleoptera, leps (minor) Galls can be simple or highly complex – They are often species specific – Gall form and maintenance is controlled by the insect – How galls are controlled is poorly understood Could be viruses, insect produce plant hormones, etc Massive expansion of genes in gall formers unique for this function Galls come in an amazing variety of forms Seed predation Seeds are often highly nutritious and many insects specialize on them Harvester ants – Stores seeds in granaries – Have barbed stingers like honey bee – Most toxic venom per weight To defend against rodents that want to get into the seed granaries Seed dispersal by ants Some seeds are hard and inedible, but they have attached an elaiosome that attracts ants The ants eat the elaisome and disperse the seed Not co-evolution because the ants have not changed, only the plants – It is a mutualism, though Digestion Most insects can digest cellulose Early studies thought only insects with microbial symbionts can digest cellulose Now we know that most insects make endogenous cellulases – Walking sticks, beetles, termites make a lot – Dipterans, lepidopterans make none Danger of the model system approach – Caterpillars do not digest cellulose so the consensus was insects do not digest cellulose