Plant-Insect Interactions. Plant-Insect Interactions. Specialization and Diversification. Fig. 3.1

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1 Plant-Insect Interactions 1 Plant-Insect Interactions! Insect attack more like parasitism than predation Do not kill or do so slowly Seeds and seedling herbivory is predation! Small size leads to long-term relationship with plants Often highly specific host choice 2 Specialization and Diversification Fig

2 Insect Adaptations for Access Fig Generalization vs. Specialization! The number of taxa that are interacting! Disadvantage of specialization: if food resource scarce due to bad year Can lead to extinction in extreme cases 5 Specialization Hypotheses! 1) Jack of all trades is a master of none! 2) Plants are protection, not just food! 3) Evolutionary dead-end 6 2

3 Testing the Dead-end Hyp! Dendroctonus bark beetles Fig What Limits Insect Populations?! Rarely is all plant material consumed! Top-down or bottom-up control?! Nutrient requirements and modes of digestion the same in most animals Proteins, fats, carbs, vitamins, minerals! Limited by energy transfer along trophic chain? 8 Role of Microbial Endosymbionts Fig

4 Macronutrients! Liebig s Law: growth is limited by the nutrient that is in the shortest supply! For herbivores, protein usually limited Insects over-eat to compensate for low N 2 content Hemi/Homopterans (phloem feeders) eat x body wgt/day 10 Plant Manipulation of Macronutrients! Storage of amino acids in forms of little use to insects! Store N 2 in non-protein amino acids Usually D-Enantiomer L-Enantiomer is toxic Dioclea megacarpa only attacked by Caryedes brasiliensis 11 Micronutrients 12 4

5 Plant Defenses Against Insects! 1) Physical barriers Trichomes, sometimes glandular Fig Plant Defenses Against Insects! 2) Toxins and Deterrents Huge diversity of noxious cmpds possible (C, H, N, O, P, S)! Secondary compounds, allelochemicals when defensive 1) must affect degree to which plant is attacked 2) plants with the attribute must have greater fitness than those without 14 Delivery! Bursera schlechtendalii! Terpenes under pressure, shoot up to 150cm Fig

6 Plant Defenses Against Insects! 3) Abscission: Dropping leaf! Useful for small or immobile insects! Loss of nutrients and energy 16 4) Recruitment! Acacia and Pseudomyrmex feruginea! Extrafloral nectaries (7.5% of tropical plants)! Beltian bodies 17 4) Recruitment! Synomones! Cotesia wasp finds host using volatiles of injured plant! Experiment: Attraction twds Insect damage (yes) Mechanical damage (no) Mech. + saliva (yes) 18 6

7 4) Recruitment! Fungal endosymbionts! Produce herbivore resistance by producing toxins ergot 19 5) Tolerance! Avoid decrease in fitness from damage by tolerating it Grow back without suffering consequences! May be a trade-off between resistance and tolerance Fig Insect Adaptations to Plant Defense! 1) Behaviour Avoid defense by not consuming it Deactivate defense: Milkweed beetle trenching and cutting on Asclepiad 21 7

8 Insect Adaptations to Plant Defense! 2) Detoxification Phase I: conversion into something less toxic Phase II: conjugation with sugars, amino acids! to make more hydrosoluble! Phase I: Cytochrome P450 Multi function oxidase Flexibility due to receptivity change with single aa substitution! Phase II: tranferases Increase polarity 22 Insect Adaptations to Plant Defense! 3) Conjugation Joins toxin with another molecule to render it harmless Tannins: bind to proteins and prevent their digestion Saliva contains proline which binds to dietary tannins, preventing interference 23 Insect Adaptations to Plant Defense! 4) Target Site Insensitivity Some toxins specific to particular receptors Pyrethrins target sodium-ion channels providing knockdown Mutation in kdr gene leads to resistance in diamond-backed moth, house fly, german cockroach 24 8

9 Insect Adaptations to Plant Defense! 5) Excretion Getting rid of toxins without altering them at all Tobacco Hornworm Manduca sexta Combination of absorption via Malpighian tubules and detox by P Insect Effects on Plants! 1) Synergy from multiple attacks Prior attack may influence future ones May alter scenario physically or chemically Solidago branching and aphids Grapevine and mites 26 Insect Effects on Plants! 2) Vectors of disease Some insects have symbiosis with pathogens Bark beetle Ips pini carries fungus Ophiostoma ips in pits on elytra! Insects inadvertently vector over 380 plant viruses 27 9

10 Insect Effects on Plants! 3) Disrupting relationships with mutualists! Pollination: Direct effects on flowers or floral display Indirect effects on plant physiology (reduced floral size, pollen viability, increased 2 cmpds in flowers)! Micorrhizal symbionts Above ground herbivory leads to decreased root colonization by fungi and bacteria 28 Evolution of Plant Defense! 1) Optimal Defense Theory 2 cmpds are costly to produce and variable within and among individual plants Production governed by susceptibility to attack and fitness value of attacked plant part Plants can modify visual, chemical or lifespan apparency! Resource Availability Hyp: when resources are abundant, regrowth is cheaper than protection 29 Evolution of Plant Defense! 2) Inducible defense Regardless of constitutive defense, many plants can increase toxins when attacked Protease inhibitors in tomato/ potato increase by 3-fold when attacked (systemic)! Can carry over Birch leaves protected year following attack 30 10

11 Evolution of Resistance in Insects! 1) Sequestration and adoption of toxicity Arctiid moth Utetheisa ornatrix and pyrolizidine alkaloids 31 Plant Phytochemistry! Quite conservative phylogenetically Used by taxonomists Used by insects to distinguish hosts Oviposition or feeding stimulants Cucumber beetle and cucurbitacin B Fig Coevolution as Diversifying Force Erlich and Raven (1964)! Reciprocal adaptations require: Plant and herbivore can influence each other s fitness Traits are genetically variable Each taxa must respond to the selection from the other Should lead to parallel phylogenesis 33 11

12 Parallel Phylogenesis Fig Coevolution Hypothesis Fig Other Hypothesis! Hybrid Bridge Hypothesis Hybrid zones provide opportunities to switch between plants Mixed attributes allow for gradual selection to new plant resource! 2 assumptions: Defense traits are additive (some dom/rec) Insects are limited in host-jumping by phenotype of defense 36 12

13 Community Level Effects! Most evidence comes from introduction of exotics Cactus Opuntia stricta introduced to Australia in 1839 from SAm and grew out of control Moth Cactoblastis cactorum introduced in 1925 and managed to control! In Caribbean, introduced moth jumped to Florida and threatened endangered O. spinosissima 37 Janzen-Connell Hypothesis Fig Ecosystem Nutrient Cycling! Herbivory affects timing and availability of nutrients Leaf litter usually ~80-90% of nutrient content of soil (end of season) After infestation, leaves account for ~30% (the rest is frass) N and P leach from frass and are lost from ecosystem 39 13