1. Plant Defense 1- Physical Defense 6 th Lecture Plant-insect Interactions

2. Plant Defense It seems that plants are unmovable and silent. Thus, it thoughts that no quick responses have been expected, when plants are under attack.

3. Introduction Plant defense against herbivory or host-plant resistance (HPR) includes a range of adaptations evolved by plants that improve their survival and reproduction by reducing the impact of herbivores.

4. Introduction Plant defenses can be classified generally as induced or constitutive. Constitutive defenses are always present in the plant species, while induced defenses are synthesized or mobilized to the site where a plant is injured

5. Introduction Most external mechanical defenses and large quantitative defenses are constitutive Induced defenses include secondary metabolic products, as well as morphological and physiological changes.

6. Quantitative and qualitative Quantitative defenses are dose dependent compounds which are considered to be costly to produce. Feeding deterrents such as tannins, terpenes, and flavanoids are quantitative defenses. These defenses are termed "immobile" defenses in the RAH vernacular. Conversely, qualitative defenses are the toxins which are lethal in small concentrations and are thus considered to be less costly to produce than quantitative defenses. These types of defenses are known as "mobile" defenses in the RAH. RAH = The resource availability hypothesis HM what is RAH

7. Facts Plant defense responses to herbivore attack are rapid and highly dynamic.

8. Introduction Plant-insect interaction is a dynamic system, subjected to continual variation and change. In order to reduce insect attack, plants developed different defense mechanisms including chemical and physical barriers

10. Physical Defense Plant Structure

11. Morphological or physical resistances Plant structures that interfere physically with the insect's locomotive, feeding, or reproductive functions. These functions may involve plant color and shape or more specialized defensive adaptations such as trichomes, tough tissues, surface waxes, or cell silication

12. 1- Trichomes Trichomes are cellular, hair-like out- grwoths of the plant epidermis, which may occur on leaves, shoots, or roots. –Water conservation –Probably the most important morphological defense against insects attack Insects vary in their response to trichomes Trichomes may interfere with insect oviposition, attachment of the insect to plant, feeding, or ingestion

14. Trichomes The mechanical effects of trichomes depends on four main characteristics:- –Density –Erectness –Length –Shape

15. Trichomes Some trichomes possess glands that exude secondary plant products Physical and chemical defense in one structure

16. Trichomes In other cases, the exudates is sticky material that acts physically to glue the insect's legs or other body parts together and thus reduce locomotion.

17. Trichomes Insect of piercing-sucking mouth parts generally unable to feed on plants that are pubescent Xylem and phloem feeders Mesophyll feeders Short trichomes an effective barrier to first group (Xylem and phloem feeders)

18. Trichomes Potato leafhopper Trichomes on soybean Proboscis length 0.2-0.4 mm 1 mm in length at density of 8 trichomes per mm Trichomes barrier to feeding Conclusion

19. Trichomes Chewing mouth-parts Cereal leaf beetle –Egg viability, larval growth and survival were reduced on varieties of wheat with having densely pubescent leaves. –BUT! –Table 3.2

21. Trichomes Trichomes may slow the searching rate of predators and parasitoids, or make the herbivore inaccessible to the point where enemies become ineffective. Example –Whitefly on cucumber plant and Encarsia formosa Fig 3.6

23. Trichomes Some insects species have been able to counter the problem that may pose. The aphid Myzocallis schreiberi has a pair of claws and a pair of flexible empodia that help it to get a good grip on the short woolly trichomes

24. Trichomes Trichomes density may vary not only with abiotic growth conditions, but also with plant damage caused by herbivory. Example After larvae of pieris rapae had consumed parts of young black mustard, trichomes densities on some newly expanded leaves increased.

25. 2- Epicuticular waxes The cuticle of most plants are covered by a thin layer of wax Functions of wax layers –Maintaining the water balance of plants –Reduce attacks by some pathogens and insects fig 3.3 fig 3.3

27. Wax layers Brassica oleracea plants are more resistances to attack by the cabbage flea beetle on normal plant than on glossy-leaf plant. Wax layers cause leaf slipperiness. As results the survival rate decreased due to starvation. In contrast to, the cabbage aphid and the whitefly developed a large colonies on the waxy leaves of broccoli plants. In contrast to, the cabbage aphid and the whitefly developed a large colonies on the waxy leaves of broccoli plants.

28. As indirect effect upon herbivores, wax may impair the adhesion, mobility, and effectiveness of predatory insects in an increasing of herbivore populations. Larvae of the lacewing reduced population of its prey on glossy than normal waxbloom cabbage plant Fig 3.4 & table 3.1

31. 3- Leaf toughness Plant cell walls strengthened by cellulose, ligin, … make a plant resistant to mechanical injury, tearing actions by mandibles, penetration of piercing-sucking mouthparts, and oviposition. Leaf toughness acts as an effective factor reducing herbivory.

32. Leaf toughness Insect species vary in strength of mandibles. Plant leaves vary in toughness. Grass are three times tougher than herbs. Table 3.3 There are also differences in leaf toughness between different climatic zone. Thus, leave toughness of tropical forest shows 3-fold greater value than those in temperate zones.

33. Leaf toughness Differences in leaf toughness affect insect feeding and growth. Fig. 3.7

35. Leaf toughness The size of leaf fragments swallowed by chewing insect species varies with size (instar) of the insect and hardness of the food. Thus, the alimentary tract of saturniid caterpillars feeding on tough leaves contain leaf particles that are large and very regular in size, whereas, sphingid larvae generally feeding on soft leaves of herbaceous host bite off small leaf particles independent of caterpillar size. Fig 3.8

37. Leaf toughness Saturniid

38. Leaf toughness Sphingids

39. As most insect digest cell wall only to very limits degree. The frass of lepidopteran (Paratrytone melane) contains leaf pieces with 76-86% uncrushed cells.

40. Leaf toughness Within certain limits an insect may adapt its head morphometric to the toughness of its food. The larvae of Pseudaletia unipunctata, in which g the head is twice as large when fed on hard grass than on soft artifcial food.

41. Leaf toughness Pseudaletia unipunctata

42. Leaf toughness Beside cellulose, silicon at high concentrations, such as in Poaceae and Palmae, enrich the toughness of the plants Fig 3.10 A and B

44. 4- Structure involved in mutualistic relations Domatia and plant extrafloral nectaries Domatia and extrafloral nectaries are plant structures that provide shelter and food to predaceous arthropods and thus affect herbivorous insects only indirectly. Domatia occur in plant species belonging to over 90 families

45. Domatia

46. Extrafloral nectaries Plants can provide carnivores with extrafloral nectar (Fig. 10.8). Its production can be induced by herbivory or mechanical wounding.