Sap sucking

The feeding activities of insects that chew or mine leaves and shoots cause obviious damage. In contrast, structural damage caused by sap-sucking insets often is incospicuous. As the withdrawal of cell contest from plant tissues usually leave the cell walls intact. Damage to the plant may be diflicult to quantify even though the sap sucker drains plant resources  (by removing phloem or xylem contents ). Causing loss of condition such as retarded root growth. Fewer leaves. Or less overail biomass accumulation compared with unaffected plants. These effects may be detectable with conlidence only by controlled experiments in which be growth of infested and uninfected plants is compared. Certain sap-sucking insects do cause conspicuous tissue necrosis either by transmitting diseases. Especially viral ones. Or by injecting toxic saliva, whereas others induce obvious tissue distortion or growth abnormalities called galls.

Most sap-sucking insects belong to the hemitera. All hemitera heve long. Therad – like mouthparts consisting of appressed mandibular and muxillary stylets forming a bundle lying in a groove in the labium (taxobox 20). The maxillary stylests contain a salivary canal that directs saliva into the plant, and a food canal through which plant juice or sap is sucked up into the insect’s gut. Only the stylets enter the tissuesof the host plant  (fig. 11.4a). they may penetrate super- ficially into a leaf or deeply into a plant stem or leaf midrib. Following either an intracellular or intercellular path. Depending on species. The feeding site reached by the stylet tips may be in the parenchyma (e.g. some immature scale insects, many heterptera). The phloem (c.g most aphids, mealybugs, soft scales, psyllids, and leafhoppers). Or the xylem (e.g. spittle bugs and cicades). In addition to a hydrolyzing type of saliva. Many species produce a solidifying saliva that forms a sheath around the stylets as they enter and penetratc the plant tissue. This sheath can be stained in tissue sections and allows the feeding tracks to be traced to the feeding site (fig. 11.4b.c). the two feeding strategies of hemipterans. Stylets- shcath and maccrate-and-flush feeding. Are described in section 3.6.2, and the gut specializations of hemipterans for dealing with a watery diet are discussed in box 3.3 many species of plant feeding hemiptera are considered serious agricultural and horticultural pest. Loss of sap leads to wilting. Distortion. Or stunting of shoots. Movement of the insect between host plants can lead to the efficient transmission of plant viruses and other diseases. Especially by aphids and whitefiles. The sugary excreta ( honeydew) of phloem- feeding hemiptera. Particularly coccoids, is used by black sooty molds. Which soil leaves and fruits and can imfair photosynthesis.

Thrips (thysanoptera) that feed by sucking plant juice penetrate the tissues using their stylets  (fig2.15) to pierce the epidermis and then rupture individual cells below. Damaged areas discolor and the laef. Bud flower. Or shoot may wither and die. Plant damage typically  is concentrated on rapidly growing tissues. So that flowering and leaf flushing may be seriously disrupted. Some thrips inject toxic saliva during feeding or transmit viruses, such as the tospovirus  (bunyaviridae) carried by the pestiferous western flower thrips. Frank liniella accdentalis. A few hundred thrips species have been recorded attacking cultivated plants. But only 10 species transmit tospoviruses.

Outside the himaptera and thysanoptera, the sap-sucking habit is rare extant insect. Many fossil species. Howower, had a rostrum with piercing-and sucking mountparts. Palaeodiciyoteroids (fig.8.2), for example, probably fed imbibing juices from plant organs.

Gall induction

Insect-induced plant galls result from a very specialized type of insect-palnt interaction in which the morphology of plant parts is alterad, often subtantially and characteristically, by the influence of the insect. Generally, galls are defined as pathologically developed cells, tissues, or organs of plant that have arisen by hypertophy (increase in cell size) and or hyperplasia (increase in cell number) as a result of stimulantion from forcign organims. Some galls are induced by viruses.

Bacteria, fungi, nematodes, and mites, but insects cause many more. The study of plant is called cecidolgy, galls-causing animals (insects, mites, and nemotodes) are cecidozoa, and galls induced by cecidozoa are referred to as zoocecidia. Cecidogenic insects accountfor about 2% of all described insect species, with perhaps 13.000 spicies known. Although galling is a worldwide phenomenon acroos most plant groups, global survey shows an eco-geographical patteren with gall incidence more frequent in vegetation with a sclerophyllous habit, or at least living on plants in wet dry seasonal  environments.

            On a world basis, the principal cecidozoa  in terms of number of species are representatives of just three orders of insects the himaptera. Diptera, and hymenoptera. In addition, about 300 species of mostly tropical thysanoptera (thrips) are associated with galls, although not necessarily as inducers, and some spicies of coleoptera (mostly weevils) and microlepidoptera  (small month) induce galls. Most hemipteran galls are elicited by sternorrhyncha. In particular aphids. Coccoids, and psyllids: their galls are struturally devirse and those of gall-inducing eriococcids (coccoidera: eriococciodae) often exhibit spectacular sexual dimor-phism, with galls of female insects much larger and more complex than those of their conspecific males (fig. 11.5a.b). worldwide there are several hundred gall-inducing coccoid species in about 10 families, about 350 gall-forming psylloidea. Mostly in two families, and perhaps 700 gall-inducing aphid species distributed among the three families, phylloxcridae (taxobox 20), adelgidae, and aphididae.

            The diptera contains the highest number of gall-inducing species, perhaps thousands, but the probable number is uncertain because many dipteran gall inducers are poorly known taxonomically. Most cecidogenic flies belong to one family of at least 4500 species, the cecidomyiidae (gall midges), and induce simple or complex gall on leaves, stems, flowers, buds, and even roots. The other fly family that includes some important cecidogenic species is the tephritidae, in which gall inducers mostly affect plant buds, often of the asteraccae. Galling species of both cecidomyiids and tephritids are of actual or potential use for biological control of some weeds. Three superfamilies of wasps contain large numbers of gall-inducing species: cynipoidae contains the gall wasp (cynipidae, at leats 1300 species). Which are among the best-known gall insects in europe and North America, where hundreds of species from often extremely complex galls, especially on oaks and roses (fig.11.5c.d): tenthredinoidae has a number of gall-forming sawllies, such as pontana species (tethredinidae) (fig.11.5g): and chalcidoidae includes several families og gall inducers, especially species in the agaonidae (wasps: box 11.4. below). Eurytomidae  and pteromalidae.

            There is enormous diversity in the patterns of development, shape, and celular complexity of insect galle (fig.11.5). they range from relatively undifferentiated masses of cells (“indeterminate” galls) to highly organized structures with distinct tissue layers (“determinate” galls). Determinate galls usually have a shape that is specific to each insect species. Cynipids, cecidomyiids, and eriococcids form some of the most histologically complex and specialized galls, these galls have distinct tissue layers or types that may bear little resemblance to the plant part from which they are derived. Among the determinate galls, different shapes correlate with mode of gall formation, which is related to the initial position and feeding method of the insect (as discussed below). Some common types of galls are:

·         covering galls, in which the insect becomes enclosed withim the gall, either with an opening (ostiole) to the exterior, as in coccoid gall (fig. 11.5a.b). or without any ostiole. As in cynipid galls (fig. 11.5c):

·         filz galls, which are characterized by their hairy epidermal outgrowths (fig.11.5d).

·         roll and fold galls, in which differential growth provoked by insect feeding results in rolled or twisted leaves, shoots, or stems, which are often swollen, as in many aphid galls (fig. 11.5e):

·         pouch galls, which develop as a bulge of the leat blade, forming an invaginated pouch on one side and a promincnt bulge on the other, as in many psyllid galls (fig. 11.5f):

·         mark galls, in which the insect egg is deposited inside stems or leaves so that the larva is completely enclosed throughout its development, as in sawfly galls (fig. 11.5g).

·         pit galls, in which a slight depression, sometimes surrounded by a swelling, is formed where the insect feeds:

·         bad and rosette galls, which vary in complexity and cause enlargement of the bud or sometimes multiplication and miniaturization of new leaves. Forminga pinc-conc-like gall.

            Gall inducation may involve two separate proccesses: (a) initiation and (b) subsequent growth and maintenance  of structure , usually, galls cab be stimulated to develop only from actively growing plant tissue. Therefore galls are initiated on young leaves. Flower buds, stems, and roots, and rerelyon mature plant parts. Some complex galls develop only from undifferentiated meristematic tissue.which becomes molded into a distinctive gall by the activities of the insect. Development and growth of insect- induced galls (including, if present, the nutritive cells upon which some insect feed) depend upon continued stimulation of the plant. Control most aspects of gall formation. Largely via their feeding activities.

            The mode of feeding differs in different taxa as a consequence of fundamental differences in moughpart  structure. The larvea of gall-inducing beetles, moths. And wasp have vestigial mouth parts and largely absorb nourishment by suction. Thus, these different insect mechannically damega and deliver chemicals (or perhaps genetic material, see below) to the plant cells in a variety of ways.

            Little is known about what stimulates gall indution and growth. Wounding  and plant hormones (such as cytokinins) appear important in indeterminate galls, but the stimuli are probably more complex for determinate galls. Oral secretions have been implicated in different insect-plant interaction taht result in determinate galls. The best-studied compounds are the salivary secretions of hemiptera. Salivary substances, including amino acids. Auxins (and other plant growth regulators), phenolic compounds, and phenol oxidases, in various concentrasions may have a role either in gall initiation and growth or in overcoming the defensive necrotic reactions of the plant. Plant hormones. Such as auxins and cytikinins. Must be involved in cecidogenesis but it is equivocal whether these homones are produced by the insect. By the plant as a directed response to the insect. Or are incidental to gall induction. In certain complex galls, such as those of criococoids and cynipids, it is concevable that the development of the plant cells is redirected by semiautonomous genetic entities (viruses, plasmids, or transposos) transferred from the insect to the plant. Thus, the initiations of such gall may involve  the insect acting as a DNA or RNA donor, as in some wasps that parasitize insect hosts (box 13.1). unfortunately, in comparison witht anatomical ang physiological studies of galls genetic ivestigations are in their infancy.

            The gall-inducing habit may have evolved either from plant mining and boring (especially likely for lepidoptere, hymenopter, and certain diptera) Or from sedentary surface feeding (as is likely for hemiptera, thysanoptera, and cecidomyiid  diptera). It is believed to the insect, rather than defencive response of the plant to the insect attack. All gall insect derive their food from the tissues of the gall and also same  shelter or protection from natural enemies and adverse conditions of temperature or moisture. The relative importance of these enviromental fasctors to the origin of the galling habit is difficult to ascertain because current advantages of gall living may differ from those gained in the early stages of gall evolution. Clearly, most galls are “sinks” for plan t assimilates: the nutritive cells that line the cavity of wasp and fly galls contain higher concentratioan sof sugars, protein, and lipids than ungalled plant cells. Thus. One advantage of feeding on gall rather than normal plant tissue is the availability of high- quality food. Moreover. For sedentary surface feeders, such as aphids, psyllids, and coccoid, gall furnish a more protected microenvironment than the normal plant surface. Some cecidozoa may “escape” from certain parasitoids and predators that are to unable penetrate galls, particularly gall with thick woody walls.

            Other natural enemics, however, specialize in feeding o gall- living insect or their galls and sometimes it is difficult to determine which insect were the original inhabitants. Some galls are remarkable for the association of an extremely complex community of species other than the gall causer belonging to diverse insect groups. These other species may either parasitoids of the gall inducer (i.e. parasites that cause the eventual death of their host chapter 13. Or inquilines  (‘guests’ of the gall inducer ) that obtain their nourishment from ussues of the gall. In some cases. Gall inquilines cause the original inhabitant  to die through abnormal growth of the gall: this may obliterate the cavity in which the gall inducer lives or preventemergence from the gall. If two species are obtained from a single gall or a single type of gall, one of these insect must be a parasitoid. An inqiline, or both. There are even cases of hyperparasitism. In which the parasitoids them selves are subject to parasitization (section 13.3.1)

Seed predation

 Plant seeds usually contain higher levels of nutrients than other tissues. Providing for the growth of the seedling. Specialist seed- eating insects use this resource. Notable seed-eating insects are many beetles (below). Harvester ants (especially species of messor monomorium and pheidole), which store seeds in under ground granaries, bugs(many corcidae, lygacidae, pentatomidae, pyrrhocoridae adn scutelleridae) that suck out the contents of developing or mature seeds, and a few moths (such as some Gelechiidae and Oecophoridae).

Hasverter ants are ecologycally significant seed predator. These are the dominant ants in terms of biomass and/or colony numbers in desert and dry grasslands in many parts of the world. Ussualy the species are highly polimorphic with the larger individuals possessing powerful mandibles capable of cracking open seeds. Seed fragments are fed to larvae but probably many harvested seeds ascape destruction either by being abandoned instores or by germinating quickly within the ant nests. Thus, seed harvesting by ants, which could be viewed as exclusively detrimental. Actually may carry some benelits to the plant through dispersal and provision of local nutrients to the seedling.

            An array  of beetles (especially curculionidae and bruchine Chrysomelidae) develop entirely within individual seeds or consume several seeds within one fruit. Some bruchine seed beetles. Particularly those attacking leguminous food plants such as peas and beans are serious pests. Species that eat dried seeds  are preadapted to be pests of stored products such as pulses and grains. Adultbeetles typically oviposit onto the developing ovary or the seeds or fruits, and some larvae the mine through the fruit and /or seed wall or coat. The larvae develop and pupate inside swwds. Thus destroying them. Successful development usually occurs only in the final stages of maturity of seeds. Thus, there appears to be a ‘window of opportunity’ for the larvae : a mature  seed may have an impenetrable seed coat but if young seeds are attacked the plant can abort the infected seed or even the whole fruit or pod if little investment has been made in it. Aborted seeds and those shed to the ground (whether mature or not) generally are less attractive to seed beetles than those retained on the plant but evidently stored product  pests have no difficulty in developing within cast (i.e. harvested and stored) seeds. The larvae of the granary weelvil. Sitophilus granarius (taxobox 22) and rice weevil   Sitophilus oryzae  develop inside dry grains of corn wheat, rice, and other plants.

            Plant defence against seed predation includes the provision of protective seed coatings or toxic chemicals (allelochemicals) or  both. Another strategy is the synchronous production by a single plant species of an abundance of seeds. Often separated by long intervals of time. Seed predator either cannot synchronize then life cycle to the cycle of glat and scarcity or are overwhelmed and unable to lind and consume the total seed production.

Insects as biological control agents for weeds

Weeds are simply plants that are growing where they are not wanted. Some weed species are of little economic or ecological consequence. Where as the the presence of others results in significant  losses to agriculture or causes detrimental effects in natural ecosystems. Most plant are weedsonly in areas outside their native distribution, where suitable climatic and edaphic conditions, usually in the absence of natural enemies. Favor their growth and survival. Sometimes exotic plants that have become weeds can be controlled by introducing host-specific phytophagous insects from the area or origin of the weed, this is called classical biological control of weeds and it is analogous to the classical biological control of insects pests ( as explained in detail in Section 16.5). another form of biological control. Called augmentasi (section 15.5) involves increasing  the natural level of insect enemies of a weed and thus requeres mass rearing of insect for inundative release. This method of controlling weeds  is unlikely to be cost-effective for most isect plant system. The tissue damage caused by introduced or augmented insect enemies of weeds may limit or reduce vegetative growth (as shown for the weed discussed in box 11.3). prevent or reduce reproduction, or make the weed less competitive than other plants in the environment.

            A classical biological control program involves a sequence of steps that include biological as well as sociopolitical considerations. Each program is initiated with a review of available data (including taxonomic  and distributional information) on the weed, its plant relatives, and any known natural enemies. This forms the basis for assessment of the nuisance status of the target weed and a strategy for collecting. Rearing, and testing the utilitiy of potential insect enemies. Regulatory authorities must then approve the proposal to attempt control of the weed. Next foreign exploration and local surveys must determine the potential control agents attacking the weed in both its native and introduced ranges. The weeds ecology, especially in relation to its natural enemies, must be studied in its native range. The host-specificity of potential control agents must be tested, either inside or outside the country of introducation and in the former case always in quarantine. The result of these test will determine whether the regulatory authorities approve the infortation of the agents for subsequent release or only for further lesting, or refuse approval. If approved and the agent is imforted, there is a period of rearing in quanrantine to evminate any imported diseases or parasitoids. Prior to mass rearing in preparation for lield release. Release is dependent on the quarantine procedures being approved by the regulatory authorities. After release the establishment, spread and effect of the insect on the weed must be monitored, if weed control is attained at the initial release sitc (s). The spread of the insects is assisted by manual distribution to other sitcs.

            There have been some outstandingly successful cases of deliberately introduced insect controling invasive weeds. A century ago St. Jhon’s Wort, hyperium perforatum  (clusiaceae). Was reported first in northern california near the Klamath river . in its native range in europe this non-invasive plant has provided herbal remedics for centuries, but is harmful to sheep. Cattle, and horses. In contrast, in north amarica. What became known as the klamath weed spread rapidly