Rice Insect Pests and their Arthropod Parasites and Predators
Overview
Order Araneae (spiders)
The spiders are separated from the insect orders by their eight legs and two body regions; cephalothorax (fused head and thorax) and unsegmented abdomen. There is an abundance of spider species in the global rice ecosystems where they serve as a major factor in the biological control of rice insect pests. Adult spiders are commonly found near the base of the rice plant. The hunting spiders (Lycosa) are of major importance and considered the most important predators of rice fields. Hunting spiders actively hunt their prey, especially nymphal and adult leaf-and planthoppers. A single wolf spider, L. pseudoannulata can easily consume between 7 to 45 brown planthoppers, Nilaparvata lugens, per day. The male hunting spider is distinguished by its large pedipalps (the second pair of appendages in the cephalothorax used to hold down and crush prey). The web spinning spiders, Argiope and Tetragnatha, although present in rice, are not considered important predators of rice pests. Key characters used in the identification of spider species are the fangs of the chelicerae, pedipalps, carapace, caudal tubercle, claws, clypeus, eyes, spinnerets, legs and abdomen.
TopOrder Coleoptera
There are many beetle species that occur in rice ecosystems. They are primarily foliage feeders, attacking rice in the vegetative stage, but some are predacious. Some African species mechanically transmit the Rice Yellow Mottle Virus (RYMV) while feeding. The major foliage feeders are the blister beetles (Meloidae), and the leaf beetles (Chrysomelidae). The family Curculionidae contains a number of species that are important root feeding pests. The rice water weevil, Lissorhoptrus oryzophilus, is the most important invertebrate pest of rice in the USA. Populations of this species have invaded Japan (1970s), Korea and China (1980s) and Italy (2004). Various other species in this genus are reported as pests of rice in Central and South America, Cuba and Puerto Rico. Most of the Coccinellidae species are considered as beneficial insects, because many prey on herbivorous rice pests. However, some species such as the ladybird beetle Chnootriba similis, are herbivorous feeding on the foliage of upland rice in West Africa. The tiger beetles (Cicindellidae), and the ground beetles (Carabidae), are important predators of rice insect pests. The Hydrophilidae are primarily scavengers but Hydrophilus acuminatus larvae are also known to feed on aquatic prey. The Coleoptera are separated from other insect orders by their forewings which form a highly chitinous or leathery, non-folded, rigid plate called elytra. The elytra are hard and serve as covers for the hind wings; the elytra meet in a line down the middle of the back; the hind wings are large and membranous, folded beneath the elytra. They have chewing mouthparts (sometimes located at the tip of a beak or snout) and the tarsi are 2- to 5-segmented. Beetle species identification requires familiarity with antennal shapes, tarsi (formulas, shapes of segments), mouthparts (labial and maxillary palpi), ventral characters (sterna, pleura, coxae), elytra, prothorax, pronotum, and other morphological characters.
TopOrder Dermaptera
The Dermaptera (earwigs) are elongated, slender insects with a flattened and heavily sclerotized body. They are beetle-like in appearance but can easily distinguished from Coleoptera by the presence of distinct cerci, or pincers that look like forceps at the tip of the abdomen. The forceps are used in defense; to fold the soft membranous hindwings beneath the forewings and to catch and hold prey while it is consumed. Dermaptera means ‘skin wings’ referring to the leather -like texture of the short forewings. Although the earwigs are primarily scavengers, their food varies considerably, ranging from living and dead insects to decaying and living plants. Adults feed on the floral parts of the rice plant (pollen, pistils, stamens) when the glumes open causing sterility and abortion of the grain. The damage, they cause is minimal and they are considered minor pests of rice. Euborellia annulata, is used for the biocontrol of the Asian Corn Borer, a pest of corn in East Asia. Dermapterous species can be separated as based on their cerci, wings and abdomen.
TopOrder Diptera
The major dipterans attacking rice are the African rice gall midge, Orseolia oryzivora, Asian rice gall midge, Orseolia oryzae, and the stalk-eyed flies, Diopsis spp. The adult gall midge is mosquito-like in appearance and does not cause plant damage. The gall midge is primarily a pest of irrigated rice. The larva produces a distinctly characteristic plant damage symptom- a gall resembling an onion leaf, called a ‘silver shoot’, in place of a panicle. The Diopsis adults are easily identified by the characteristic eyes which are borne on stalks. The diopsid larvae damage the rice plant by boring (stem borer) in the stem which results in a dead heart (dead central shoot). Damage differs from that of the pyralid stem borers as the damage only causes dead hearts, not whiteheads. Many dipterous species are parasitoids. Tomosvaryella spp. attack the green leafhoppers Nephotettix spp.and tachinid flies are parasitic on scarabaeoid larvae, pentatomid bugs, rice skippers and the stem borers- Chilo spp.and Scirpophaga nivella. Among the many key characters used to separate the species are the antennal segments, head shape, genitalia, stigma maxillary palpi, wing venation, ptilinal suture, bristles and setae.
TopOrder Hemiptera
The Hemiptera consist of the leafhoppers, planthoppers, tree hoppers and the true bugs. It is a very large order consisting of numerous species in rice ecosystems. Most hemipterans feed on plants, using their sucking and piercing mouthparts to extract plant sap while others are predators that feed on other insects or small invertebrates. The green leafhoppers Nephotettix spp. and the brown planthopper Nilaparvata lugens are major pests of rice in Asia where they both feed on rice and transmit rice viruses. The rice delphacid Tagosodes orizicolus is a serious pest in Central American, Caribbean and South American rice-growing countries where they cause direct damage by the removal of plant and indirect damage via the transmission of the hoja blanca virus. The pentatomids (stink bugs) can cause severe damage where both the nymphs and adults feed on the liquid content (‘milk’) in the developing rice grains from anthesis (the flowering period of a plant, from the opening of the flower bud) to the grain hardening stage. Feeding causes both quantitative damage (decrease in rice yield) and qualitative damage (decrease in rice grain quality). The mirid bug, Cyrtorhinus lividipennis is the most abundant egg predator of green leafhoppers and the planthoppers in Asia. The name Hemiptera means 'half wing’, referring to the forewings of many heteropterans which are hardened near the base, but membranous at the ends. The antennae in Hemiptera typically consist of four or five segments, although they can still be quite long, and the tarsi of the legs have two or three segments. The defining feature of hemipterans is their "beak" in which the modified mandibles and maxillae form a "stylet" which is sheathed within a modified labium. Key characters used to separate the hemipteron species are the forewing, abdomen, antennae, siphunculi, cauda, tegula, vertex, ocelli, pronotum, clavus, eyes, tylus, arolia, legs, hemelytra, corium, scutellum, coloration and rostrum.
TopOrder Hymenoptera
The hymenopteran species in rice field ecosystems includes the ants (Formicidae) and wasps. The ants are phytophagous soil pests, and the wasps are parasitoids. Ants are most prevalent in upland environments where they store, below the ground, rice seeds removed from the rice field. The result is a loss in plant stand. The wasps are parasitic on insect eggs, larvae and pupae. Brown planthopper eggs are parasitized by mymarids, trichogrammatids and eulophids. Stem borer larvae and pupae are parasitized by braconids and elasmids. Key characteristics in separating the Hymenoptera from other insect orders are (1) the distinct waist referred to as the petiole, (2) two pairs of membranous (hymen, thin, see-through) wings, (3) the forewings and hindwings are held together by small hooks (hamuli), (4) the hindwings are smaller than the forewings and differ in shape and (5) hindwing venation is often much reduced. The petiole is the thin structure (narrow waist) that connects the thorax and abdomen in some hymenopteran insects, especially ants and wasps. The ants are distinguished from the wasps by the abdomen connected to thorax with a dorsal node. The petiole can consist of either one or two segments, a characteristic that separates major subfamilies of ants. The fore and hind hymenopteran wings have key characters used in the identification of species. Other characters used in the separation of hymenopterous species include the antennae, gaster, tarsi, hypopygium, propodeum, clypeus, areola, hind coxae, ovipositor, eyes, abdomen, scutellum, ocellar setae, aedeagus, funicular segment and body color.
TopOrder Isoptera
The African subterranean termites, Microtermes and Macrotermes species, are common pests of rice throughout Africa where they cause plant damage during dry periods. Termites are primarily upland rice feeders but can occur in light textured soils in rainfed lowland areas. They cannot survive in flooded fields. Microtermes feed on the plant’s root system, whereas Macrotermes cut seedlings at the base of the stem just below the soil surface. Macrotermes build mounds while Microtermes, which are strictly subterranean, do not build mounds. The Formosan subterranean termite Coptotermes formosanus is probably endemic to Taiwan and southern China but now occurs in Japan, South Africa and USA (Hawaii, Texas, Louisiana, South Carolina and Florida). Because of its need for cellulose, C. formosanus primarily feeds on structural lumber but it also attacks many species of living plants, including rice. The Isoptera are separated from other orders by their forewing which is different from the hindwing in size, shape and venation and extended well beyond the tip of the abdomen at rest; absence of cross veins; wing scales (= stumps) present; fontanelle usually present; antenna moniliform; tarsi 4-segmented and with characteristic node forms. Key characters used to identify the Coptotermes, Macrotermes and Microtermes species are the anterior wing scales, wing venation, pronotum, ocelli and fontanelle.
TopOrder Lepidoptera
The major lepidopterous pests of rice consist of the skipper butterflies, noctuid and pyralid moths. Plant damage is caused by the larvae whereas the adult moths are nectar feeders. The larvae damage rice plants through the act of defoliation or boring in the stem. The stem borer larvae cause dead hearts (dead central shoots) when attack occurs in the vegetative stage and whiteheads (dead panicles) when attack occurs in the reproductive stage. The rice skippers (Hesperiidae) are pests at the rice reproductive stage. Skippers are so named because of their fast and erratic flight behavior-they skip from plant to plant. Skipper larvae feed on leaves at night. The noctuid (Noctuidae) moths are heavy bodied with a thick tuft of hairs on the thoracic dorsum. The noctuids, Sesamia spp., are the most polyphagous among the rice stem borers. Of the Sesamia species that feed on rice in West Africa, the African pink borer, Sesamia calamistis, is the most common. The Asiatic pink stem borer, Sesamia inferens is widely distributed in Asia, North America and Oceania and has a wide range of host plants in addition to rice. The noctuid, Spodoptera frujiperda (Fall Armyworm) is an emerging invasive pest that is attacking many crops throughout the world. There are several pyralid (Pyralidae) species that are common in rice fields. The pyralid moths are moderately small, delicate and slender built. The pyralid group consists of both defoliating species and stem boring species. The pyralid, Elasmopalpus lignosellus, the lesser cornstalk borer, is a polyphagous pest that attacks several crops and has many weed hosts. It occurs in upland rice in Mexico, Central America and South America. Larvae spend time in soil and is most common in sandy soil and upland rice. Key characteristics that separate the Lepidoptera from other orders are the membranous wings with largely overlapping scales, mouthparts in a coiled proboscis and the many segmented antennae. Key characteristics for separating Lepidopterous species are the antennae, eyes, wings, palpi, frenulum, face, aedeagus, haustellum, genitalia and juxta.
TopOrder Neuroptera
Neuroptera are rather fragile insects with two pairs of many-veined wings of about the same size with marginal branching. Antennae are long and threadlike and prominently visible. Most Neuroptera hold their wings roof-like over the abdomen. The term Neuroptera refers to “nerve-wings” because it alludes to the extensive branching found in the wing veins. The green lacewing, Chrysopa basalis is the only neuropteran included in the key. It has a green body and wings and gold or bronze compound eyes. Lacewings and their immature forms, known as aphid lions, are considered beneficial as they are predatory, feeding on other insects, especially aphids.
TopOrder Odonata
The Odonata order consists of the dragon- and damselflies. They are beneficial insects causing no plant damage. The larvae are aquatic while the adults are predacious feeding on a wide variety of adult rice insect pests including lepidopterous moths, adult leafhoppers and planthoppers. Coenagrionid damselfly adults prey on hoppers resting on rice foliage, while libellulid dragonfly adults capture hoppers in flight. The adults are easily recognized by their two pairs of narrow, transparent membranous wings with numerous veins, large compound eyes, short, bristle-like antennae, sloping thorax, and long, usually slender body; the abdomen is almost always longer than any of the wings. In damselflies the front and hind wings are stalked (narrow at the base) whereas in dragonflies the hind wings have an enlarged anal region (broader toward the base than front wing). When at rest, damselfly wings are held closed over the abdomen, while dragonfly wings are held open, horizontally or downwards. Male Odonata have complex genitalia, different from those found in other insects. These include grasping cerci for holding the female and a secondary set of copulatory organs on the abdomen in which the sperm are held after being produced by the primary genitals. The wing venation serves as a key character in the identification of Odonata species. The pterostigma, a prominent cell, usually opaque and colored, near the tip of each wing, is characteristic of the Odonata and is particularly noticeable in dragonflies. Other characters used to separate species are the body size, prothoracic lobe, apical ventral spine, anal appendages and anal bridge.
TopOrder Orthoptera
The Orthoptera, grasshoppers and locusts, are foliage feeders (defoliators) in both the nymphal and adult stages. The mole crickets, Gryllotalpa africana and G. orientalis, are polyphagous insects. They occur in all rice environments but are most prevalent on young seedlings in non-flooded upland rice when the fields are damp. Both the nymphal and adult stages attach to rice stems below the ground and close to the roots where they cut the tillers. Damage is evident a few days later when tillers begin to dry. The entire seedling dies if attack is severe. There are many grasshopper species attacking rice in addition to other crops. With their chewing mouthparts, they feed on rice foliage at all stages of crop growth. Grasshoppers are localized in dry regions and rarely cause significant damage other than along field margins where they chew angular holes in leaves causing damage like that caused by leaffolders and army worms. There are numerous grasshopper species belonging to the short-horned (Acrididae and Pyrgomorphidae) and long-horned (Tettigoniidae) families that feed on rice. The Gryllidae family consists of both phytophagous and predaceous species. Key characters for the identification of the orthopteran species include the antennae, cerci, tarsi, head, foreleg, fastigium, elytron, femur, eyes, supra-anal plate, tegmina and pronotum.
TopOrder Strepsiptera
The Strepsiptera (‘twisted wing’) families Elenchidae and Halictophagidae are common in rice ecosystems. They are distinguished by having pseudohalteres in place of the forewing, the large hind wing with few veins, and the flabellate antennal segments (fan shaped with long thin processes lying flat on each other like the folds of a fan). Adult male Strepsiptera have eyes unlike those of any other insect. They consist of hundreds to thousands of ommatidia, that each produce a pixel of the entire image. Strepsipterons are endoparasites feeding within insect hosts e.g., leafhoppers and planthoppers where they cause sterility of the insect hosts. In field studies conducted in farmers’ fields in Côte d’Ivoire, Halictophagus parasitism of the white leafhopper Cofana spectra averaged 21%.
TopOrder Thysanoptera
Thrips are small insects barely visible to the naked eye. They are present in all rice environments, but they are most abundant during periods of dry weather. Heavy rainfall washes them off the plants. Thrips are sporadic pests of rice infesting mostly young plants. Larvae and adults, both of which feed on the leaves, have rasping mouthparts. They have only one mandible which is used to puncture leaf tissue. The maxillae and mouth cone, which form a tube, are used to suck leaf sap. Damaged leaves which have silvery streaks curl inward longitudinally from the edges forming a protective chamber for the nymphs and adults. Leaf tips dry up, particularly when the crop is under drought stress. The extensive removal of green tissue causes only a translucent epidermis to remain. Thrips outbreaks are normally small in scale and plants can recover from much of the damage. Adult thrips are minute elongate insects, about 1 mm long. Wings are short and narrow with reduced venation and a characteristic wide marginal fringe. The antennae are 6- to 9- segmented. Wing venation, antennae, and setae are key characters used to separate the species. The species are divided into two suborders, Terebrantia and Tubulifera which differ in the presence or absence of longitudinal veins in the forewings.
Top