Rice: Major Insects Pests
Rice is the staple food of Sri Lankan inhabitants. About 1.8 million farm families are engaged in paddy cultivation in Sri Lanka. In both yala & Maha season, paddy cultivation is done. Because of the attacking of major insect-like Stem borers, planthoppers, paddy bugs, thrips to the rice plants, a considerable amount of yield losses can be seen per year. In here discuss those major insects pests associated with paddy cultivation & their adaptation for doing harmful effects & their behaviours.
Paddy Bug:- Leptocorisa acuta & Leptacorisa oratoria
In subtropical & tropical areas, most popular paddy bugs belong to the genus of Leptocorisa. Paddy bugs are most abundant in Asian lowland rice crops. Favourable conditions for them are 270 C – 280C & 80% – 82% RH. The population is built up with the flowering of rice crops, warm weather. But due to the heavy rain, their population is going down. They are broad-headed insects & belong to the family Alydidae. Adults are 14 – 17mm long & slender (3 – 4mm wide).
They are light yellow-green to yellow-brown colour, often similar in length & width to the pronotum & scutellum. The pronotum is the upper surface of the first plate on the thorax & the scutellum is a triangular-shaped plate on the thorax, posterior to the pronotum. Also, these bugs have globular, protruding eyes in addition to the small ocelli which are difficult to see. Ocelli mean simple eyes. The fourth antennal segment is curved & longer than the third segment. Adults emit an unpleasant odour when disturbing them. That odour is stronger than the odour emitted by true stink bugs. Adults can disperse by flying from plant to plant in the field. Eggs are oval with the tops slightly flattened. Females lay eggs in batches of 10 to 20 in rows on the upper surface of the leaf blade. Just after finishing the laying eggs are cream-yellow colour & turning into the reddish-brown approximately after one week. Nymphs are pale yellow-green colour and have long antennae. Paddy bugs are mostly found during the flowering stage of rice crops.
Nymphs & adult use their piercing & sucking type mouthparts to feed on developing rice grains. Because of that piercing & sucking mouthparts puncture the plant tissues & grains. In here they are inserting their needle-like mouthpart in to the new leaves, tender stems & developing grains. Attacked grains & plants are deformed. Excessive feeding can cause yellow spots on the leaves. Because of the above damages, the photosynthetic rate becomes low. As well as vascular system can be damaged. Puncture holes also serve as an entry point for plant pathogens. This kind of damages causes reduced yield & reduced market price.
Rice Thrips :- Stenchaetothrips biformis
Most of rice-growing countries have thrips attack in their paddy fields. There are two most common species associated with rice are Stenchaetothrips biformis & Haplothrips aculeatus belong to the family Thripidae. In Sri Lanka most common species is Stenchaetothrips biformis. They attack young rice seedlings as well as newly transplanted plants. Nymph & adult stages have damaged the plants.
Nymphs are transparent & towards the second molting, they turn to pale yellow. The adults have a slender body & dark brown in colour. Usually1-2mm long with well pronounced five to eight segmented antennae. Two types of adults exist such as winged or wingless. The winged form has two pairs of elongated narrow wings that are fringed with long hairs. Their mouth cone composes of labium, labrum & maxillae. There are three stylets derived from two maxillae & left mandible. Right mandible is absent. Both maxillary palp & labial palp are present. Larvae & adults have rasping type mouthparts. They exhibit a punch & suck feeding technique. The single mandible punches a hole in the plant surface. Then paired maxillary stylets are inserted to imbibe plant sap. Typical symptoms of thrips damage include inward rolling of the leaves along the margin. Severe damages under water stress condition. Short duration traditional varieties are resistant to thrips.
Brown Planthopper:- Nilaparvata lugens
Two species of planthoppers can be seen in rice cultivation. They are a brown plant hopper & white-backed planthopper. Mostly abundant in irrigated wetlands & rainfed environments. Occurs in areas with continuous submerged conditions in the field, high shade & humidity. Closed canopy of rice, excessive use of nitrogen, early-season insecticides using also favourable for the development of insects.
Brown planthopper adults occur in macropterous & brachypterous forms. The macropterous form is about 3.5-4.5mm in length. The body is brown, & the wings are transparent, with very conspicuous veins. Young nymphs are white but they gradually become darker in older instars. Macropterous females lay groups of 5-15 eggs into the sheaths or midrib of leaves. Macropterous females can produce about 100 eggs & brachypterous females produce 300 to more than 700 eggs. They are having piercing & sucking mouthparts & have three segmented labium with a deep groove in the anterior side, a stylet fascicle consisting of two mandibular & two maxillary stylets & uppermost small cone-shaped labium. Both nymphs & adults penetrate the tissues of their rice host plants with their piercing-sucking mouthparts in order to ingest phloem sap. Loss of nutrients & obstruction of vessels causes yellowing of leaves. Rice is most sensitive to attack at late vegetative & reproductive stages.
Yellow Stem Borer:- Scirpophaga incertulas
Stem borer most serious pest related to paddy cultivation worldwide, occur & infest plants from seedlings stage to maturity. There are fifty identifiable species in three families, as Pyrallidae, Noctuidae & Diopsidae. Yellow stem borer belongs to the family Pyrallidae. They are primarily distributed in tropics where the temperature remains above 100C & annual rainfall is more than 1000mm are favourable conditions. They are pests of deepwater rice & most abundant in aquatic environments where is continuously flooding.
The damaging stage of the insect is larvae, because this is the feeding stage, as they feed the internal tissues of the stem of the paddy. The adult moths aren’t doing any harmful effects to the plants. Female moths lay 400-600 eggs in 2-3 egg clusters. After 5-8 days of rest, the eggs hatch to the larva. They are pale white whereas, the head with a prominent pro-thoracic shield is dark brown. The full-grown larvae make a thin silken case over themselves inside the stem & thus cocoons are formed. Moths are inactive during the day & active in the evenings. The female moth is white to yellowish in colour. It has a pair of clear black spots in the middle of each forewing. The male moth is smaller & dull in colour. It has two rows of black spots at the tip of the forewing. Larvae feed upon tillers causes dead hearts or drying of the central tiller during vegetative stage & causes whiteheads at reproductive stage, where the emerging panicles are whitish & unfilled or empty. Larvae bore at the base of the plants during the vegetative stage. In older plants, they bore through the upper nodes & feed towards the base. Tiny holes on the stems & tillers can be seen.
Each insect has different adaptations to harm the rice plants & have different behaviours. All of them are responsible for yield losses at different stages. Therefore should need to prevent their damages. Following specific biological or chemical control methods for each insect can reduce the damages & increase the yield.