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Plantwise Technical Factsheet

bladderwort (Utricularia aurea)

Host plants / species affected
Oryza sativa (rice)
Description
U. aurea is a free-floating aquatic plant 12-50 cm long. The stem is green, minutely pubescent and functions as a rhizoid, with many air ducts. The stem dies slowly at the posterior end, while growing at the anterior. Young leaves form a pale-green tuft at the apex; adult leaves are 1-6 by 0.67-2 cm, patent, and occurring in groups of 2-5, alternating at one side of the stem and finely divided into filiform segments. Each side branch near the base of the stem has 1-3 bladder traps which are 3-4 mm long on a short, thin stalk. The bladders are slightly compressed, with the ventral side flat and the dorsal side convex; initially reddish and later bluish-black.

The bladders have a ventral opening which closes with a hinged valve, the free end of which rests against a thickened lip. The valve bears stiff, erect bristles outside and glandular hairs inside, the latter excreting water so that the water pressure becomes lower inside than outside. If an object touches the sensitive bristles of the valve, it opens inwards and water rushes in, carrying the prey with it. The valve closes again in some 30 minutes and water is excreted again to reset the trap. Glandular papillae inside the bladder excrete proteolytic enzymes.

The flowers are held in erect, peduncled (2-8)-flowered racemes in the axil of a bract. The peduncle is filiform, 3-12 cm long; the bracts on the peduncle are ovate-orbicular, convex, acute and 1.5-2.5 mm long. The pedicels are filiform, 5-20 mm long and ultimately nodding. The calyx is 3-lobed, with the lobes sub-equal, ovate, obtuse or acutish, green, striped, 3-4 mm long, strongly accrescent (to 9 mm), and reflexed or patent in the fruit. The corolla is 8-15 mm long, 2-lipped and yellow, with the upper lip erect, broadly ovate, entire and slightly shorter than the transversely elliptic lower lip, which has a distinct, sometimes red-veined bulge at its base. The spur is cylindrical from a conical base, parallel and slightly shorter than the lower lip; the tube is pale and very short.

Stamens (2), are attached to the corolla throat; filaments are linear, 2 mm long and thickened upwards, strongly bent. The anthers are 2-celled and confluent. The ovary is superior, one-celled, many ovuled (placentas basal), globose and glandular. The style is long, with a conical base, and the stigma is formed of two unequal lobes. The capsule is globose, nodding, crowned by the elongated style, thick, fleshy and 4-6 mm in diameter. Seeds are angular, compressed, brownish-yellow, obscurely reticulate-ribbed, 0.72-2 mm in diameter and narrowly winged on the angles.

(After Soerjani et al., 1987.)
Prevention and control
In Indonesia, control is predominantly manual (Soerjani et al., 1987). Kaul (1986) notes that U. aurea generally occurs in association with other aquatic weeds and, once the plants form a complete mesh in shallow waters, manual eradication is extremely difficult.

Most vascular aquatic weeds, including Utricularia spp., are controlled by the herbicide fluridone (Tarver, 1985).
Impact
U. aurea is found throughout South-East Asia (Pancho and Soerjani, 1978). It is reported to occur as a weed in irrigated and tidal ricefields in a variety of systems including direct-seeded (both dry and wet), transplanted and deepwater rice (IRRI, 1989).

In China it is a common weed of paddy (Wang, 1990), but in Indonesia it is said to be a weed of minor agricultural importance (Soerjani et al., 1987).

When reaching high densities in water courses, it can impede the flow of water and may render the water body useless for navigation and fishing (Pancho and Soerjani, 1978).
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