Nitrogen and establishment of a beetle for biological control of the floating weed Salvinia in Papua New Guinea

  • Published source details Room P.M. & Thomas P.A. (1985) Nitrogen and establishment of a beetle for biological control of the floating weed Salvinia in Papua New Guinea. Journal of Applied Ecology, 22, 139-156.


The floating fern Salvinia molesta native to southeastern Brazil, has been introduced accidentally by man to many tropical and subtropical countries where it has become a serious weed. It was first noted in Papua New Guinea (PNG) in 1977. The weed began to causes problems in lakes and narrow channels between the lakes and the Sepik River, with floating, thick mats of the plant being impenetrable to small boats thus causing serious difficulties for the local people reliant on waterways for transport and fishing, and killing water plants and other fauna. A field enclosure experiment was undertaken to investigate the potential of a beetle Cyrtobagous sp. (Coleoptera: Curculionidae) for biological control of the fern.

Study area: Most of the work was carried out at Binatang Lagoon (completely covered by S.molesta) near Angoram (40º4'S 144º404'E) along the Sepik River in East Sepik Province, PNG.

First Cyrtobagous introduction: On 7 February 1982, 590 adult Cyrtobagous sp. were collected from S.molesta plants at a site in Queensland, Australia. Three days later the 570 survivors were divided between two floating cages (each 2 x 2 x 1 m high) anchored near the edge of the lagoon. The fern appeared yellowish, suggesting nutrient deficiency, so a plastic bag containing 2 kg of NPK fertilizer was pierced in several places and hung just below water level in each cage to act as a slow-release dispenser. These bags were recharged on 3 June 1982.

Samples of fern tissues for NPK analysis were collected every week at 30 m from the cages and NPK content determined. Samples were not taken from inside cages as the priority at this time was to establish the beetles. On 2 September 1982, samples were taken from the cages for NPK analysis and to estimate beetle numbers using Berlese funnels. Buds were examined for beetle damage within 30 cm of the bags of fertilizer, and at the opposite sides of the cages 150 cm away.

Second Cyrtobagous introduction: Samples suggested that the beetles were dying out and that the remaining individuals were concentrated around the bags of fertilizer. Analyses had confirmed the initial impression that levels of N in S.molesta were lower at Binatang Lagoon than from the Queensland site where Cyrtobagous had established rapidly. Consequently, on 3 September 1982 a further 296 beetles were introduced into each cage and an extra bag of NPK fertilizer hung underneath. A weekly application of 25 g urea (a convenient nutrient source) in 200 ml water sprayed in each cage was initiated.

On 16 November 1982, samples showed that beetle numbers had doubled and that 90% of buds had been damaged. On 26 November 1982, a quarter of the plant material inside each cage was replaced with undamaged S.molesta from outside the cages to maintain the supply of food to the insects. The material removed was placed next to the cages in the hope that the beetles it contained would multiply and spread. This procedure was repeated twice in December 1982.

A boom of floating logs was placed 5 m from the cages to stop S.molesta in the vicinity from being moved away by the wind. On 16 November 1982, 200 buds were inspected weekly at 1 m and 20 m, towards the centre of the lagoon from the cages. From 22 April 1983, inspections were made at c. 5 m (just inside the boom), 10 m, 20 m and 35 m from the cages. Numbers of beetles and damaged buds were recorded.

Additional trials were undertaken to assess the effects of N fertilizer on Cyrtobagous establishment.

The initial 570 adult Cyrtobagous released into the two field cages declined to about 40 adults over nearly 7 months or about four generations. Eleven weeks after adding another 592 adults to the cages, adding an additional bag of NPK fertilizer under each cage, and starting weekly application of urea, there were approximately 3,000 adults present.

On release from cages, and in the unsprayed treatment, beetle numbers increased exponentially despite there being very low N concentrations in the fern. Evidence suggests that by damaging its host, the beetle increases the quality of the host as food.

An enclosure experiment in which insects were placed on unsprayed control plants, on plants sprayed with N fertilizer and on plants sprayed with N + P fertilizer produced additional evidence that the rate of beetle population increase could be manipulated by application of N to the fern.

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