Effects of adult and larval Cyrtobagous salviniae on the floating weed Salvinia molesta

Published source details Julien M.H., Bourne A.S. & Chan R.R. (1987) Effects of adult and larval Cyrtobagous salviniae on the floating weed Salvinia molesta. Journal of Applied Ecology, 24, 935-944.
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Summary

Salvinia molesta (Salviniaceae) is a sterile floating fern which has become an invasive pest in some regions of tropical and southern-hemisphere temperate freshwaters. Biological control has been achieved in tropical areas of Australia and elsewhere through introduction of the herbivorous weevil Cyrtobagous salviniae (Coleoptera: Curculionidae). Damage to the fern is caused by both larvae and adult weevils; larvae feed mainly inside rhizomes and adults on buds. It has been suggested that the most destructive feeding is by larvae. The aim of this study was to measure and compare the response of S. molesta to damage by adults and larvae at densities that occur in the field in Australia.

Study site: The experiments were conducted at Wappa Dam, Queensland, Australia.

Methods: Five adult and three larval experiments, involved 36 and 16 floating cages, respectively, placed 1 m apart and 15 m from the bank of the dam. The cages were 50 x 50 x 50 cm with plywood sides, a fine nylon mesh top and an open base. Each contained a floating sheet of styrofoam with a 20 cm diameter hole cut out, within which five ferns trimmed from mats growing in the dam, were placed, each plant in loose contact with each other.

Adult weevil experiments: There were three replicates of four beetle and three urea (additional nitrogen) treatments. Adult laboratory-reared, weevils were placed in the cages at densities of 0, 0.6, 1 and 1.4 per plant. Urea (46% N) was applied at 0, 15 and 30 g per cage as granules in dialysis tubes suspended among the ferns roots and replaced every third day. Experiments were conducted between April 1983 and January 1984. Each was limited to 14 days so that any larvae developing from eggs would not influence plant growth.

Larval weevil experiments: There were four replicates of four larval densities: 0, 2, 4 and 8 per plant in experiment 1; and 0, 1, 2 and 4 in experiments 2 and 3. Laboratory-reared larvae were placed singly in buds, except with eight larvae per plant when two were placed in each of the four buds (experimental ferns had four buds). Experiments ran for 16-29 days, between October 1983 and April 1984, depending on the larvae development time.

Fern monitoring: At the beginning and end of the experiments, the fern rhizomes were mapped. Each bud, node and root was assessed as undamaged, damaged or destroyed, and leaf damage was rated either, 0, <25%, < 50%, <75%, <95% or
100%. New growth was scored as 1 for a new ramet and 0.25, 0.5 or 0.75 for buds,
depending on the stage of development. Thirty plants at the beginning, and all at
the end of each experiment, were washed in deionized water, dissected into buds, tops and roots, and each portion was dried. Prior to the dissection of plants, adults and pupae were removed; some were not recovered and a few larvae may have remained in the fern tissues. The percentage dry weight of N, P and K in the combined material from each cage was determined.

Effect of adult feeding: Adult weevils fed on meristematic tissues in buds and on young leaves and roots. Lost meristematic tissues was compensated for by increased development of buds, but was only fully compensated for at high levels of N availability. Their feeding for 14 days did not reduce relative growth rate of ramets or whole plant weight. Though roots were destroyed, compensatory growth maintained the root weight relative growth rate.

Effect of larval feeding: Larvae tunnelled through buds, rhizomes and roots. Plants were not able to compensate for ramets killed through larvae destroying vascular tissues. Relative growth rates were reduced by 0.0018 ramets per ramet per day and 0.0014 g per g per day by each larvae present during the period from hatching to pupation, on ferns initially containing seven ramets and four buds.

Conclusions: The internal damage to S.molesta plant tissues caused by weevil larvae was more debilitating and plants were less able to compensate for such damage than when only meristematic tissue was destroyed, as by adults.

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