Added soil nitrogen does not allow Rumex obtusifolius to escape the effects of insect-fungus interactions

Published source details Hatcher P.E., Paul N.D., Ayres P.G. & Whittaker J.B. (1997) Added soil nitrogen does not allow Rumex obtusifolius to escape the effects of insect-fungus interactions. Journal of Applied Ecology, 34, 88-100.
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Summary

The rust fungus Uromyces rumicis and chrysomelid beetle Gastrophysa viridula are considered biocontrol agents against a common weed in the UK, broad-leaved dock Rumex obtusifolius. As this weed grows in widely varying soil nitrogen conditions, and the effect of insects and fungi on their hosts has been shown to vary greatly with changes in plant nitrogen fertilization in some species, the influence of nitrogen fertilization regimes on the efficacy of these biocontrol agents, both individually and combined against R. obtusifolius seedlings, was investigated.

Study site: The influence of three nitrogen fertilization regimes on performance of two biocontrol agents of broad-leaved dock Rumex obtusifolius was investigated in a glasshouse experiment conducted at Lancaster University Field Station, Lancashire, north-west England.

Rumex cultivation: At the end of April 1995, Rumex obtusifolius were germinated in a glass house, and 23 days later on 18 May were potted out in plastic tubs (38 cm x 26 cm depth). A combination of slow- and fast-release mineral compounds was used in the soil mix. The tubs were sunk into the ground in rows, spaced 1.5 m apart.

Nitrogen addition: Three nitrogen fertilization regimes (0, 200, 400 kg/ha/year added nitrogen) were tested. Nitrogen was applied in four occasions monthly from 23 May. A 500 ml solution of three parts ammonium nitrate to two parts sodium nitrate was applied.

Damage treatments: Three damage treatments were imposed on each nitrogen treatment: insect herbivory (Gastrophysa viridula); rust infection (Uromyces rumicis); and both combined. Controls were left untreated.

Non-destructive measurements were made at monthly intervals from May, and plants were harvested at the end of October.

The addition of 200 kg/ha/year nitrogen increased Rumex obtusifolius leaf area by 70% by September. At harvest, leaf weight was increased by 19% and root weight by 43% compared with plants with no nitrogen added. There was no significant difference between the effect of 200 and 400 kg/ha/year added nitrogen.

Gastrophysa viridula herbivory alone caused a greater reduction in leaf area and decrease in root and shoot weight than rust fungus infection alone.

The effects of nitrogen fertilizer addition, herbivory and infection were additive throughout the experiment. The combination of herbivory and infection had an increasing effect on leaf area as the year progressed, from a 46% reduction in July to 62% in October, compared with healthy plants. Plants receiving no additional nitrogen with combined herbivory and infection had a 65% reduction in leaf area and weight, and a 72% reduction in total harvested weight, compared to healthy plants with 400 kg/ha/year added nitrogen.

Conclusions: in this study, increasing nitrogen fertilizer additions did not reduce the effects of the insect herbivore and rust fungus pathogen upon R.obtusifolius and did not alter the additive nature of the insect-fungus interaction.

Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://links.jstor.org/sici?sici=0021-8901%28199702%2934%3A1%3C88%3AASNDNA%3E2.0.CO%3B2-8