Predicting numbers of an insect (Neophilaenus lineatus: Homoptera) in a changing climate.

Measurements of the density of an upland population of the spittlebug Neophilaenus lineatus (L.) (Homoptera, Auchenorrhyncha) were made from 1961 to 1997. Population change from year to year is shown to be largely density-independent. Analyses of meteorological records over the 37-year period showed that the weather variable with the highest correlation with population change was mean minimum temperature in September. A simple model was constructed relating numerical changes from 1961 to 1992 to temperature, and this was found to explain 75% of the change in this population. The model was used to predict numerical change from 1993 to 1997 over which period it explained 70% of the change. The model predicts that a projected rise in mean temperature of 1°C would increase the mean population density of N. lineatus by 50%. Experimental cloches at the upland site which raised mean temperature by 1°C, had N. lineatus densities which were 157% higher than in adjacent control quadrats. An experimental raising of field temperature at a lowland site where population change had been shown to be density dependent did not result in density changes, although dates of hatching were earlier.

The effect of an insect herbivore and a rust fungus individually, and combined in sequence, on the growth of two Rumex species.

The chrysomelid beetle Gastrophysa viridula and the rust fungus Urmnyces rumicis both occur on leaves of Rumex crispus and R. obtusifolius. We investigated the effect of beetle grazing or rust infection individually and when combined in sequence on the growth of their hosts in the field. Singly, beetle or rust reduced leaf area and plant biomass; the effect was greater on R. crispus, and rust caused greater damage than the beetle. Beetle grazing with subsequent rust infection caused damage no greater than that caused by rust alone, although on R. obtusifolius damage was greater than that from beetle grazing alone. Rust infection of R. obtusifolius with subsequent beetle grazing produced damage similar to that from other treatments; involving rust infection. In R. crispus this treatment produced the greatest reduction in biomass, The reductions in root and total plant weight from rust infection with subsequent beetle grazing were accurately predicted by a model including the damage produced by beetle and rust alone and the length of time each was present on the plant. This model also predicted accurately the damage to R. obtusifolius from the beetle followed by rust treatment, but over-estimated by up to 40% the damage to R. crispus. This can be explained mainly by an inhibition of rust infection by beetle grazing.

Interactions between Sitka spruce, the green spruce aphid, sulphur dioxide pollution and drought.

Sitka spruce trees, with and without the aphid Elatobium abietinum and/or drought treatment, were subjected to 25 nl litre(-1) of sulphur dioxide over a 2-month period. Aphids became three times as abundant on the fumigated trees if they were well watered and twice as abundant on trees from which water was withheld, compared with unfumigated controls. Growth parameters of the trees were little affected by pollution alone, but were substantially reduced by either aphids or drought. There was a significant interaction between SO(2) and aphids in a further reduction of both leader extension and root weight. Root weight was also reduced by 24% more than expected from the additive effects of the combined SO(2) and drought treatment.

Responses to ozone of insects feeding on a crop and a weed species.

The influence of ozone on insect herbivore growth and population development was investigated. Fumigation of both pea (Pisum sativum L.) and dock (Rumex obtusifolius L.) at a range of O(3) concentrations between 21-206 nl litre(-1) produced changes in mean relative growth rates of the aphids Acyrthosiphon pisum Harris and Aphis rumicis L. of between 24 and -6% relative to controls. However, there was no evidence of a dose-related response to O(3) fumigation and no clear differences in aphid response when fumigated with the plant on prefumigated or previously unfumigated plant material. It is suggested that this may, in part, be due to the presence of NO contamination during O(3) fumigation. However, the MRGR of dock aphids was found to be greater on new compared to old leaves as well as the increase on the new growth and decrease on the old growth of fumigated plants relative to unfumigated controls. The size of egg batches of the chrysomelid beetle Gastrophysa viridula Degeer were found to be larger, survival and productivity of larvae was higher, and the food consumption lower on R. obtusifolius fumigated with 70 nl litre(-1) O(3) compared with unfumigated controls. This meant that these beetle larvae consumed less leaf area per mg of production on fumigated leaves probably because of their better nutritional quality and/or reduced leaf defences. However, the rate of development of larvae was similar on fumigated and control plants.

Effects of insect damage on photosynthesis, transpiration and SO2 uptake by sycamore.

This study investigated the effects of damage by the typhlocybine leafhopperOssiannilssonola callosa on the physiology of sycamore seedlings. Feeding by 20 adult leafhoppers for four days produced stippling damage of about 10% of total leaf area. Measurements immediately following removal of the leafhoppers showed a 22% decrease in photosynthesis, 25% decrease in daytime rate of water loss and a 34% increase in night-time rate of water loss, probably due to impaired stomatal function. Sixteen days later control of stomatal function was largely regained with rates of photosynthesis similar to the control plants and slightly higher rates of water loss (6.6% day, 18.0% night) attributable to the still-open feeding punctures of the leafhoppers. Daytime SO2 uptake was similar in damaged and control plants but at night the damaged plants showed a 35% increase in uptake. The hypothesis that leafhopper damage is responsible for this increase is supported by a positive relationship between uptake rate and area of leaf stippled. The physiological effects of leafhopper feeding are greater than the visible damage suggests and the effects are particularly serious when the damage is fresh. Increased rate of air pollutant entry to damaged plants is also a problem, again most marked when damage is fresh.

Effect of SO2 on the reproduction of pea aphids, Acyrthosiphon pisum, and the impact of SO2 and aphids on the growth and yield of peas.

Pea aphids feeding from birth to maturity on pea plants (Pisum sativum) exposed to SO(2) concentrations of 50 nl litre(-1) or 80 nl litre(-1) showed a significant 19% increase in the rate of nymph production during the reproductive period, compared to control aphids feeding on plants in charcoal-filtered air. The higher nymph production resulted in a mean 4.6% increase in the intrinsic rate of population increase (rm). In longer term glasshouse fumigation experiments pea aphid populations were, on average, 1.8 times greater on pea plants in ambient air plus 45 nl litre(-1) SO(2) than in ambient air alone. Aphid infestation in ambient air caused a 42% reduction in pea yield and affected most plant parameters adversely. Ambient air plus SO(2) had no direct effect on yield, but, in combination with aphid infestation, a further 10% reduction in yield was recorded.

The effects of chrysomelid beetle grazing and plant competition on the growth of Rumex obtusifolius.

The interaction between grazing by Gastrophysa viridula and interspecific plant competition was investigated for Rumex obtusifolius growing in the field. During an eight-month growing season non-competing R. obtusifolius grew larger than competing plants although herbivore loads (beetle productivity per unit leaf area) were similar for competing and non-competing plants. However, grazing significantly reduced the growth of R. obtusifolius only in competing plants thus demonstrating a synergism between plant competition and invertebrate herbivory. The relevance of these findings for the distribution of plant species is discussed.