J. de Kraker

Impact of nitrogenous-fertilization on the population dynamics and natural control of rice leaffolders (Lep.: Pyralidae.

The effect of nitrogenous-fertilization on the population dynamics and natural control of rice leaffolders was studied in an irrigated rice area in the Philippines. Nitrogen was applied at three levels (0, 75 and 150 kg N ha-1), and its impact on crop growth and yield, arthropod abundance, and rates of leaffolder parasitism and survival was assessed with weekly samples. Rice plants were taller and had a higher leaf nitrogen content with increasing levels of nitrogenous fertilization, but grain yield was highest at the medium nitrogen level. Herbivores, predators, and parasitoids increased in abundance with nitrogenous-fertilization level. The average density of rice leaffolder larvae at the highest nitrogen level was eight times the density at zero nitrogen level, and the peak percentage injured leaves increased from 5 to 35%. The strong increase in larval density was due to the positive effect of nitrogenous-fertilization on egg recruitment and survival of medium-sized larvae. The percentage parasitism of eggs and larvae was not affected by nitrogenous-fertilization. The increase in survival of medium-sized larvae with nitrogen levels was associated with lower predator to leaffolder ratios. The strong effect of nitrogenous-fertilization in the present small-scale experiment was attributed mainly to allowing the moths an oviposition choice between plots with different application levels of nitrogen. Therefore it is hypothesized that the effect of increasing nitrogenous-fertilization level on leaffolder larval densities will be less pronounced when implemented over a large area.

Biocontrol of grey mould by Ulocladium atrum applied at different flower and fruit stages of strawberry

Grey mould is an important disease ofstrawberries resulting from flower and fruitinfection by Botrytis cinerea Pers. Thesaprophytic fungus Ulocladium atrumPreuss is a promising biological controlagent for control of B. cinerea instrawberry and other crops. The objective ofthis research was to determine the efficacy ofU. atrum to control grey mould by asingle application of a spore suspension (2 ×106 conidia/ml) at different flowerand fruit development stages. Four experimentswere carried out in 1999, two under natural andtwo under enhanced inoculum levels of B.cinerea. In each experiment, flowers and youngfruits in six distinct stages of developmentwere sprayed with either water or U.atrum suspension. U. atrum suppressedB. cinerea sporulation on petals by 15 to54%. One to four days after spraying, U.atrum was present on less than 30% of stamensand did not affect the incidence of B.cinerea on these flower parts. The efficacy ofthe U. atrum sprays in controlling greymould was low to moderate, and resulted onaverage in a reduction of 21% in diseaseincidence on ripe fruits. Low control efficacywas probably due to poor coverage with orcolonisation of stamens by U. atrumspores, and a relatively low level ofsuppression of the colonisation of flower partsby B. cinerea. Significant reductions ofgrey mould in comparison to the control(p ≤ 0.10; on average 41% reduction) werefound most frequently when the antagonist wasintroduced at late flowering or early fruitstages. Therefore, these are the most suitablestages to apply U. atrum. Further studiesare needed to improve the spray coverage andpersistence of U. atrum inoculum.

Population dynamics of rice leaffolders (Lepidoptera: Pyralidae) and their natural enemies in irrigated rice in the Philippines

Populations of rice leaffolders and their natural enemies were studied in eight crops of irrigated rice in Laguna Province, the Philippines. The rice leaffolder complex consisted of three species: Cnaphalocrocis medinalis (Guenee), Marasmia patnalis Bradley and M. exigua Butler. Leaffolder population dynamics were characterized by an egg peak at maximum tillering and a broad larval peak around booting stage. Peak densities ranged from 0.2 to 2.0 larvae per hill. Most larvae originated from immigrant moths and there was no substantial second generation. The seasonal percentage egg parasitism by Trichogramma sp. ranged from 0 to 27°and percentage larval parasitism from 14 to 56ÐThe braconid Macrocentrus philippinensis Ashmead was the most commonly reared larval parasitoid. Forty natural enemy taxa that may attack rice leaffolders were identified from suction and sweepnet samples: 24 predator taxa and 16 parasitoid taxa. The estimated survival rates from leaffolder egg to larval stages and between larval stages showed large variation between rice crops, but were not clearly correlated with observed levels of parasitism, natural enemy abundance, or natural enemy to leaffolder ratios. It is suggested that the generally low densities of rice leaffolders in Philippine transplanted rice are caused by their ovipositional preference for crops at the maximum tillering stage, allowing for only one generation, and by high immature mortality caused by the abundant and diverse complex of natural enemies.

Conidial Persistence and Competitive Ability of the Antagonist Ulocladium atrum on Strawberry Leaves

The persistence of conidia and competitive ability of Ulocladium atrum , an antagonist of Botrytis cinerea , were studied in field plots of strawberry in The Netherlands. Single application of conidial suspensions (2 - 10 6 conidia ml -1 ) of U. atrum were sprayed on the canopy and repeated 14 times, each in a different plot during the spring, summer or early winter of 1997 and 1998. Leaf samples were taken approximately weekly in the summer crops and monthly in the overwintering crops up to 30-120 days after application, to assess total conidial density, conidial viability, and competitive colonization of necrotic leaf tissue by U. atrum in relation to naturally occurring B. cinerea and other necrotrophic fungi. The total density of U. atrum conidia on green strawberry leaves declined exponentially after application by an average relative rate of -0.10/day. Variation in the rate of decrease between individual sprays was large and partly associated with the average daily precipitation during the first week after application. For pooled data, percentage viability of U. atrum conidia declined linearly at a rate of 1%/day after application. Colonization of necrotic strawberry leaf tissue by U. atrum increased with total conidial density and, at densities > 1500 conidia cm -2 , leaf colonization by naturally occurring B. cinerea was consistently reduced. With the exponential decline in total conidial density of U. atrum , effective suppression of B. cinerea on strawberry leaves can only be expected when U. atrum is applied less than one week before the leaves become necrotic. Further research needs to focus on enhanced rain-fastness of deposited conidia of U. atrum to reduce spore loss.

Egg mortality of rice leaf folders Cnaphalocrocis medinalis and Marasmia patnalisin irrigated rice fields

Egg mortality of rice leaf folders Cnaphalocrocis medinalisand Marasmia patnalis was studied in unsprayed irrigated rice fields in Laguna Province, the Philippines. Mortality was assessed by field exposure of laboratory-laid eggs for two days and by monitoring of field-laid eggs. Egg disappearance, the major mortality factor, was low in the first four weeks after transplanting and then increased. Egg parasitism by Trichogrammajaponicum was highest at the start of the crop and decreased to a low level towards crop maturity. Non-hatching of eggs was of minor importance. Over the total duration of the egg stage, the average disappearance of exposed laboratory-laid eggs was40%, and of field-laid eggs 46%. Egg mortality due to parasitism averaged 15% and 18%, respectively. The potential impact of egg parasitism is probably partly obscured by the disappearance of parasitized eggs. Mortality rates were highly variable between egg cohorts, but with multiple regression analysis several factors were identified that statistically explained a significant part of this variation. The results suggest that the predatory crickets Metiochevittaticollis and Anaxipha longipennis play a major role in egg disappearance, and that egg parasitism is positively dependent on the overall density of host eggs of Trichogramma in the field.

Effect of prey and predictor density on prediction of rice leaffolder eggs by the cricket Metioche vittaticollis.

Cage experiments were conducted to quantify the predation rate of the cricket Metioche vittaticollis (Sta § l) on the eggs of rice leaffolder Marasmia patnalis Bradley. Egg predation by adult females was measured in response to changes in egg density, predator density and leaf area per cage. The number of eggs consumed per predator increased with egg density, without reaching a plateau. The predation rate decreased with increasing leaf area. The functional response could be adequately described with a linear Type I model, with the effect of leaf area included. This type of response to leaffolder egg density means that predation was not limited by prey handling time or satiation, but by the search rate. The search rate is here interpreted as the leaf area effectively searched for leaffolder eggs by a single predator in one day. Estimated search rates averaged 0.13 m 2 day -1 for M. vittaticollis females. The search rate of the predators increased with prey density, but a model describing a density dependent search rate explained only 3% of the total variation in search rate. Increasing predator density per cage led to a decrease in the per capita egg predation rate when predator density was more than two per m 2 leaf area. Interference might thus reduce the potential to enhance leaffolder egg predation by conservation or augmentation of predatory cricket populations.