R. Rabbinge

Influence of temperature on bionomics of cotton aphid, Aphis gossypii, on cotton

Life table parameters of Aphis gossypii Glover (Homoptera: Aphididae) on Gossypium hirsutum L. were determined at six temperatures (10, 15, 20, 25, 30, and 35 ± 0.5 °C) in the laboratory. Relationships of life table parameters with temperature were described with mathematical equations. Development was fastest at 30 °C, with a pre‐larviposition period of 4.6 d. Survival to adult was greatest at 25 °C (81%). Fecundity was highest at 25 °C, with a total fecundity of 28.3 nymphs per female and a mean reproductive rate of 3.1 nymphs per female per day. Threshold temperatures for development in the first through fourth instar and the adult were 8.2, 8.0, 7.2, 6.2 and 7.9 °C, respectively. The durations of these stages, expressed as temperature sums above these thresholds, were 24.2, 23.7, 23.0, 25.5 and 168.8 degree‐days (D°), respectively. A. gossypii achieved its maximum net reproductive number (24.4 nymphs per female) and greatest intrinsic rate of increase (0.386 d−1) at 25 °C. The high relative rate of population increase at 25 °C results in a daily population increase of 47% and a doubling time of only 1.8 d, illustrating the tremendous growth capacity of A. gossypii populations under favourable conditions. Compared to literature sources, our source of A. gossypii, fed on cotton, showed a comparatively great heat tolerance.

Agriculture and its environment: Are there other ways?

Abstract Although the Common Agricultural Policy has served its original purposes very well, the problems with overproduction, the environment and the EC budget are, by now, so profound that the need for a major overhaul is widely recognised. The aim of the present paper is to contribute to the discussion of this technical and political problem. For this purpose some fundamental aspects of the agricultural production process are treated first. Within the European Community, there appear to be considerable possibilities for a simultaneous increase of production per surface unit and of its efficiency, so that in all situations where agriculture is practised as an economic enterprise the yield per surface unit will continue to increase. This increase in production has to be kept in line with the stagnating demand for use of land for other purposes. This is necessary, irrespective of whether prices are adjusted downward or production quotas are introduced. Against this background two opposing perspectives of development are presented, in such a way that the dilemmas the European Community is faced with are clarified. These perspectives of separation and integration of functions were originally developed by the Dutch Scientific Council for Government Policy (WRR, 1983). They differ especially in their view of agriculture and its relation to nature and the environment and in the way that the problem of overproduction is coped with. The discussion of the advantages and disadvantages of these opposite perspectives paves the way for a broad outline of another common agricultural policy that is characterised by both market orientation and solidarity and may lead to a competitive and sustainable agriculture and a fair reconciliation of conflicting agricultural and environmental goals.

Damage effects of cereal aphids in wheat

Laboratory and field studies on aphid damage in wheat are described. Both direct and indirect effects of aphids on the behaviour of the crop were studied. In the laboratory a clear effect of honeydew and yeasts on photosynthesis was found. This effect could not be demonstrated under field conditions, since even at yield loss levels of 700 kg ha−1 the size of this effect may be undetectable with crop enclosures reaching an accuracy of 10%. The effect of honeydew on ageing, measured in the laboratory trials was also found in the field and may form a major cause of the yield losses found in the field. Yield losses under field conditions reached 700 kg ha−1 and were for 72% due to direct sucking damage of the 35 aphids, found maximally per culm, and direct and indirect honeydew effects. Saprophytic and possibly also some necrotrophic fungi caused 28% of the yield losses. The exact contribution of each of the damage factors was not revealed but it was demonstrated that direct and indirect effects contribute to the final yield losses.SamenvattingLaboratorium- en veldproeven over de schade door bladluizen in tarwe worden beschreven. Een analyse van de directe en indirecte effecten van bladluizen op de groei van het gewas werd verricht. In het laboratorium werd een duidelijk effect van honingdauw en gisten op de fotosynthese gevonden. Dit effect kon niet worden gedemonstreerd in het veld, hetgeen waarschijnlijk wordt veroorzaakt door de orde van grootte van dit effect dat valt binnen de meetfout van de gewasfotosyntheseapparatuur.Het effect van honingdauw op veroudering, dat in het laboratorium werd vastgesteld, kon ook worden aangetoond in het veld en is waarschijnlijk een van de hoofdoorzaken van de oogstverliezen. Deze oogstverliezen in het veld bedroegen 700 kg ha−1 bij een maximale dichtheid van 35 bladluizen per halm en werden voor 72% veroorzaakt door direct zuigsschade en honingdauweffecten. Van de totale schade werd 28% veroorzaakt door saprofytische en wellicht ook door enige necrotrofe schimmels. De precieze bijdrage van ieder van de schadefactoren tot de schade werd niet vastgesteld, maar de wijze waarop directe en indirecte effecten hun bijdrage leverden aan de uiteindelijke oogstverliezen werd zichtbaar gemaakt.

Multistage functional responses in a ladybeetle-aphid system: scaling up from the laboratory to the field

Abstract Experiments and models are described that quantify the functional responses of the larval and adult stages of the sevenspot ladybeetle, Coccinella septempunctata L. toward mixed stage populations of the cotton aphid, Aphis gossypii Glover, in cotton. In the laboratory, functional responses were measured for five beetle stages and three size groups of prey at five temperatures (15 to 35°C). The 75 resulting functional responses were each characterized by a search rate (cm2/predator/d) and a handling rate (prey/predator/d). Both search and handling rates increased with predator stage. Search rates increased, while handling rates decreased, with prey size. Search rate increased linearly with temperature, while handling rate showed an optimum response to temperature. Field cage studies were conducted to verify whether the search rates determined in the laboratory were valid under field conditions. Search rates of predators derived from observations in field cages yielded parameter estimates that were similar to those found in the laboratory. A comprehensive model, allowing for effects of temperature, stage distribution of the aphid population, and plant leaf area on predation rate, was then constructed to calculate predation by larvae and adults of C. septempunctata on multi stage populations of prey. This model gave good correspondence to the cage observations if two-sided leaf area was input into the model as search substrate for the predator. The model appears suitable for calculating predation rates of C. septempunctata on A. gossypii under field conditions. A sensitivity analysis of the functional response model shows the crucial effect of crop leaf area on predation.

Monitoring for cereal aphids in winter wheat

A simple monitoring method for cereal aphids is described which diminishes observation time and can be used by farmers themselves. By repeated observations the farmers can follow the population upsurge of the aphids. A description of the yield decrease — aphid density relation is given which may help to decide whether control measures against cereal aphids are needed.SamenvattingUit een evaluatie van veldproeven die gedurende de laatste jaren werden verricht is gebleken dat de in wintertarwe het frekwentst voorkomende bladluizensoortenMetopolophium dirhoduM, enSitobion avenae, zowel in combinatie als per soort afzonderlijk flinke oogstreducties in kg/ha kunnen veroorzaken. De opbrengstderving wordt veroorzaakt door een combinatie van directe zuigschade door de bladluizen, honingdauwafscheiding en de wijziging in verouderingssnelheid en netto assimilatie van de bladeren.Een eenvoudige waarnemingsmethode voor bladluizen in wintertarwe wordt beschreven, welke de waarnemingstijd vermindert en door de telers zelf kan worden gebruikt. Door herhaling van de waarnemingen kan de teler de populatieopbouw van de bladluizen vogen. Een beschrijving van de relaties tussen bladluizendichtheid en opbrngstderving is gegeven. Deze kan behulpzaam zijn bij het nemen van een beslissing over bestrijdingsmaatregelen.

The use of incidence counts for estimation of aphid populations. 1. Minimum sample size for required accuracy

In attempts to reduce the time required for monitoring cereal aphid populations, it has been suggested that direct counting be replaced by the use of incidence counts: estimation of aphid density from the proportion of wheat tillers infested. It is important, however, that the use of this simpler and more rapid method does not result in a loss of accuracy. This paper calculates the minimum sample size (number of tillers to be inspected) required to yield a given accuracy, using the two alternative sampling methods. Over the range of aphid densities close to the economic thresholds for pesticide application, the use of incidence counts does not require a much greater sample size than that needed for direct counting.SamenvattingTeneinde de bemonsteringstijd voor graanluizen te reduceren en een eenvoudige betrouwbare bemonsteringsmethode te ontwikkelen wordt telling van het aantal individuen per halm vaak vervangen door bepaling van het bezettingspercetage. Deze eenvoudige methode mag evenwel niet leiden tot een onaanvaardbaar verlies in nauwkeurigheid. Daarom wordt voor de directe telmethode en de methode met infectiepercentages de minimale monstergrootte bepaald bij een van te voren vastgestelde nauwkeurigheid. Het blijkt dat bij bladluizendichtheden, die dicht bij de economische schadedrempel liggen, de monsters bij de twee methoden niet in groote behoeven te verschillen.

Modeling Spatial Characteristics in the Biological Control of Fungi at Leaf Scale: Competitive Substrate Colonization by Botrytis cinerea and the Saprophytic Antagonist Ulocladium atrum

ABSTRACT A spatially explicit model describing saprophytic colonization of dead cyclamen leaf tissue by the plant-pathogenic fungus Botrytis cinerea and the saprophytic fungal antagonist Ulocladium atrum was constructed. Both fungi explore the leaf and utilize the resources it provides. Leaf tissue is represented by a two-dimensional grid of square grid cells. Fungal competition within grid cells is modeled using Lotka-Volterra equations. Spatial expansion into neighboring grid cells is assumed proportional to the mycelial density gradient between donor and receptor cell. Established fungal biomass is immobile. Radial growth rates of B. cinerea and U. atrum in dead cyclamen leaf tissue were measured to determine parameters describing the spatial dynamics of the fungi. At temperatures from 5 to 25 degrees C, B. cinerea colonies expanded twice as rapidly as U. atrum colonies. In practical biological control, the slower colonization of space by U. atrum thus needs to be compensated by a sufficiently dense and even distribution of conidia on the leaf. Simulation results confirm the importance of spatial expansion to the outcome of the competitive interaction between B. cinerea and U. atrum at leaf scale. A sensitivity analysis further emphasized the importance of a uniform high density cover of vital U. atrum conidia on target leaves.

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.

Analysis of Foraging Behaviour of the Whitefly Parasitoid Encarsia formosa on a plant: A Simulation Study

The foraging behaviour of Encarsia formosa was analyzed using a stochastic simulation model of the parasitoids behaviour. Parasitoids were allowed to search during a day on a tomato plant infested with immatures of the greenhouse whitefly, Trialeurodes vaporariorum. The model simulates searching, host selection, host handling and patch leaving behaviour, and the physiological state of the parasitoid. The outputs of the model are the number of visited leaflets and the number of hosts encountered, parasitized or killed by host feeding. The simulation results agreed well with observations of parasitoids foraging on tomato plants. The number of encounters and ovipositions on the plant increased with host density according to a type II functional response. At a clustered host distribution over leaflets and low host densities, the most important parameters affecting the number of ovipositions were the leaf area, the parasitoids walking speed and walking activity, the probability of oviposition after encounter...

Analysis of foraging behavior of the whitefly parasitoid Encarsia formosa (Hymenoptera: Aphelinidae) in an experimental arena: a simulation study.

Foraging ofEncarsia formosa was analyzed using a stochastic simulation model of the parasitoids behavior. Parasitoids were allowed to search during a fixed time in an experimental arena with immatures of the greenhouse whitefly,Trialeurodes vaporariorum. The model simulates host searching, selection, and handling behavior and the physiological state (egg load) of the parasitoid. The simulated number of hosts encountered, parasitized, or killed by host feeding agreed well with observations on leaf disks. The hypothesis of random host encounter seems to be correct. The number of ovipositions on the leaf at low host densities was strongly affected by the parasitoids walking speed and walking activity, the probability of oviposition after encountering a host, and the initial egg load. At high densities, the initial and maximum egg load were most important. A strong temperature effect was found at 18°C or below. The number of encounters, ovipositions, and host feedings increased with host density to a maximum of 25, 6.5, and 1.5, respectively, during 2 h at 25°C. The shape of the curves resembled a Holling Type II, which may be the result of the “experimental” procedure, where a parasitoid was confined to a patch during a fixed time.