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.Additional keywords: Metopolophium dirhodum, Sitobion avenae, Rhopalosiphum padi, population dynamics.The yearly rapid increase of cereal aphids requires continuous monitoring. However, continuous monitoring of aphids is a very time consuming procedure and therefore methods which reduce sampling and monitoring activities are urgently needed. Population models which simulate the population curve of the aphids from immigration until the flattening of the curve may help in this. Detailed population models of Sitobion avenae (Rabbinge et al., 1979, Carteret al., in prep.) explain the upsurge of the population and the period of flattening but are still inadequate in explaining completely the collapse of the aphid population. Based on a sensitivity analysis with these explanatory models, simple formulas were developed which are used to produce short term predictions (Rijsdijk et al., in prep.).With these short term predictions of the population density of the aphids it is possible to determine at what time measures are needed to prevent the aphids from reaching density levels at which they may cause economic damage. Definition of the damage threshold (costs of treatment = costs of potential yield loss) requires also knowledge of the level of yield loss and the way it is generated (Vereijken, 1979).It appears that the relation between aphid numbers per ear in the peak and yield loss has a correlation coefficient of 0.75 (Vereijken, 1979). The maximum number of aphids per culm (main shoot and tillers) has a higher correlation coefficient and a slightly lower regression coefficient (r = 0.83; y = 26.1 x + 8.02). Figure 1 based on an analysis of 20 experiments (PAGV, CABO, IPO*; ten Hag, de Vos and Ubels, pers. comm.) during the years [1974][1975][1976][1977][1978][1979] shows that Metopolophium dirhodum and S.avenae may cause considerable yield loss. Situations in which M. dirhodum
ln 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 mu~h greater sample size than that needed for direct counting.
Hyacinth bulbs are propagated by hollowing them out or by making deep cuts. The bulbs treated in this way are stored in a special room ("hollowing room") and they produce daughter bulbs. The bulbs in this room can be infected by Thrips tabaci Lind., penetrating into the room from outdoors. Bulbs infected at an early date by this thrips produce no or only a few daughter bulbs. If infection takes place when daughter bulbs have already developed, then these young bulbs may rot away after planting. INLEIDINGIn 1963 en 1964 werd advies gevraagd over de bestrijding van tripsen, die in de zogenaamde holkamers de voor de vermeerdering bestemde hyacintebollen hadden beschadigd. De soort werd geidentifieeerd als Thrips tabaci Lind. forma pulla Uzel. Nader onderzoek wees uit, dat deze aantasting geregeld voorkomt, doch meestal niet wordt opgemerkt. Later, nadat de uitgeplante bollen ,,van de wortel gaan" en de tripsen zijn verdwenen, wordt het euvel ten onrechte aan andere oorzaken toegeschreven.De vermeerdering van hyacintebollen komt in grote trekken op het volgende neer. De bollen worden beginjuli gerooid. Er zijn twee wijzen van vermeerderen. Bij de ene methode wordt aan de onderzijde van de bol met een speeiaal mesje een uitholling gemaakt, bij de andere worden drie door bet middelpunt lopende diepe sneden aangebraeht. Op de besehadigde rokken ontwikkelen zich dochterbolletjes. De voor het hollen bestemde bollen worden v66r de bewerking tien dagen gedroogd; het snijden geschiedt in verse toestand. De geholde bollen worden vijf/~ zes dagen onder een afdak gedroogd om de wonde te laten besterven. Daarna worden zij op rekken opgeslagen in een donkere ruimte, de zogenaamde holkamer. Hierin kunnen de temperatuur en de relatieve luchtvochtigheid worden geregeld. De gesneden bollen worden enige dagen ingekuild en daarna zonder voorafgaande droging in de holkamer gebraeht. Tot 1 september wordt de temperatuur op ongeveer 20~ gehouden, daarna op 23 tot 25~ Drie tot vier weken na het inbrengen in de holkamer wordt de relatieve luchtvochtigheid meestal tussen 70 en 90 ~ gehouden.Na verloop van tijd komen er dochterbolletjes tot ontwikkeling, op de geholde bollen maximaal 40 en op de gesneden bollen maximaal 20. Omstreeks 10 november worden de bollen uitgeplant; vlak daarvoor worden zij ontsmet met een organische kwikverbinding. De moederbol rot gedurende de winter grotendeels weg, waarna de dochterbolletjes zelfstandig doorgroeien. Met de holmethode worden na vier jaar leverbare bollen verkregen, met de snijmethode na drie jaar. BIJZONDERHEDEN OVER THRIPS TABACIDe nominaatvorm van deze soort is geel, de forma pulla daarentegen bruin. Volgens PRIESN~g (1928) is de donkere vorm sleehts een vari~teit; deze opvat-1 Aangenomen voor publikatie 23 december 1964. 67
Two approaches have been used in the monitoring of local populations of cereal aphids: direct counting of aphids on a number of tillers, and the less time-consuming assessment of incidence levels (the proportion of tillers infested). This paper compares the accuracy of these two methods, using a fixed sample size, as is desirable in a practical monitoring system. It is shown that the two methods are equally reliable over a range of incidence levels, but that the confidence intervals for the population density are very broad. Reduction of these sampling errors requires a considerable increase in sample size, so they should instead be taken into account in the management decision-making process.
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