Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Effects of dietary protein and carbohydrate on life-history traits and body protein and fat contents of the black soldier fly Hermetia illucens
Barragan-Fonseca, Karol B. ; Gort, Gerrit ; Dicke, Marcel ; Loon, Joop J.A. van - \ 2019
Physiological Entomology 44 (2019)2. - ISSN 0307-6962 - p. 148 - 159.
Body nutrient composition - fecundity - food quality - larval performance - macronutrients - nutrition

We investigate how the black soldier fly Hermetia illucens L. (Diptera: Stratiomyidae) responds to dietary protein (P) and carbohydrate (C) contents and the P:C ratio in terms of both immature and adult life-history traits, as well as effects on larval body composition. Nine chicken-feed based diets varying in their P:C ratio are formulated. We test three protein concentrations (10%, 17% and 24%) and three carbohydrate concentrations (35%, 45% and 55%) and their combinations. All nine diets support the complete development and reproduction of this species. Survival is high on all diets. Development time, larval yield, larval crude fat and egg yield are more influenced by P and C contents than by the P:C ratio. Low contents result in a shorter development time. Larval yield is higher on diets with higher C-contents. Pupal development is faster on a low dietary P-content for all three C-contents. Egg yield only increases when P-content increases, although it also varies with the P:C ratio. Larval crude protein content is similar on all nine diets but increases when C-content is low (10%) in P10 and P17. Larval crude fat content is high at P24-diets irrespective of C-content. We conclude that a high macronutrient content combined with a low P:C ratio positively affects H. illucens performance. The diet P17:C55 supports the highest larval and adult performance and results in a high larval body protein content and an intermediate crude fat content.

Linking growing up and getting old : plastic and evolutionary effects of developmental diet on adult phenotypes and gene expression in the fruit fly
May, C.M. - \ 2016
Wageningen University. Promotor(en): Bas Zwaan, co-promotor(en): Fons Debets. - Wageningen : Wageningen University - ISBN 9789462577633 - 183
drosophila melanogaster - gene expression - nutrition - evolution - diet - lifespan - fecundity - biological development - drosophila melanogaster - genexpressie - voeding - evolutie - dieet - levensduur - voortplantingspotentieel - biologische ontwikkeling
A bio-economic analysis of harvest control rules for the Northeast Arctic cod fishery
Eikeset, A.M. ; Richter, A.P. ; Dankel, D.J. ; Dunlop, E.S. ; Heino, M. ; Dieckmann, U. ; Stenseth, N.C. - \ 2013
Marine Policy 39 (2013). - ISSN 0308-597X - p. 172 - 181.
reaction norms - gadus-morhua - management - stock - economics - model - recruitment - maturation - evolution - fecundity
Harvest control rules (HCRs) have been implemented for many fisheries worldwide. However, in most instances, those HCRs are not based on the explicit feedbacks between stock properties and economic considerations. This paper develops a bio-economic model that evaluates the HCR adopted in 2004 by the Joint Norwegian-Russian Fishery Commission to manage the world's largest cod stock, Northeast Arctic cod (NEA). The model considered here is biologically and economically detailed, and is the first to compare the performance of the stock's current HCR with that of alternative HCRs derived with optimality criteria. In particular, HCRs are optimized for economic objectives including fleet profits, economic welfare, and total yield and the merging properties are analyzed. The performance of these optimal HCRs was compared with the currently used HCR. This paper show that the current HCR does in fact comes very close to maximizing profits. Furthermore, the results reveal that the HCR that maximizes profits is the most precautionary one among the considered HCRs. Finally, the HCR that maximizes yield leads to unprecautionary low levels of biomass. In these ways, the implementation of the HCR for NEA cod can be viewed as a success story that may provide valuable lessons for other fisheries
Don’t put all your eggs in one basket! : reproductive strategies and fecundity regulation in temperate marine teleosts
Damme, C.J.G. van - \ 2013
Wageningen University. Promotor(en): Adriaan Rijnsdorp, co-promotor(en): O.S. Kjesbu; Mark Dickey-Collas. - Wageningen : Wageningen UR - ISBN 9789461736253 - 183
teleostei - zeevissen - voortplanting - voortplantingspotentieel - oöcytrijping - kuitschieten - biomassa - eierproductie - visstand - visserijbiologie - visserijbeheer - teleostei - marine fishes - reproduction - fecundity - oocyte maturation - spawning - biomass - egg production - fish stocks - fishery biology - fishery management

In fisheries management the spawning stock biomass (SSB) is an important indicator of the status of exploited fish stocks. Knowledge on the reproductive biology is essential to estimate SSB. A large variety of reproductive strategies is found. In marine fish two extreme strategies are known, capital spawners which have a determinate fecundity (no de novo oocyte recruitment during spawning), and income spawners which have an indeterminate fecundity (de novo oocyte recruitment during spawning). In this thesis fecundity regulating mechanisms are studied in commercial fish species with contrasting life history.

In capital spawning plaice Pleuronectes platessa and herring Clupea harengus , which spawn in autumn and/or winter, oocyte maturation starts around April when daylight length increases. Both species recruit a high number of oocytes which are down-regulated in the course of time in relation to the available energy. After the summer feeding period, when energy levels are highest, plaice shows a second recruitment phase. In herring, no difference was observed in the oocyte development between autumn and winter spawners, although winter spawners continue developing oocytes and spawn fewer but larger eggs. The income breeding horse mackerel Trachurus trachurus utilises food resources during spawning although the first batch of spawned eggs is developed on stored energy.

Food availability, through the body condition, is the most important factor regulating fecundity. In situations where food is available during the spawning season traditional determinate spawners may switch to a pseudo-indeterminate fecundity style. In conclusion this thesis shows that fecundity type of marine fish females is not fixed at the species level but represents a plastic response to the environment through food availability and energy allocation.

Optimisation of selective breeding program for Nile tilapia (Oreochromis niloticus)
Trong, T.Q. - \ 2013
Wageningen University. Promotor(en): Johan van Arendonk, co-promotor(en): Hans Komen. - S.l. : s.n. - ISBN 9789461735447 - 176
oreochromis niloticus - selectief fokken - veredelingsprogramma's - genetische parameters - voortplantingskenmerken - kuitschieten - voortplantingspotentieel - vruchtbaarheid - heritability - genotype-milieu interactie - groeitempo - dierveredeling - visteelt - aquacultuur - oreochromis niloticus - selective breeding - breeding programmes - genetic parameters - reproductive traits - spawning - fecundity - fertility - heritability - genotype environment interaction - growth rate - animal breeding - fish culture - aquaculture

The aim of this thesis was to optimise the selective breeding program for Nile tilapia in the Mekong Delta region of Vietnam. Two breeding schemes, the “classic” BLUP scheme following the GIFT method (with pair mating) and a rotational mating scheme with own performance selection and natural group spawning, were investigated. In the latter scheme, the aim was to mimic natural spawning conditions of Nile tilapia to reduce the time for family production; however reconstruction of pedigrees using DNA markers to monitor inbreeding is required. Parental assignment using microsatellites and SNPs showed that exclusion- and likelihood-based methods are equally good for parental assignment, provided that good marker sets with high exclusion power, such as SNPs, are available and that all parents are sampled. Prolonged family production is problematic in BLUP breeding value estimation and could be a consequence of selection for harvest weight in Nile tilapia. Using a natural mating design with single males mated to multiple females in groups, 85% of the successful spawns were collected within 20 days. Genetic correlations between harvest weight and spawning success ranged from 0.48 to 0.52, provided that the mating period is limited to 20-32 days. We conclude that Nile tilapia favour mating in groups, and that selection for harvest weight in GIFT should improve spawning success of Nile tilapia. Moreover, harvest weight and body weight at spawning have favourable genetic correlations with number of eggs, relative fecundity, and number of swim-up fry, which are the desired characteristics for Nile tilapia seed production. High-input cages and low-input ponds are the dominant production systems for tilapia in the Mekong Delta. We show that selection in nucleus ponds will produce desired correlated responses in Nile tilapia grown in river-cages. Moreover, they are expected to develop a more rotund and thicker body shape at the same length compared to fish grown in ponds. In conclusion, we recommend the use of the ‘single male, multiple females’ mating as this will reduce the generation interval by 2 months, thereby increasing genetic gain by about 20%. A rotational mating scheme, with at least 4 cohorts, can be incorporated into the GIFT selection scheme to further reduce inbreeding, to estimate pond effects and to secure the breeding material. Finally, a reliable multiplier system is important to sustain the current Nile tilapia breeding program, which can provide sufficient improved fry (>50 million per year) for the whole Mekong Delta Nile tilapia production.

Effects of a botanical larvicide derived from Azadirachta indica (the neem tree) on oviposition behaviour in Anopheles gambiae s.s. mosquitoes
Howard, A.F.V. ; Adongo, E.A. ; Vulule, J. ; Githure, J. - \ 2011
Journal of Medicinal Plants Research 5 (2011)10. - ISSN 1996-0875 - p. 1948 - 1954.
malaria vector mosquitos - western kenya - body-size - diptera-culicidae - culex mosquitos - sensu-stricto - giles - volatiles - stephensi - fecundity
More focus is given to mosquito larval control due to the necessity to use several control techniques together in integrated vector management programmes. Botanical products are thought to be able to provide effective, sustainable and cheap mosquito larval control tools. However, bio-larvicides like Azadirachta indica (neem) could repel adult mosquitoes from laying their eggs in the treated larval habitats. In this study the response of Anopheles gambiae s.s. mosquitoes towards varying doses of crude aqueous neem extracts was examined. Non-choice oviposition tests were used to measure the proportion of mosquitoes laying on the first or second night, or not laying at all, when compared to the control. For each individual mosquito, the number of eggs laid and/or retained in the ovary was counted to determine the relationship between wing length and egg production. Larger female mosquitoes produced larger egg batches. The results show that at a dose of 0.1 g/l, a concentration previously found to be effective at controlling mosquito larvae, the oviposition behaviour of adult female mosquitoes was not significantly affected. The results indicate that the mosquitoes would expose progeny to this neem control tool, making the use of these simple neem wood extracts effective and potentially sustainable.
Linking individual phenotype to density-dependent population growth: the influence of body size on the population dynamics of malaria vectors
Russell, T.L. ; Lwetoijera, D.W. ; Knols, B.G.J. ; Takken, W. ; Killeen, G.F. ; Ferguson, H.M. - \ 2011
Proceedings of the Royal Society. B: Biological Sciences 278 (2011)1721. - ISSN 0962-8452 - p. 3142 - 3151.
plasmodium-falciparum malaria - anopheles-gambiae - multimodel inference - culicidae - diptera - transmission - arabiensis - mosquitos - fecundity - abundance
Understanding the endogenous factors that drive the population dynamics of malaria mosquitoes will facilitate more accurate predictions about vector control effectiveness and our ability to destabilize the growth of either low- or high-density insect populations. We assessed whether variation in phenotypic traits predict the dynamics of Anopheles gambiae sensu lato mosquitoes, the most important vectors of human malaria. Anopheles gambiae dynamics were monitored over a six-month period of seasonal growth and decline. The population exhibited density-dependent feedback, with the carrying capacity being modified by rainfall (97% wAICc support). The individual phenotypic expression of the maternal (p = 0.0001) and current (p = 0.040) body size positively influenced population growth. Our field-based evidence uniquely demonstrates that individual fitness can have population-level impacts and, furthermore, can mitigate the impact of exogenous drivers (e.g. rainfall) in species whose reproduction depends upon it. Once frontline interventions have suppressed mosquito densities, attempts to eliminate malaria with supplementary vector control tools may be attenuated by increased population growth and individual fitness
Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice
Rijnsdorp, A.D. ; Damme, C.J.G. van; Witthames, P.R. - \ 2010
ICES Journal of Marine Science 67 (2010). - ISSN 1054-3139 - p. 1931 - 1938.
cod gadus-morhua - pleuronectes-platessa l - maturation reaction norms - annual egg-production - atlantic cod - broadcast spawner - fecundity - growth - size - age
A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed.
The effects of, and interactions between, Cardinium and Wolbachia in the doubly infected spider mite Bryobia sarothamni (Acari: Tetranychidae)
Ros, V.I.D. ; Breeuwer, J.A.J. - \ 2009
Heredity 102 (2009)4. - ISSN 0018-067X - p. 413 - 422.
induced cytoplasmic incompatibility - tetranychus-urticae - drosophila-simulans - host genotype - nasonia-vitripennis - bacterial symbiont - haplodiploid mite - dna amplification - genome sequence - fecundity
Many arthropods are infected with vertically transmitted, intracellular bacteria manipulating their host's reproduction. Cytoplasmic incompatibility (CI) is commonly observed and is expressed as a reduction in the number of offspring in crosses between infected males and uninfected females (or females infected with a different bacterial strain). CI is often related to the presence of Wolbachia, but recent findings indicate that a second reproductive parasite, Cardinium, is also capable of inducing CI. Although both Wolbachia and Cardinium occur in arthropods and may infect the same host species, little is known about their interactions. We observed Wolbachia and Cardinium in the sexual spider mite Bryobia sarothamni (Acari: Tetranychidae) and investigated the effects of both bacteria on reproduction. We performed all possible crossing combinations using naturally infected strains, and show that Cardinium induces strong CI, expressed as an almost complete female mortality. B. sarothamni is the third host species in which Cardinium-induced CI is observed, and this study reveals the strongest CI effect found so far. Wolbachia, however, did not induce CI. Even so, CI was not induced by doubly infected males, and neither singly Wolbachia-infected nor doubly infected females could rescue CI induced by Cardinium-infected males. Possibly, this is related to the differences between Cardinium strains infecting singly and doubly infected individuals. We found a cost of infection in single infected individuals, but not in doubly infected individuals. We show that infection frequencies in field populations ranged from completely uninfected to a polymorphic state. In none of the populations infections were fixed
Stock and recruitment in North Sea herring (Clupea harengus); compensation and depensation in the population dynamics
Nash, R.D.M. ; Dickey-Collas, M. ; Kell, L.T. - \ 2009
Fisheries Research 95 (2009)1. - ISSN 0165-7836 - p. 88 - 97.
cod gadus-morhua - pelagic fisheries - fish stocks - atlantic - management - growth - environment - fecundity - patterns - recovery
The recovery of a stock after severe exploitation is of major interest to fish ecologists and managers alike. Understanding the dynamics of recruitment at low stock sizes is crucial to the simulation of stocks as they recover. Compensation in recruitment has occurred in North Sea herring, and it was stronger after the collapse of the stock. The compensation appears to be a product of both increased production of larvae per spawner and increased survival to the juvenile stage. There is only slight evidence for depensation and the point at which North Sea herring has zero recruitment appears close to the origin. There is more variability in recruits per unit spawning stock size when the stock is smaller, this is probably as a result of the potential larger diversity in contributions from spawning components in an unexploited stock compared to an overexploited stock. Mimicking this dynamic in population models will increase the uncertainties in a projection of stock recovery. The lack of observations at higher stock sizes hinders our ability to compare the dynamics of recruit to SSB across the full range of stock sizes, however, recent apparent over-compensation at higher spawning biomass has resulted in less than one recruit per mature adult being produced per year.
The presence of Plasmodium falciparum gametocytes in human blood increases the gravidity of Anopheles gambiae mosquitoes
Ferguson, H.M. ; Gouagna, L.C. ; Obare, P. ; Read, A.F. ; Babiker, H. ; Githure, J.I. ; Beier, J.C. - \ 2005
American Journal of Tropical Medicine and Hygiene 73 (2005)2. - ISSN 0002-9637 - p. 312 - 320.
western kenya - yoelii-nigeriensis - aedes-aegypti - seasonal transmission - malaria transmission - egg development - body-size - infection - fecundity - stephensi
We conducted a field study in an area of endemic malaria transmission in western Kenya to determine whether mosquitoes that feed on gametocyte-infected blood but do not become infected have reduced or enhanced fecundity in comparison to mosquitoes fed on uninfected blood. Fifteen paired membrane-feeding experiments were conducted in which two strains of Anopheles gambiae mosquitoes were simultaneously fed on either Plasmodium falciparum¿infected blood from children or uninfected control blood from adults. The presence of noninfecting gametocytes in blood increased the probability that An. gambiae would produce eggs after one blood meal by sixfold (odds ratio for control relative to infected blood group 0.16; 95% CI 0.10¿0.23). This result could not be explained by variation in blood meal size or hemoglobin content between hosts. When children cleared their infections, the difference in gravidity between mosquitoes fed on their blood and uninfected adults disappeared, suggesting this phenomenon is due to the presence of Plasmodium gametocytes in blood and not to host-specific factors such as age. This result was observed in two mosquito strains that differ in their innate fecundity, suggesting it may apply generally. To our knowledge, this is the first time that Plasmodium has been implicated as enhancing vector gravidity
Fisheries-induced adaptive change in reproductive investment in North Sea plaice (Pleuronectes platessa)?
Rijnsdorp, A.D. ; Grift, R.E. ; Kraak, S.B.M. - \ 2005
Canadian Journal of Fisheries and Aquatic Sciences 62 (2005)4. - ISSN 0706-652X - p. 833 - 843.
cod gadus-morhua - life-history evolution - beam-trawl effort - reaction norms - somatic growth - trade-offs - maturation - fecundity - size - age
Life history theory predicts that fishing may select for increased reproductive investment. A model of the reaction norm for reproductive investment in a capital breeder was developed to disentangle changes in reproductive investment from changes in growth rate in North Sea plaice (Pleuronectes platessa). Trends in reproductive investment since 1960 were estimated as (i) the decrease in body weight of mature males and females between the start and end of the spawning period, (ii) the difference in weight of ripe and spent females, and (iii) the ovary weight of prespawning females. These estimates were related to somatic growth estimated by back-calculation of otoliths and temperature. The ovary weight and weight loss of females that had just started and just finished spawning did not reveal any trends. There was a significant increase in weight loss over the spawning season in both sexes, but much of this increase was likely due to changes in environmental conditions. Evidence for a fisheries-induced change in reproductive investment from our analyses thus remained inconclusive. However, fecundity and ovary-weight data from previous studies tentatively suggest that an increase in reproductive investment occurred between the late 1940s and the 1960s. Such an increase is consistent with a fisheries-induced evolutionary change
Reduced competitive ability due to Wolbachia infection in the parasitoid wasp Trichogramma kaykai
Huigens, M.E. ; Hohmann, C.L. ; Luck, R.F. ; Gort, G. ; Stouthamer, R. - \ 2004
Entomologia Experimentalis et Applicata 110 (2004)2. - ISSN 0013-8703 - p. 115 - 123.
microbe-associated parthenogenesis - cytoplasmic incompatibility - drosophila-melanogaster - physiological cost - sex allocation - hymenoptera - fecundity - superparasitism - populations - california
Several hymenopteran parasitoids are infected with parthenogenesis-inducing (PI) Wolbachia. Infected wasps produce daughters instead of sons from unfertilized eggs. Thus far, little is known about the direct effects of PI Wolbachia on their host's fitness. Here, we report reduced competitive ability due to Wolbachia infection in a minute parasitoid wasp, Trichogramma kaykai Pinto and Stouthamer (Hymenoptera: Trichogrammatidae). Immature survival of infected individuals in a host parasitized by a single infected female, laying a normal clutch of eggs, was lower than those parasitized by a single uninfected individual. When the offspring of infected and uninfected females shared the same host, the infected immatures had significantly lower survival rates than their uninfected counterparts. The survival rate of infected immatures was higher when they competed with other infected immatures from a different infected parent than in competition with uninfected immatures of conspecific wasps. Thus, the host Trichogramma can suffer a substantial reduction in fitness when it is infected with the PI Wolbachia. We discuss why such a reduction is to be expected when populations of infected and uninfected individuals co-occur, and how the reduced competitive ability of PI Wolbachia influences the spread of the bacteria in the field
Mosquito appetite for blood is stimulated by Plasmodium chabaudi infections in themselves and their vertebrate hosts
Ferguson, H.M. ; Read, A.F. - \ 2004
Malaria Journal 3 (2004). - ISSN 1475-2875 - 8 p.
leishmania-mexicana-amazonensis - anopheles-stephensi mosquitos - la-crosse virus - malaria parasite - aedes-aegypti - yoelii-nigeriensis - brugia-malayi - fecundity - gallinaceum - gambiae
Background - Arthropod vectors of disease may encounter more than one infected host during the course of their lifetime. The consequences of super-infection to parasite development are rarely investigated, but may have substantial epidemiological and evolutionary consequences. Methods - Using a rodent malaria model system, behavioural avoidance of super-infection was tested by examining whether already-infected Anopheles stephensi mosquitoes were less responsive to new vertebrate hosts if they were infected. Additionally, a second dose of parasites was given to malaria-infected mosquitoes on a biologically realistic time scale to test whether it impeded the development of a first infection. Results - No effect of a second infected blood meal on either the prevalence or parasite burden arising from a first was found. Furthermore, it was found that not only were infected mosquitoes more likely to take a second blood meal than their uninfected counterparts, they were disproportionately drawn to infected hosts. Conclusions - The alterations in mosquito feeding propensity reported here would occur if parasites have been selected to make infected vertebrate hosts more attractive to mosquitoes, and infected mosquitoes are more likely to seek out new blood meals. Although such a strategy might increase the risk of super-infection, this study suggests the cost to parasite development is not high and as such would be unlikely to outweigh the potential benefits of increasing the contact rate between the parasite's two obligate hosts.
Host preference of Callosobruchus maculatus: a comparison of life history characteristics for three strains of beetles on two varieties of cowpea
Boeke, S.J. ; Loon, J.J.A. van; Huis, A. van; Dicke, M. - \ 2004
Journal of Applied Entomology 128 (2004)6. - ISSN 0931-2048 - p. 390 - 396.
f coleoptera - fabricius coleoptera - egg-production - seed beetle - bruchidae - oviposition - susceptibility - resistance - fecundity - size
The reproductive success of Callosobruchus maculatus Fabricius, the main insect pest of stored cowpea, may vary between strains of this beetle and between varieties of the host seeds. Life history parameters of beetle strains from three different origins in West Africa were compared on two susceptible varieties of cowpea, Vigna unguiculata (L.) Walp. All beetle strains were assayed in a no-choice and a two-choice test. No major differences were found between the beetle strains. In a no-choice situation, the developmental period from egg to adult was prolonged on the bean variety Kpodjiguegue. In a two-choice situation, the beetles showed a strong preference for the Californian blackeyed bean variety to oviposit on. Here again the development took longer on Kpodjiguegue beans and the intrinsic rate of increase of the beetle population was lower. Using either equal numbers of beans of the same size or equal weights of beans of undetermined size of the two bean varieties did not affect the outcome of the test.
Fitness effects of Alternaria dauci on wild carrot in The Netherlands
Schouten, H.J. ; Tongeren, C.A.M. van; Bulk, R.W. van den - \ 2002
Environmental Biosafety Research 1 (2002). - ISSN 1635-7922 - p. 39 - 47.
biosafety - carrot leaf blight - Daucus carota - fecundity
Comparison of a Canadian and a Dutch strain of the parasitoid Aphelinus mali (Hald) (Hym., Aphelinidae) for control of woolly apple aphid Eriosoma lanigerum (Haussmann) (Hom., Aphididae) in the Netherlands: a simulation approach
Mols, P.J.M. ; Boers, J.M. - \ 2001
Journal of Applied Entomology 125 (2001)5. - ISSN 0931-2048 - p. 255 - 262.
pieris-rapae - host - requirements - temperature - fecundity - size
Woolly apple aphid, Eriosoma lanigerum is one of the important apple pests in the Netherlands. Weather conditions and natural enemies determine whether woolly apple aphid (WAA) will reach pest status. WAA may escape control by natural enemies and therefore it must be controlled using chemical insecticides. To prevent unnecessary applications of insecticides and to promote biological and natural control of WAA more knowledge is needed about the role of natural enemies, weather and their effects on the development of WAA populations. The monophagous parasitoid Aphelinus mali (Hald.) has been introduced into most of apple growing areas to control WAA, but success is variable and depends on climatological conditions. In the Netherlands the level of parasitization is often too low, especially after warm winters. The biological control potential of a strain of A. mali from Nova Scotia (Canada) was compared with a Dutch strain by simulating population growth of both WAA and the Dutch and Canadian strain of the parasitoid for three different years The results indicate that the Canadian strain would perform in general better than the Dutch strain under Dutch weather conditions.
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