Host instar and host plant effects on Aphidius colemani

Abstract:  The effect of Myzus persicae age (I–IV instar nymphs) on parasitism by Aphidus colemani was investigated in choice experiments with either aubergine ( Solanum melongena L.) or pepper ( Capsicum annuum L.) as the aphid host plant species. Aphidius colemani could develop successfully to the mummy stage in all ages of aphid tested, on both pepper and aubergine plants. The greatest proportion of parasitoid mummies was found with II and III aphid instars on pepper and on I, II and III aphid instars on aubergine. On aubergine, I instar nymphs were also parasitized to an appreciable degree. Successful parasitism of IV instar nymphs was consistently low on both crops. The potential implications of host plant effects on the effectiveness of A. colemani as a biological control agent are discussed.     

Host selection by winged colonisers within the Myzus persicae group: a contribution towards understanding host specialisation

Abstract.  1.  Myzus persicae sensu lato demonstrates considerable genetic variation in respect to adaptation to host plants. The subspecies M. persicae nicotianae shows a preference for tobacco, while M. persicae sensu stricto ( s. str. ) for other herbaceous plants. Given that winged colonisers of several aphid species play an important role in selecting host plants, here their role in the host specialisation observed in M. persicae was examined in choice and no-choice tests conducted outdoors, in performance studies, and in DC Electrical Penetration Graph (DC-EPG) studies.
2. In outdoor choice tests, 77% of spring migrants of M. persicae nicotianae chose tobacco, whereas equal proportions of M. persicae s. str. selected tobacco and pepper. In no-choice tests, spring migrants settled more quickly after alighting on host rather than on non host plants, and significantly more alate M. persicae s. str. (27%) than M. persicae nicotianae (2%) left tobacco after walking briefly on the leaf surface, whilst no significant differences were found on pepper. Cross-host transfers significantly reduced the fecundity of both summer and spring migrants of the two subspecies. Finally, the results of no-choice tests and DC-EPG studies showed that winged aphids distinguished their host through cues located on the plant surface or in subcutaneous tissues perceived prior to the initiation of feeding.
3. This study demonstrates the important role of winged colonisers in the evolution of host specialisation in M. persicae . The multifarious divergent selection that the two host forms experience, i.e. the selection against cross-migrants and their subsequent generations, is a crucial factor involved in the development and maintenance of host specialisation and promotes the parallel evolution of improved host-recognition ability.     

The effect of parasitoid host-size preference on host population growth rates: an example of Aphidius colemani and Aphis glycines

Abstract.  1. Direct observations of Aphidius colemani foraging for Aphis glycines demonstrated that A. colemani attacks large aphid size classes selectively, in contrast to other Aphidiinae (Hymenoptera: Braconidae) species that generally prefer small or medium-sized nymphs of different host species.
2. To determine the effect of this size preference on the potential ability of A. colemani to control A. glycines outbreaks, the stage-dependent survival and fecundity of A. glycines were measured to parameterise a stage-structured demographic model for the density-independent population growth rate of the aphid.
3. Compared with hypothetical parasitoids that show either no size preference or preference for medium-sized host nymphs, the preference of A. colemani for large hosts caused a greater reduction in the population growth rate of A. glycines . This occurs in the model because, by attacking reproductive adults, A. colemani kills those aphids that have the greatest immediate effect on the population growth rate.
4. The strong effect of size preference by A. colemani is diminished somewhat by the continued reproduction of A. glycines adults, which can reproduce for up to 3 days following parasitism. Nonetheless, reproduction by parasitised aphids is not enough to compensate for the stronger, negative effect of the preference of A. colemani for large aphids, which removes individuals with the greatest reproductive value from the A. glycines population.     

Population dynamics of cereal aphids: influence of a shared predator and weather

1 Aphid populations may show strong year-to-year fluctuations, but questions remain regarding the dominance of factors that cause this variation, especially the role of natural enemies. To better understand the dynamics of aphid species that occur as pests in cereals, we investigated the relative influence of top-down control by a predator and weather (temperature and precipitation) on population fluctuations of three cereal aphid species.
2 From 1987 to 2005, populations of Metopolophium dirhodum , Sitobion avenae and Rhopalosiphum padi in insecticide-free stands of winter wheat were monitored in the Praha-Ruzyné region of the Czech Republic. Densities of an aphidophagous predator, the ladybeetle Coccinella septempunctata , were recorded from an overwintering site in the landscape. Weather was quantified using historical records.
3 A significant bottom-up effect of densities of aphids on those of C.   septempunctata was found, but evidence of direct top-down regulation of aphids by C.   septempunctata was only significant in the case of R.   padi . There was no significant periodicity in the dynamics of the aphid or C.   septempunctata , suggesting that there was no clear predator-prey cycle. Combinations of C.   septempunctata and weather variables could be used to explain M.   dirhodum and R.   padi per capita rate of change. There were also indications that weather directly affected peak density of M.   dirhodum .
4 We conclude that regional estimates of C.   septempunctata densities are not sufficient to determine whether aphid population dynamics are driven by predator–prey interactions. Feasibility of time series analysis as an investigative tool in aphid population dynamics studies is discussed.     

Barley exposed to aerial allelopathy from thistles (Cirsium spp.) becomes less acceptable to aphids

Abstract.  1. Recent studies have shown that plant–plant interaction via chemicals (allelopathy) can affect insects. Here the effects on aphid acceptance of barley after exposure to volatiles and root exudates from two common weeds, the thistles Cirsium arvense and Cirsium vulgare , were investigated.
2. Settling by bird cherry-oat aphid, Rhopalosiphum padi , was significantly reduced on barley plants that had been exposed to volatiles from Cirsium species for 5 days. Settling by Sitobion avenae was also reduced on Cirsium -exposed plants, whereas settling by Metopolophium dirhodum was not.
3. In olfactometer tests, Cirsium -exposed barley was significantly less attractive to R. padi than was unexposed barley, indicating that exposure causes a change in the volatile profile of barley.
4. Exposure of barley to root exudates from Cirsium species had no effect on R. padi settling.
5. The results lend weight to the theory that the effects of plant–plant allelopathy can extend to higher trophic levels.     

Behavior of bird cherry-oat aphid and green peach aphid in relation to potato virus Y transmission

Potato virus Y is transmitted to potato in a nonpersistent manner by many aphid species, some of which do not colonize this crop. The behavior of bird cherry-oat aphid, Rhopalosiphum padi (L.) on potato, Solanum tuberosum L., a plant species that is not colonized by this aphid, was described and compared with that of the potato-colonizing green peach aphid, Myzuspersicae (Sulzer). A higher proportion of winged morph of R. padi than M. persicae left the plant, but aphids that stayed in contact with the plant took the same mean time to initiate the first probe and it lasted the same mean time compared with M. persicae. Electronic penetration graph technique was used to study the probing behavior of the aphids during Potato virus Y (family Potyviridae, genus Potyvirus, PVY) transmission tests. Transmission rate decreased from 29 to 8% when the acquisition time increased from 5 min of continuous probing to 1 h with M. persicae, but it remained low (2 and 1%) with R. padi. Most of the difference in transmission rate between acquisition time with M. persicae and between aphid species was related to the change in the time and behavior taking place between the last cell puncture of the acquisition phase to the first cell puncture of the inoculation phase. Results presented here clearly demonstrated the importance of host plant selection and probing behavior in the transmission of nonpersistent plant viruses. They also stress the need to consider the behavior of the aphid in the design of laboratory tests of virus vector efficacy.     

The effect of banker plant species on the fitness of Aphidius colemani Viereck and its aphid host (Rhopalosiphum padi L.)

Banker plants, a type of open-rearing unit, are increasingly used in greenhouse crops to sustain natural enemy populations at times of low pest abundance. The most common banker plant system is a non-crop, cereal plant which supports Rhopalosiphum padi L. as an alternative host for Aphidius colemani Viereck. Although bottom-up effects of plants are known to affect natural enemies, this aspect has generally been ignored in previous investigations of banker plant efficacy. Here, we tested four cereal plant species with three varieties each to investigate host plant effects on R. padi and A. colemani. Though limited differences were observed in laboratory experiments spanning one aphid or parasitoid generation, longer greenhouse experiments spanning several generations revealed significant plant effects on both insects. R. padi performed poorly on oats (Avena sativa L.), resulting in wasps with the longest female development time, lowest emergence rates, and the lowest number of wasps produced per unit. Rye (Secale cereal L.) – intermediate in terms of aphid performance – produced a significantly male-biased wasp population with the smallest males. Conversely, R. padi placed onto either wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) performed consistently well in terms of aphid and parasitoid fitness and abundance, though neither species was obviously superior over the other. Overall, cultivars within each plant species did not significantly affect outcomes. As each plant species tested had different positive effects on aphid and parasitoid phenotypes, the potential benefits of mixing of cereal species is an area for future investigation.     

Strategies for the Control of Aphis gossypii Glover (Hom.: Aphididae) with Aphidius colemani Viereck (Hym.: Braconidae) in Protected Cucumbers

Aphis gossypii Glover (Hom.: Aphididae) is a damaging pest of protected cucumbers in the UK, and control measures are required which are compatible with other components in the overall cucumber integrated pest management programme. Two methods of establishing the parasitoids, Aphidius colemani Viereck (Hym.: Braconidae), in cucumber crops prior to invasion of the aphids were developed. The first involved weekly releases of small numbers of parasitoids beginning before A. gossypii became established on the plants. The second method used open-rearing units based on maize, wheat and rygrass plants infested with Rhopalosiphum padi L (Hom.: Aphididae), which is a common host to many natural enemies of A. gossypii but not a threat to the cucumber crop. Both methods were found to be more efficient in summer than in late spring. Parasitoid release rates were established for the two control methods at both of these times of the year. Like all biological control measures, these methods will require careful management in practice and some fine-tuning to suit individual crop production systems.     

Snowdrop lectin (Galanthus nivalis agglutinin) in aphid honeydew negatively affects survival of a honeydew- consuming parasitoid

1 Insecticidal proteins can be excreted in the honeydew when sap-sucking insects feed on insect-resistant transgenic plants. Honeydew can be an important source of carbohydrates, thus potentially exposing a broad range of honeydew-feeding insects to transgene products.
2 Snowdrop lectin ( Galanthus nivalis agglutinin; GNA) dissolved in a 2 m sucrose solution had no antifeedant effect on female aphid parasitoids ( Aphidius ervi ) but had a direct negative effect on their longevity.
3 When feeding on honeydew from Rhopalosiphum padi feeding on a GNA-containing artificial diet, Aphidius ervi suffered a longevity reduction that was more pronounced than was to be expected based on the detected GNA concentration in the honeydew.
4 Analysis of carbohydrate and amino acid composition revealed that a change in honeydew composition caused by a GNA-effect on the aphids could be a possible explanation for the additional reduction in parasitoid longevity.
5 When comparing the effect of honeydew from Sitobion avenae and R. padi feeding on GNA-expressing or nontransformed wheat plants on A. ervi longevity, aphid species was found to have a significant effect, whereas the wheat variety had no effect. The latter result was probably due to low GNA expression levels in the plants. Differences in nutritional suitability between honeydew from R. padi and S. avenae could be explained by differences in carbohydrate and amino acid composition.
6 This is the first study to demonstrate that GNA ingested by aphids and transported into the honeydew can negatively affect the parasitoids consuming this honeydew.
7 We recommend that honeydew should be considered as a route of exposure to transgene products in future risk assessment studies.     

Development and parasitism of Lysiphlebus testaceipes (Cresson) and Aphidius colemani Viereck (Hymenoptera: Braconidae) on Aphis gossypii Glover (Hemiptera: Aphididae) on two chrysanthemum cultivars

The parasitoids Lysiphlebus testaceipes (Cresson) and Aphidius colemani Viereck play an important role on the regulation of aphid populations on chrysanthemum grown in greenhouses. This study evaluated the interaction of biological control and plant resistance by the development and the parasitism of L. testaceipes and A. colemani on Aphis gossypii Glover. The aphid host was kept on commercial cultivars of chrysanthemum susceptible Yellow Snowdon (YS) and resistant White Reagan (WR). The experiment was carried out in climatic chamber at 22 +/- 1 masculineC, 70 +/- 10% RH, and photophase 12h. Nymphs of 2nd and 3rd instars of A. gossypii attacked only once by the parasitoids were used in the experiment. The development time was 15.0 and 12.9 days for L. testaceipes and 17.0 and 16.3 days for A. colemani kept on A gossypii on cvs. YS and WR, respectively. The parasitoid L. testaceipes showed greater parasitism rate (68.4%) on A. gossypii on cv. YS than on WR (50%), and for A. colemani was not found significative difference on parasitism (46.8 and 35% to YS and WR, respectively). The emergence rates of L. testaceipes and A. colemani were 91.6% and 85.3% on A gossypii reared on YS, and 78.1% and 100% on WR, respectively. The longevity of A colemani and the parasitism rate of L. testaceipes were affected in a negative way by the cv. WR. The results indicated a positive interaction of biological control with parasitoids and the resistant cv. WR on A. gossypii control in protected cultivation.     

Limited scope for maternal effects in aphid defence against parasitoids

Abstract.  1. A mother's environment frequently affects her offspring's phenotype. Such maternal effects may be adaptive, in particular with respect to pathogens or parasites, for example if maternal exposure increases offspring resistance.
2. In aphids, maternal effects are likely to occur as a result of their telescoping generations. This study investigated whether maternal effects influence the susceptibility of the peach-potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), to its parasitoid Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae: Aphidiinae).
3. In a first experiment, susceptibility was compared among offspring of aphid mothers that had either no contact to parasitoids, had contact but were not attacked, or were attacked but not mummified. Mothers from the last group had successfully resisted the parasitoid.
4. In a second experiment using two different clones, maternal and progeny environment were manipulated by rearing each generation either on a benign (radish) or a more stressful host plant (silver beet) before progeny exposure to parasitoids.
5. The first experiment revealed no significant effect of the maternal treatment on offspring susceptibility to parasitoids and thus no evidence for trans-generational defence. In the second experiment, maternal environment effects were also weak, yet with a trend towards less susceptible offspring of aphid mothers reared on the more stressful plant. However, there was a significant difference among clones and a strong clone × progeny host plant interaction, illustrating that the outcome of a parasitoid attack may be determined by a complex interplay of genetic and environmental factors.
6. Overall, the results suggest that there is limited scope for maternal effects in aphid defence against parasitoids.     

Natural enemy ravine revisited: the importance of sample size for determining population growth

Abstract. 1. The population growth of three aphid species, Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.), and Sitobion avenae (F.), on winter wheat, was analysed by regression. The calculations were based on censuses of aphids made in 268 plots at 3- or 7-day intervals for 10 years on leaves and 6 years on ears. The calculations were made separately for each plot each year, then repeated on the pooled data from all plots monitored in a year.
2. At the level of individual plots, no population growth was detected at very low densities. At high densities, the populations grew exponentially and the growth rates did not decrease with increasing aphid density.
3. Significant growth was always detected in the pooled data. These growth rates decreased significantly at the highest densities. Field estimates of the intrinsic rate of increase derived from these data ranged from 0.010 to 0.026 for M. dirhodum , 0.0071–0.011 for R. padi , and 0.00078–0.0061 and 0.0015–0.13 for S. avenae , on leaves and ears respectively .
4. The apparent lack of growth in the individual plots at low densities is attributable to small sample size. It is concluded that the natural enemy ravine in the population dynamics of cereal aphids, identified by Southwood and Comins (1976), is a consequence of low population densities at which population increase is undetectable unless very large samples are taken.     

Endophytic fungi decrease available resources for the aphid Rhopalosiphum padi and impair their ability to induce defences against predators

Abstract.  1. The production of winged morphs is a well known mechanism of induced defence in aphids to escape from natural enemies, and is also a reaction to poor resource quality.
2. Host plants of aphids often associate with endophytic fungi that have been shown to reduce the fitness of some species of aphids.
3. It was hypothesised that endophyte infection of host plants that represent a low quality plant resource should increase the aphid's induced response to a predator because both low plant quality and predator presence represent a stronger cue for wing production than predator presence alone.
4. In a laboratory experiment, bird cherry-oat aphids Rhopalosiphum padi L. were exposed to the factors predator threat and endophyte infection and the effects of these factors on the proportion of winged morphs produced by the aphid colonies was analysed.
5. The presence of endophytic fungi strongly decreased aphid colony sizes. When a predator threat was present, all colonies on endophyte-free grasses produced winged morphs whereas only a few colonies were able to produce winged morphs on endophyte-infected grasses. However, these few colonies produced larger proportions of winged morphs than colonies on endophyte-free grasses. Without a predator threat, no colonies on endophyte-infected grasses produced any winged morphs.
6. These results show that aphids in stressed conditions and with reduced fitness will only invest in inducible defences when predators are present but are unable to produce winged morphs in response to endophyte presence.     

Seasonal abundance of aphids (Homoptera: Aphididae) in wheat and their role as barley yellow dwarf virus vectors in the South Carolina coastal plain

Aphid (Homoptera: Aphididae) seasonal flight activity and abundance in wheat, Triticum aestivum L., and the significance of aphid species as vectors of barley yellow dwarf virus were studied over a nine-year period in the South Carolina coastal plain. Four aphid species colonized wheat in a consistent seasonal pattern. Greenbug, Schizaphis graminum (Rondani), and rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), colonized seedlingwheat immediately after crop emergence, with apterous colonies usually peaking in December or January and then declining for the remainder of the season. These two aphid species are unlikely to cause economic loss on wheat in South Carolina, thus crop managers should not have to sample for the subterranean R. rufiabdominalis colonies. Bird cherry-oat aphid, Rhopalosiphum padi (L.), was the second most abundant species and the most economically important. Rhopalosiphum padi colonies usually remained below 10/row-meter until peaking in February or March. Barley yellow dwarf incidence and wheat yield loss were significantly correlated with R. padi peak abundance and aphid-day accumulation on the crop. Based on transmission assays, R. padi was primarily responsible for vectoring the predominant virus serotype (PAV) we found in wheat. Pest management efforts should focus on sampling for and suppressing this aphid species. December planting reduced aphid-day accumulation and barley yellow dwarf incidence, but delayed planting is not a practical management option. English grain aphid, Sitobion avenae (F.), was the last species to colonize wheat each season, and the most abundant. Sitobion avenae was responsible for late-season virus transmission and caused direct yield loss by feeding on heads and flag leaves during an outbreak year.     

Life history of the bird cherry-oat aphid, Rhopalosiphum padi, on transgenic and non-transformed wheat challenged with Wheat streak mosaic virus

The life history of the bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), was studied via laboratory assays on Wheat streak mosaic virus (WSMV)-infected and non-infected transgenic and non-transformed wheat [ Triticum aestivum L. (Poaceae)]. Although R. padi is not a WSMV vector, it is known to colonize WSMV-infected wheat plants. Two transgenic soft white winter wheat genotypes, 366-D03 and 366-D8, that express the WSMV coat protein gene, and the WSMV-susceptible non-transformed cultivar Daws were tested. All genotypes showed disease symptoms when infected with WSMV. Whereas plant height was significantly reduced on virus-infected compared to non-infected plants of all genotypes, virus-infected transgenic plants exhibited lower virus titer and lower disease rating scores than Daws. No significant effects of WSMV infection or genotypes were observed on the length of R. padi nymphal development period, nor on their pre-, and post-reproductive periods. Rhopalosiphum padi reproductive period was significantly longer on Daws infected with WSMV than on non-infected plants of this cultivar. In contrast, there were no significant differences in length of R. padi reproductive period between virus-infected and non-infected transgenic plants within a genotype. Rhopalosiphum padi daily fecundity was significantly lower and adult longevity significantly longer on virus-infected than on non-infected plants of all genotypes. Total aphid fecundity and intrinsic rate of increase were not significantly different among treatments. The percentage of winged aphids that developed was greater on WSMV-infected compared to non-infected plants within a genotype. Results indicate that both virus infection status of plants and wheat genotype influence the life history of R. padi.     

Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.
2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well-watered A. elatius was, however, lower than that of well-watered D. glomerata , correlating with the decreased performance of aphids on well-watered A. elatius .
3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.
4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well-watered D. glomerata , there was a reduction in aphids feeding on drought-stressed D. glomerata and drought-stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well-watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.     

Contact with the host plant enhances aphid survival at low temperatures

1. The hypothesis, suggested by previous studies, that host plant contact reduces the cold tolerance of anholocyclic aphids was tested under laboratory conditions. Adult and first-instar Rhopalosiphum padi (L.) were exposed to temperatures of 0, –5 and –10 °C on intact plants, excised leaves and in the absence of contact with plant material.
2. Median lethal time (LT₅₀) values at all three temperatures indicate that aphids exposed in association with plant material survive longer than aphids that have no contact with their host plant. The difference in survival was most pronounced at –10 °C. Therefore, the above hypothesis is rejected for aphids on cereals because host plant contact apparently enhances cold tolerance.
3. Exposure on excised leaves also enhanced aphid survival at low temperature but was less effective than the intact plant. This suggests that plant quality as well as the presence or absence of plants is important in the cold tolerance of aphids on cereals.     

Performance of Myzus persicae (Hemiptera: Aphididae) clones on different host-plants and their host preference

The performance of eighteen clones of Myzus persicae (Sulzer) on pepper and tobacco plants at 20 degrees C and L16:D8 and the choice of young adult apterae between tobacco and pepper leaf-discs were examined. The clones were collected from weeds and peach in two tobacco-growing regions: Katerini, northern Greece and Karditsa, central Greece (only from weeds) and from Lehonia, central eastern Greece where tobacco is not cultivated. All clones did well on both hosts. However, the analysis of data revealed a significant effect of "region / host plant origin" on aphid performance. The mean values of adult weight, intrinsic rate of increase and fecundity of the clones collected in Lehonia and reared on tobacco were significantly lower than the observed values for clones from Katerini and Karditsa. Aphids from Lehonia had significantly higher mean values for developmental time on tobacco than clones from the other regions whereas the opposite was observed when aphids were reared on pepper. Aphids collected in Lehonia performed better on pepper than those originating from the tobacco-growing regions. A choice test revealed differences among the clones originating from different regions. Fifty three percent and 43% of aphids from weeds and peach from Lehonia, respectively, chose pepper. By comparison 41.5% and 40.0% of aphids from peach and weeds from Katerini, respectively and 49.5% of aphids from Karditsa preferred tobacco. The results are discussed in relation to host specialization in M. persicae.     

Host-related fitness trade-offs in a presumed generalist parasitoid, Diaeretiella rapae (Hymenoptera: Aphidiidae)

Abstract.  1. Host ranges of parasitoid wasps are mediated by behavioural responses to hosts and their environment (infectivity), and development in hosts (virulence). Determinants of host range were measured in Diaeretiella rapae (Hymenoptera: Aphidiidae), which has been described as a generalist that attacks more than 60 species.
2. In northern Colorado, this wasp mainly attacks two hosts: cabbage aphid ( Brevicoryne brassicae ) and Russian wheat aphid ( Diuraphis noxia ). Here, laboratory experiments are described in which D. rapae originating from these two hosts were offered several hosts for oviposition. Both infectivity and virulence were measured.
3. Infectivity included host acceptance and handling time, while virulence was measured as productivity (number of progeny), survival of immatures within hosts, development time, and sex ratio.
4. Wasps had higher productivity and survival when attacking 'home' hosts than 'alternate' hosts, and trade-offs were found by quantitative genetic analyses to be genetically determined. Sex ratio and development times also showed trade-offs, but mainly related to the host environment in which females were reared.
5. In previous genetic studies in northern Colorado, populations were genetically subdivided on the scale of 1 km. The fitness differences described here could be strong enough to create populations adapted to different hosts, but it appears that gene flow is sufficient to prevent formation of separate lineages on the two hosts.
6. Rather than being a generalist with a broad host range, D. rapae is a serial specialist, attacking particular hosts according to availability in different seasons or in different geographical areas.