大气二氧化碳浓度变化对禾谷缢管蚜种群动态的影响

利用开顶式熏气室研究了大气CO₂浓度和土壤水分对禾谷缢管蚜Rhopalosiphum padi (L.)种群动态的影响,并分析了禾谷缢管蚜密度与被处理小麦叶片化学成分的关系。结果表明：（1）禾谷缢管蚜种群密度随CO₂浓度升高而持续增大并与土壤水分密切相关,各CO₂浓度下均以60%田间持水量时的密度最大;（2）CO₂和土壤水分对小麦叶片化学成分有明显的影响,麦叶水分、可溶性蛋白质、可溶性糖、淀粉含量随CO₂浓度和土壤水分含量上升而增加,纤维素含量随CO₂浓度上升而增加、随土壤水分含量上升而降低,单宁、丁布（DIMBOA）含量在CO₂浓度为550 μl/L时最高,但单宁含量随土壤水分上升而增加,丁布含量在60%田间持水量时最低;（3）禾谷缢管蚜密度与叶片水分、可溶性蛋白质、可溶性糖、淀粉含量呈正相关,与丁布、单宁含量呈负相关。结论：在未来的气候条件下,随着CO₂浓度升高禾谷缢管蚜种群可能会持续增长,这种增长在半干旱区更加突出。禾谷缢管蚜种群增长的原因之一是大气CO₂和土壤水分条件改变了植物的化学成分构成。     

Co‐occurrence of defensive traits in the potato aphid Macrosiphum euphorbiae

1. Insecticide usage selects strongly for resistance in aphid populations, but this could entail fitness costs in other resistance traits. The potato aphid Macrosiphum euphorbiae Thomas exhibits intraspecific variation in susceptibility to parasitism by braconid wasps and provides a suitable species to study the relation between the defensive traits of parasitism and insecticide resistance. 2. Clonal lines (23 in total) of M. euphorbiae were established from aphids collected in 2013 from geographically separate populations in the U.K. Clonal lines belonged to five aphid genotypes, but one genotype predominated (78% of samples), and the facultative endosymbiont Hamiltonella defensa was detected in c. 40% of lines. 3. Total esterase activity in aphid tissues varied significantly between aphid genotypes and collection areas, but there was no clear pattern in relation to H. defensa infection or between collection sites likely to differ in insecticide pressures. 4. Five clonal lines representing low to moderate levels of enzyme activity, which included different aphid genotypes and presence/absence of H. defensa infection, were assayed for their susceptibility to the parasitoid wasp Aphidius ervi Haliday. Aphid mummification varied significantly between aphid genotypes, with low values in one genotype of aphids irrespective of H. defensa presence. 5. The results revealed that aphid lines belonging to the parasitism‐resistant genotype exhibited moderate levels of total esterase activity, indicating a competitve advantage for this genotype of M. euphorbiae when exposed to chemical and biological control factors in agroecosystems.     

Some factors affecting the build-up of cereal aphid infestations in winter wheat

Aphid populations were measured at 14-day intervals during May to July 1974 in 17 fields of wheat at 10 sites along an 85 km transect running east from Hertfordshire to the Essex coast. Metopolophium dirhodum was the most numerous species initially, but was later superseded by Sitobion avenae. Rhopalosiphum padi was third in order of abundance. Several characteristics of each site and crop were examined in relation to the development of aphid infestations: two were associated with inter-site differences in aphid infestation. Infestations started earlier at the eastern than at the western sites due to the earlier arrival of alate migrants, and this resulted in higher peak populations densities of R. padi but not of M. dirhodum or S. avenae. Greater aphid densities were associated with crops having low nitrate concentrations in plant sap in the early stages of growth.     

Effect of exposure to fluoride,nitrogen compounds and SO2 on the numbers of spruce shoot aphids on Norway spruce seedlings

Summary Development of spruce shoot aphid (Cinara pilicornis Hartig) populations was monitored in natural and artificial infestations of Norway spruce (Picea abies Karst.) seedlings, exposed to air pollutants in an experimental field. The pollutants, applied both singly and in mixtures, were gaseous sulphur dioxide, NaF (30 mg l^-1 F) and Ca(NO₃)₂ or (NH₄)₂SO₄ in aqueous solutions (200 mg l^-1 N). Aphid numbers on 10 seedlings in each treatment and two control plots were counted at 2-week intervals. At the beginning of the experiment aphid numbers did not differ between treatments. Aphid populations peaked in late June and early July. All the pollutants and their combinations significantly increased the numbers of aphids per seedling. Four apterous females were transferred to spruce seedlings which were growing in containers in the same plots. After 4–5 weeks aphid numbers were significantly higher in the fluoride treatment and in the combined treatment of fluoride, nitrogen and SO₂. The pollution treatments did not have a significant effect on shoot growth. Concentrations of F and S in needles were higher in treatments involving these pollutants. There were no significant differences in the concentrations of free amino acids in shoot stems between control and fluoride treatment. However, the relatively low concentration of arginine in the F treatment at the end of the growing season might indicate disturbances in the nitrogen metabolism of spruce seedlings.     

Predator density manipulation and its effects on populations of Rhopalosiphumpadi (Horn.: Aphididae) in spring barley

Experiments were carried out in 1981-83 to assess the impact of polyphagous predators (e.g. Carabidae, Staphylinidae and Araneae) on populations of cereal aphids in spring barley in Sweden. Barriers were erected around mid-field and field-edge plots at different times during the aphids' establishment phase in order to manipulate the predation pressure during the aphid pre-peak period; predators were emigrating from field boundaries. Rhopalosiphum padi was the dominant species of cereal aphid in 1981 and 1982, and was abundant in both years. Reduction of the predation pressure resulted in R. padi populations that were two to six times larger compared with those in unenclosed control plots. Aphid populations were much larger when the predation pressure was reduced earlier in June than when this was done later, in both years, since late erection of barriers excludes fewer predators because most had already entered the enclosed areas. In 1983, R. padi and Sitobion avenae were equally common but occurred in low numbers, and the effects of reducing the predation pressure were less obvious. Bembidion spp. (B. lampros and B. quadrimaculatum) and linyphiid spiders were the most abundant predators during the early establishment phase of aphid populations in each year, and significant inverse correlations were found between peak numbers of R. padi per shoot and numbers of predators from these taxa in 1981 and 1982. Similar relationships were found between total aphids and numbers of Staphylinidae, B. quadrimaculatum and Coccinella septempunctata in 1983. It was concluded that differences between the manipulated numbers of polyphagous predators caused the observed differences in peak population levels of R.padiin 1981,1982 and to a lesser extent, 1983. Apparent levels of parasitism and fungal disease were low during the pre-peak period. Aphid specific predators, abundant after the peak, were considered partly responsible for the decline of aphid populations each year. The results are compared with those from similar experiments elsewhere with S. avenae and Metopolophium dirhodum in winter wheat. Reasons why the effects of polyphagous predators are greater, and occur over a much shorter period of time in Sweden are discussed.     

Relative importance of biotic and abiotic soil components to plant growth and insect herbivore population dynamics

Background

Plants are affected by several aspects of the soil, which have the potential to exert cascading effects on the performance of herbivorous insects. The effects of biotic and abiotic soil characteristics have however mostly been investigated in isolation, leaving their relative importance largely unexplored. Such is the case for the dune grass Ammophila, whose decline under decreasing sand accretion is argued to be caused by either biotic or abiotic soil properties.

Methodology/Principal Findings

By manipulating dune soils from three different regions, we decoupled the contributions of region, the abiotic and biotic soil component to the variation in characteristics of Ammophila arenaria seedlings and Schizaphis rufula aphid populations. Root mass fraction and total dry biomass of plants were affected by soil biota, although the latter effect was not consistent across regions. None of the measured plant properties were significantly affected by the abiotic soil component. Aphid population characteristics all differed between regions, irrespective of whether soil biota were present or absent. Hence these effects were due to differences in abiotic soil properties between regions. Although several chemical properties of the soil mixtures were measured, none of these were consistent with results for plant or aphid traits.

Conclusions/Significance

Plants were affected more strongly by soil biota than by abiotic soil properties, whereas the opposite was true for aphids. Our results thus demonstrate that the relative importance of the abiotic and biotic component of soils can differ for plants and their herbivores. The fact that not all effects of soil properties could be detected across regions moreover emphasizes the need for spatial replication in order to make sound conclusions about the generality of aboveground-belowground interactions.     

Protected raspberry production alters aphid–plant interactions but not aphid population size

• 1 Aphid population dynamics in crops are often driven by interactions with their host plants, which can be extensively influenced by environmental change. Protective environments (i.e. plastic tunnels) are now frequently used for soft fruit production, which may affect the localized climate and alter such interactions. This two year study on red raspberry (Rubus idaeus) addressed how protected environments affected two aphid species; the large raspberry aphid Amphorophora idaei (LRA) and the small raspberry aphid Aphis idaei (SRA).
• 2 Temperatures were higher (up to 7–10 °C) in tunnels compared with the field. Plants in tunnels grew approximately 1.4 cm/week faster and had lower (approximately 35%) foliar amino acid concentrations than plants in the field.
• 3 Aphids affected plant growth differently depending on growing environment; they promoted plant growth by 18–37% in tunnels, although they had no such effect in the field. Aphids reduced total and essential amino acid concentrations, with SRA causing greatest reductions (approximately 40% and 33%, respectively).
• 4 Aphid population sizes were similar in both environments, although individual LRA were smaller in tunnels (30% smaller in 2007) compared with those in the field. We suggest that faster aphid development rates inside warmer tunnels were not realized as a result of the variable effects of the growing environment on amino acid composition.
• 5 We conclude that the increasing use of protected environments in crop production will not necessarily cause predictable increases in aphid populations, although it may alter aphid–plant interactions in terms of aphid‐induced changes to plant growth.

     

Responses of summer cereal aphid populations to reduced rate aphicide applications in field plots of winter wheat

Abstract 1 Recommended and reduced rate applications of pirimicarb and alpha‐cypermethrin were applied to winter wheat crops to control summer infestations of grain aphid (Sitobion avenae) and rose‐grain aphid (Metopolophium dirhodum). 2 Aphid numbers were assessed weekly and the yield response to treatment application was compared with accumulated aphid days on the crop. 3 Responses to aphicide treatment varied between sites according to variations in the subsequent development of aphid populations under varying weather conditions and differential pressures from aphid natural enemies. 4 Alpha‐cypermethrin treatment reduced spider density at most sites, and also resulted in a resurgence of aphid populations at three sites.     

Bottom‐up effects of glucosinolate variation on aphid colony dynamics in wild cabbage populations

1. There is an ongoing debate about the relative importance of top‐down and bottom‐up regulation of herbivore dynamics in the wild. Secondary metabolites, produced by plants, have negative effects on survival and growth of some herbivore species, causing bottom‐up regulation of population dynamics. Herbivore natural enemies may use plant secondary metabolites as cues to find their prey, but their survival and reproduction can also be influenced by the upward cascade of secondary metabolites through the food web. Thus plant chemistry might also affect herbivore populations by mediating top‐down regulation. 2. We investigated the influence of heritable variation in aliphatic glucosinolates, a class of secondary metabolites produced by Brassica plants, on the relative importance of top‐down and bottom‐up regulation of Brevicoryne brassicae (mealy cabbage aphid) colonies in natural Brassica oleracea (wild cabbage) populations. We manipulated natural enemy pressure on plants differing in their glucosinolate profiles, and monitored aphid colony growth and disperser production. 3. Aphid colony sizes were significantly smaller on plants producing sinigrin, compared with plants producing alternative aliphatic glucosinolates. Aphid natural enemy numbers correlated with aphid colony size, but there was no additional effect of the plants' chemical phenotype on natural enemy abundance. Furthermore, experimental reduction of natural enemy pressure had no effect on aphid colony size or production of winged dispersers. 4. Our results provide evidence for glucosinolate‐mediated, bottom‐up regulation of mealy cabbage aphid colonies in natural populations, but we found no indication of top‐down regulation. We emphasise that more studies of these processes should focus on tritrophic interactions in the wild.     

Top‐down control by Harmonia axyridis mitigates the impact of elevated atmospheric CO2 on a plant–aphid interaction

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Studies on Entomophthora in populations of Aphis fabae on field beans

The population of Aphis fabae on field beans at a site in Highfield, Rothamsted in 1973 reached its peak 1 wk earlier than that at an equivalent site in Mill Dam Close, Woburn, 29 km NW of Rothamsted. Epizootics of Entomophthora caused weekly maximum mortalities of adult apterae of 71% at Highfield and 67% at Mill Dam Close. These epizootics and the periodicity of Entomophthora conidia in the air closely paralleled the development of the aphid population. There was little evidence of a consistent relationship between Entomophthora infection and any of the weather factors considered. At both sites most mortality was caused by E. planchoniana though many aphids were killed by E. aphidis and E. obscura. E. fresenii and E. virulenta killed very few aphids. Most conidia in the air were of the E. aphidis-type. Up to 44% of alatae emigrating from bean crops were infected with Entomophthora, confirming that aphid migration is an important means of distributing the fungi. Aphid numbers rose to more than 1600/plant at both sites, in spite of the action of Entomophthora, and would probably have been less had the fungi been more abundant earlier in the season.     

Temporal and Spatial Development of Papaya Ringspot in Veracruz, Mexico

The temporal progress and spatial distribution of papaya ringspot virus (PRV) and populations of aphid vectors were monitored in two papaya (Carica papaya) plantations in the state of Veracruz, México. The incidence of PRV had a typical sigmoidal curve and the logistic model was more acceptable for describing the disease progress than the Gompertz model. The rate (r^L of increase in disease incidence (0.034 and 0.023/unit/day in Plot A and B, respectively) differed (P = 0.05) between the plots; differences in number of aphids trapped could account for the difference. The initial determination of an apparent regular spatial pattern early in the epidemic, obtained with a quadrat size determined by Greig-Smith's method (n = 8 plants/quadrat) at 50 % disease incidence, was inconsistent with our biological observations in the field. An arbitrarily selected, square quadrat size (n = 9 plants/quadrat), was more consistent with visual observation for describing the spatial pattern in the field. With 9 plants/quadrat a random pattern was found. Aphid populations had a bimodal distribution at both sites with the highest population peak in December–February and a secondary peak in August–September and change in disease incidence was generally related to the aphid population level in the previous month. None of the five potential aphid vectors (Myzus persicae, Aphis gossypii A. nerii, A. citricola and Macrosiphon euphorbiae) of PRV in Mexico colonized the papaya plants, however, which may explain the absence of clustering of disease.     

Electronically monitored cowpea aphid feeding behavior on resistant and susceptible lupins

The feeding behavior of cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae) was examined on seedlings of narrow leafed lupin, Lupinus angustifolius L., and yellow lupin, L. luteus L., using electronic monitoring of insect feeding behavior (EMIF). Aphid feeding behavior was first compared between resistant (cv. Kalya) and susceptible (cv. Tallerack) varieties of narrow-leafed lupin. Aphids spent significantly more time in non- penetration and stylet pathway activities, and significantly less time in the sieve element phase on Kalya than on Tallerack, suggesting that feeding deterrence is an important component of aphid resistance in Kalya. Aphid feeding on a susceptible yellow lupin variety (cv. Wodjil) was then compared with that on two resistant lines, one (Teo) with high and the other (94D024-1) with low seed alkaloid content. There were no consistent differences in aphid feeding behavior between Wodjil and Teo. Total, mean and percentage sieve element phase times were significantly lower, and total and percentage times in non-phloem phase were greater on 94D024-1 than on Wodjil, suggesting the possibility of phloem-based deterrence in 94D024-1.     

大气二氧化碳浓度变化对禾谷缢管蚜种群动态的影响

利用开顶式熏气室研究了大气CO₂浓度和土壤水分对禾谷缢管蚜Rhopalosiphum padi (L.)种群动态的影响,并分析了禾谷缢管蚜密度与被处理小麦叶片化学成分的关系。结果表明：（1）禾谷缢管蚜种群密度随CO₂浓度升高而持续增大并与土壤水分密切相关,各CO₂浓度下均以60%田间持水量时的密度最大;（2）CO₂和土壤水分对小麦叶片化学成分有明显的影响,麦叶水分、可溶性蛋白质、可溶性糖、淀粉含量随CO₂浓度和土壤水分含量上升而增加,纤维素含量随CO₂浓度上升而增加、随土壤水分含量上升而降低,单宁、丁布（DIMBOA）含量在CO₂浓度为550 μl/L时最高,但单宁含量随土壤水分上升而增加,丁布含量在60%田间持水量时最低;（3）禾谷缢管蚜密度与叶片水分、可溶性蛋白质、可溶性糖、淀粉含量呈正相关,与丁布、单宁含量呈负相关。结论：在未来的气候条件下,随着CO₂浓度升高禾谷缢管蚜种群可能会持续增长,这种增长在半干旱区更加突出。禾谷缢管蚜种群增长的原因之一是大气CO₂和土壤水分条件改变了植物的化学成分构成。     

Host plant recognition in monophagous weevils: Specialization ofCeutorhynchus inaffectatus to glucosinolates from its host plantHesperis matronalis

Several aphid honeydews were incorporated into sucrose solutions and presented to hop aphids, Phorodon humuli (Schrank), as artificial diets in free-choice bioassays. Small additions of honeydew collected from two species of aphid feeding on hop, Humulus lupulus L., arrested the searching behavior of the hop aphid and appeared to stimulate prolonged periods of ingestion. This effect was more dependent on the host plant honeydew source than the species of aphid that produced the honeydew. Aphid honeydews collected from plants other than hop (non-hosts to P. humuli) contained hop aphid phagostimulants that were less effective. Our results indicate that analysis of aphid honeydew could help describe chemical cues involved in the recognition of appropriate host plants by aphid species.     

The life history and population biology of an intertidal aphid,Pemphigus trehernei Foster

• 1 The fluctuations in the density and composition of populations of Pemphigus trehernei, an aphid that lives on the roots of the halophyte Aster tripolium, were followed on an area of saltmarsh on the north Norfolk coast between 1971 and 1973.
• 2 The aphid has a typical Pemphigus life-cycle, but the emphasis is on the non-sexual anholocycle. Many diapausing aphids overwinter on the marsh and relatively few alates fly to or from it.
• 3 The aphids are confined to the top 11 cm of soil and are highly aggregated: the dispersion of the population can be fitted to the negative binomial. The absolute density of the populations is relatively low, with a mean value of about 2000 aphids/m² of available marsh area.
• 4 Temperature, host-plant growth and fungal attack are probably the most important factors affecting seasonal population fluctuations. Up to 50% of the populations may be infected with the fungus Metarrhizium anisopliae, which is widespread in the soil.

     

Prevalences of fungal pathogens and othernatural enemies of cereal aphids (Homoptera:Aphididae) in wheat under dryland and irrigatedconditions in South Africa

Surveys were conducted in the summer andwinter rainfall wheat producing regions of SouthAfrica in a first attempt to investigate theidentity and impact of entomopathogenic fungi withinthe cereal aphid complex. Wheat produced underdryland and irrigated conditions was surveyed duringthe 1996 and 1997 seasons. Six cereal aphid specieswere recorded of which the Russian wheat aphid, Diuraphis noxia, was the most abundantunder dryland conditions in the summer rainfallregion as opposed to the oat aphid, Rhopalosiphum padi, in the winter rainfallregion. Rose grain aphid, Metopolophiumdirhodum, was most prevalent underirrigated conditions in the summer rainfall region.Five species of entomopathogenic fungi were recordedincluding four entomophthorales and the hyphomycete,Beauveria bassiana. TheEntomophthorales included Pandora neoaphidis, Conidiobolus obscurus, C.thromboides, and Entomophthoraplanchoniana. Pandora neoaphidis wasthe most important etiological agent recorded fromD. noxia, with up to 50% mycosis recordedunder dryland conditions in the Bethlehem summerrainfall region. Similarly, P. neoaphidis wasthe most prevalent species within populations ofM. dirhodum. under irrigated conditions in theBergville/Winterton summer rainfall region (up to77% mycosis). However, mycoses of R. padi didnot exceed 1.7% in samples from these areas,suggesting that R. padi may be lesssusceptible to P. neoaphidis than M.dirhodum. Epizootics in populations of D.noxia under dryland conditions in both the winterand summer rainfall regions indicated a high levelof susceptibility to P. neoaphidis.Occurrences of hymenopterous parasitoids andpredators in populations of D. noxia were low,although a parasitism level of 25% was recorded inone small sample of R. padi collected from anirrigated field in the summer rainfall region.     

Water Permeability of the Wood of Grand Fir (Abies grandis (Doug.) Lindl.) in Relation to Infestation by the Balsam Woolly Aphid, Adelges piceae (Ratz.)

The water permeability of sapwood and heartwood of Abies grandis(Doug.) Lindl. was found for normal trees and those infestedby balsam woolly aphid Adelges Piceae (Ratz.). It was determinedas the rate of flow of water through plugs of the wood of standarddimensions and under constant suction. The permeability of normaltrees was less in the inner than the outer sapwood and thisdifference was correlated with a greater void space (gas-filledtracheids) in the inner sapwood. The permeability of the heartwoodwas less than 5 per cent that of the sapwood. Aphid infestationreduced the permeability of the outer sapwood to about the samelevel as normal heartwood. The infested wood had a high percentageof void space and again permeability was negatively correlatedwith void space. But for a given level of void space the infestedwood had a much lower permeability than normal wood. This suggeststhat there was factor additional to the air in the tracheids,which contributed to the low permeability of infested wood.The possible nature of this factor is briefly discussed.     

Metapopulation dynamics in an aphid-parasitoid system

Metapopulation theory makes a number of predictions concerning the effects of dispersal on the persistence of predator-prey or host-parasitoid systems. While the stabilising effects of dispersal have been shown in a number of laboratory studies, evidence from field studies remains scarce due to a lack of suitable model systems. I describe a host-parasitoid system that shows a classical metapopulation structure with frequent extinctions and colonisations consisting of the aphidiid Lysiphlebus hirticornisand the aphid Metopeurum fuscoviride. Both the parasitoid and the aphid are specialists on their respective hosts. I followed the dynamics of host and parasitoid on individually marked tansy (Tanacetum vulgare) plants, the host of M. fuscoviride. Dynamics of host and parasitoid populations were characterized by frequent extinctions and colonisations. Mean longevity of aphid colonies was only 3.1 weeks. Parasitism by L. hirticorniswas a main cause of extinction for the aphid as rates of parasitism often reached 100%, in particular towards the end of the field season. Patchiness in this system occurs at two spatial scales. Aphid colonies form on single tansy ramets = shoots but movements of aphid individuals among ramets within a particular tansy genet are frequent. Because aphids can persist on a genet for a large numer of generations, it is argued that local populations form on genets rather than ramets. The number of host and parasitoid extinctions described in this study exceeds the number of extinctions usually observed in field studies of host-parasitoid metapopulations. It is suggested that aphid-parasitoid systems such as the one studied in this paper may be good models to test the predictions of metapopulation theory.     

Presence of two host races of Aphis gossypii Glover (Hemiptera: Aphididae) collected in Turkey

The aphid, Aphis gossypii, is a primary pest of citrus, cotton, cucurbits and greenhouse‐grown vegetables in Turkey and throughout Europe. There is some previous empirical data suggesting that host‐adapted genotypes of this aphid exist which may in fact be host‐races. To determine if host races of A. gossypii are indeed present in the eastern Mediterranean region of Turkey, reciprocal host transfer experiments and life table analyses were performed with multiple asexual lineages (= clones) of the aphid collected from different hosts. The collection hosts included citrus, cucumber, eggplant, okra, sweet pepper and cotton. Aphid developmental times on the host from which the aphid was originally collected (= collection or natal host) were shorter (5.2–6.0 days) and had a higher intrinsic rate of population growth (r_m = 0.25–0.44) than the 6.6–7.3 days required when the aphid was reared on a non‐original collection host (= non‐collection host or non‐natal host) and had r_m = 0.03–0.30. Total immature mortality of the cotton clone, especially in the first nymphal stage, was high (51–100%) with low r_m (0–0.03) on cucumber, citrus and sweet pepper. Aphid populations transferred from citrus, eggplant and okra to cotton (r_m = 0.29–0.30) did not differ significantly in their performance from that of the cotton population on cotton (r_m = 0.34), whereas that from sweet pepper and cucumber populations (r_m = 0.22–0.24) were significantly lower. These data have allowed us to separate A. gossypii into two distinct biological groups: (a) a ‘generalist’ population obtained from cucumber, sweet pepper, citrus, eggplant and okra which exhibited statistically better development on cotton; versus (b) a population from cotton which, by comparison on reciprocal hosts, developed poorly on non‐natal hosts except on eggplant. Development of the cotton clone on cucumber and okra was not improved after four successive generations on the non‐natal host. The good development of A. gossypii from eggplant and cotton on these reciprocal hosts suggests that these particular clones were similar, if not identical, host races.