Aphid suppression by natural enemies in mulched cereals

Large populations of natural enemies are the basis for natural pest control. Effects of mulch on predator–prey interactions in arable fields are poorly known, despite its potential to enhance ground‐dwelling predators and thereby reduce pest infestations. We studied the densities of predators and parasitoids, and their impact on cereal aphids in the presence and absence of mulch. Released populations of the bird cherry aphid, Rhopalosiphum padi (L.) (Homoptera: Aphididae), and two naturally occurring aphid species, were monitored under experimentally reduced densities of: (i) ground‐dwelling predators, (ii) flying predators and parasitoids, and (iii) with straw mulch. The three treatments were applied in a 2 × 2 × 2 factorial design in a field of spring wheat (Triticum aestivum L.). The exclusion of ground‐dwelling predators increased aphid populations by 55% in June and 40% in July, respectively. Mulched plots had 25% lower aphid densities in June. This was presumably due to enhanced densities of spiders (Araneida) in mulched plots. The exclusion of flying predators and parasitoids led to 94% higher aphid populations in late July (109 vs. 56 individuals per 100 shoots), irrespective of mulch or ground predator manipulation. This was attributed to the larvae of gall midges Aphidoletes cf. aphidimyza (Rondani) (Diptera: Cecidomyiidae) and hoverflies (Diptera: Syrphidae). The results indicate that a scarcity of predators and a bare soil surface renders crops more susceptible to arthropod pests. Farming schemes should aim at enhancing both ground‐dwelling and flying predators for elevated levels of natural pest control.     

Biotic resistance limits the invasiveness of the western flower thrips,Frankliniella occidentalis (Thysanoptera: Thripidae), in Florida

The spread of the western flower thrips, Frankliniella occidentalis (Pergande), has resulted in the world‐wide destabilization of established integrated pest management programs for many crops. It is hypothesized that frequent exposure to insecticides in intensive agriculture selected for resistant populations, which allowed invasive populations in the eastern USA to overcome biotic resistance from the native community of species. Research conducted in Florida to understand the role of biotic factors in limiting the abundance of the western flower thrips is reviewed. Orius spp. (Hemiptera: Anthocoridae) are effective predators that suppress populations of thrips on crop and non‐crop hosts in southern and northern Florida. Orius are more effective predators of the western flower thrips than the native flower thrips, F. tritici (Fitch) and F. bispinosa (Morgan). The native species are competitors of the western flower thrips. Excessive fertilization and the use of broad‐spectrum insecticides in crop fields further enhances populations of the western flower thrips. Interactions with native species clearly limit the abundance of western flower thrips in Florida, but populations are abundant in fertilized crop fields where application of insecticides excludes predators and competitor species.     

Distribution of the black pecan aphid,Melanocallis caryaefoliae,on the upper and lower surface of pecan foliage

Three aphid species regularly feed on pecan [Carya illinoinensis (Wangenh.) K. Koch (Juglandaceae)] foliage: the black pecan aphid, Melanocallis caryaefoliae (Davis), the yellow pecan aphid, Monelliopsis pecanis Bissell, and the blackmargined aphid, Monellia caryella (Fitch) (all Hemiptera: Aphididae). Adults of M. caryaefoliae and both the nymphs and adults of M. pecanis and M. caryella mainly feed on the lower surface of leaves. Nymphs of M. caryaefoliae appear unique by frequently feeding on the upper surface of pecan leaves. This is risky behavior given the environmental hazards (e.g., rain, solar radiation, and dislodgement) associated with the upper surface. Thus, we determined the leaf surface distribution of M. caryaefoliae on trees in an orchard and on pecan seedlings in the laboratory. A pecan orchard survey found all three aphid species and stages predominantly on the lower leaf surface, except for the nymphs of M. caryaefoliae, which were evenly distributed between upper and lower leaf surfaces. This survey also found aphidophagous lacewing (Neuroptera) larvae predominantly on the lower leaf surface, whereas ladybird beetle (Coleoptera: Coccinellidae) larvae were more evenly distributed between upper and lower surfaces. Laboratory experiments using single or multiple pecan aphid species revealed M. caryaefoliae distribution on pecan seedlings similar to orchard data. Nymphal M. caryaefoliae require nearly 2 days to elicit chlorotic feeding lesions on leaves; without these lesions, nymphal development is hindered. The similar distribution of nymphs of M. caryaefoliae on both leaf surfaces likely reflects a strategy of predator avoidance allowing a proportion of the population to survive.     

Can polyphagous predators control the bird cherry-oat aphid (Rhopalosiphum padi) in spring cereals?: A simulation study

Predation by two carabid species,Bembidion lampros (Herbst.) andPterostichus cupreus L., on the cereal aphid,Rhopalsiphum padi L., was described using a discrete event simulation model. The results were incorporated into an aphid population growth model. The aphid model was run using actual temperatures,R. padi emigration rates, field densities and distributions from three years with distinctly different aphid population levels. Predation was incorporated at different time periods. Predation during the arrival of emigrants and establishment of aphid populations had approximately the same effect as predation over the entire aphid period, whereas predation during the 20 days prior to peak had very little effect on resulting peak populations. We conclude that a) yearly levels ofR. padi infestation are mainly determined by the volume and intensity of emigrants; b) peak levels are sensitive to changes in predator density; and c) in some cases an outbreak ofR. padi may be prevented by the action of spring active polyphagous predators alone.     

Role of aphid predator guild in controlling spirea aphid populations on apple in West Virginia,USA

Spirea aphid populations and their predators were studied on apple to identify predators of importance in controlling aphid populations. Methods included random and non-random sampling from apple orchards in West Virginia, USA, sentinel aphid colonies, laboratory feeding studies, and predator exclusion studies. Aphidoletes aphidimyza (Diptera: Cecidomyiidae), chrysopids (Neuroptera: Chrysopidae), Harmonia axyridis (Coleoptera: Coccinellidae), and Orius insidiosus (Hemiptera: Anthocoridae) were the most abundant predators associated with spirea aphid colonies on apple. Parasitoids were all but absent in the study. Abundance of all predators was density dependent with greater responses to aphid populations at the orchard scale than to tree or individual colony scales. A. aphidimyza, O. insidiosus, chrysopids, and syrphids (Diptera) had the greatest degree of density dependence on aphid populations, and spiders showed inverse density dependence. Exclusion of predators with both cages and insecticides produced significantly higher aphid populations. Because of high abundance, good synchrony with aphid populations, and high impact per individual, H. axyridis adults were the most important spirea aphid predator on apple.     

Assessment of cotton aphids, Aphis gossypii, and their natural enemies on aphid-resistant and aphid-susceptible wheat varieties in a wheat–cotton relay intercropping system

The cotton aphid, Aphis gossypii Glover (Homoptera: Aphididae), is an important cotton pest in northern China, especially in the seedling stage of cotton. After large scale commercial use of transgenic Bt cotton, cotton aphids became one of the most important cotton pests. A 2‐year study was conducted to evaluate the role of four winter wheat varieties that were resistant or susceptible to wheat aphid, Sitobion avenae Fabricius (Homoptera: Aphididae), in conserving arthropod natural enemies and suppressing cotton aphids in a wheat–cotton relay intercropping system in northern China. The results indicated that wheat–cotton intercropping preserved and augmented natural enemies more than a monoculture of cotton. The density of natural enemies in cotton was significantly different among relay‐intercropping fields with different wheat varieties. The highest density of natural enemies and low cotton aphid populations were found in the treatment of cotton in relay intercropped with the wheat variety Lovrin10, which is susceptible to wheat aphid. The lowest density of predators and parasitoids associated with high cotton aphid populations were found with the wheat variety KOK1679, which is resistant to wheat aphid. The results showed that wheat varieties that are susceptible or moderately resistant to wheat aphid might reduce cotton aphids more effectively than an aphid‐resistant variety in the intercropping system by enhancing predators to suppress cotton aphids during the cotton seedling stage.     

Non-additive effects of multiple natural enemies on aphid populations

The question of whether multiple natural enemies often interact to produce lower host mortality than single enemies acting alone has not yet been resolved. We compared the effects of four different combinations of natural enemies-parasitoids, predators, parasitoids plus predators, and no enemies-on caged aphid populations on marsh elder, Iva frutescens, in west-central Florida. Using starting densities of natural enemies commonly found in the field, we showed that parasitoid wasps reduced aphid population densities more than predatory ladybird beetles. The addition of predators to cages containing parasites reduced the ability of parasitoids to decrease aphid population densities. Because the experiments ran only over the course of one generation, such a reduction in the effectiveness of parasites is likely caused by interference of predators with parasitoid behavior. Parasitism in the cages containing both parasitoids and predators was reduced when compared to percent parasitism in parasitoid-only cages, but this could also be due to predation. Our experiments showed that ladybird beetles prey on parasitized aphids. Thus over the long-term, the effectiveness of parasites is impaired by the interference of predators on ovipositing parasitoids and by the predation of parasitized aphids. The effects of natural enemies in this system are clearly non-additive.     

Spatial patterns of goldenrod aphids and the response of enemies to patch density

Summary The two aphid species feeding on goldenrod (Solidago altissima) in northern Florida (U.S.A.) exhibited behavioral differences that resulted in characteristic spatial patterns. Uroleucon nigrotuberculatum alates (winged forms) aggregated when colonizing stems and subsequent non-winged generations were relatively sedentary, resulting in a clumped spatial pattern. U. tissoti colonized stems singly and was more mobile; these behaviors resulted in its more random spatial pattern within fields of goldenrod.Manipulations of aphid density in the field revealed that although patches with high densities of aphids accumulated more predators than patches with few aphids, predation pressure (measured as number of predators per aphid) was lower in dense patches. As a result, aphids in dense patches had a higher per capita change in density than aphids in sparse patches. However, when the fungal pathogen, Neozygites fresenii, became the dominant mortality agent, the influence of aphid density on mortality was reversed; aphids in dense patches were then more vulnerable than aphids in sparse patches. Thus the spatial patterns exhibited by the U. nigrotuberculatum and U. tissoti resulted in differences in their relative vulnerability to different natural enemies.     

Predator avoidance behavior in the pea aphid: costs,frequency, and population consequences

Induced prey defenses can be costly. These costs have the potential to reduce prey survival or reproduction and, therefore, prey population growth. I estimated the potential for predators to suppress populations of pea aphids (Acyrthosiphon pisum) in alfalfa fields through the induction of pea aphid predator avoidance behavior. I quantified (1) the period of non-feeding activity that follows a disturbance event, (2) the effect of frequent disturbance on aphid reproduction, and (3) the frequency at which aphids are disturbed by predators. In combination, these three values predict that the disturbances induced by predators can substantially reduce aphid population growth. This result stems from the high frequency of predator-induced disturbance, and the observation that even brief disturbances reduce aphid reproduction. The potential for predators to suppress prey populations through induction of prey defenses may be strongest in systems where (1) predators frequently induce prey defensive responses, and (2) prey defenses incur acute survival or reproductive costs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Predation of eggs ofSitona hispidulus [Col.: Curculionidae] in alfalfa

The predation on egg populations ofSitona hispidus (Fabricius) [Col.: Curculionidae] in a stand of alfalfa was evaluated in 1982 and 1983. By using pitfall traps, catches of reproductive adults ofS. hispidulus were correlated with catches of carabid species, and potential egg predators were identified for further evaluation. Results indicated that predators removed 28 % of eggs under field conditions.Amara aenea DeGeer was found to be particularly efficient on eggs ofS. hispidulus under caged-field conditions.      

Cereal aphids, their parasites and predators caught in cages over oat and winter wheat crops

The trapping of alate aphids in emergence cages each 1 yd² (0–83 m²) over cereal crops from mid-June to the end of July, 1964 to 1971, always revealed colonies of cereal aphids within the crop. Four species, Sitobion avenae, S. fragariae, Metopolophium dirhodum and Rhopalosiphum padi occurred every year in different proportions. Alate aphids from winter wheat were most numerous in 1968 and fewest in 1967. Alatae developed slightly earlier in cages than in the field and peak catches were a few days earlier than in a nearby 12-2 m suction trap. Cereal aphid colonies were adversely affected by bad weather in May, e.g. in 1969, and by predators. Coccinellidae (chiefly Propylea 14-punctata) were the dominant predators in 1971 and 1968, Syrphidae in 1966, 1971 and 1968 and Chrysopidae in 1970. Parasites belonging mainly to the genus Aphidius were numerous every year. When hyperparasites such as Asaphes vulgaris, Lygocerus sp., Conostigmus sp. and Phaenoglyphis sp. were abundant as in 1967, they affected numbers of aphids in the current year and increased them in the following year (1968), possibly by hindering early, heavy parasitism. Hyperparasites could have an important influence in fluctuations of cereal aphid populations from year to year. Aphids of one species or another are always present in cereal crops in sufficient numbers during the summer months to provide copious quantities of honey dew, and this is unlikely to be a limiting factor in the biology of the wheat bulb fly, Leptohylemyia coarctata.     

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.     

Abundance of the rose-grain aphid,Metopolophium dirhodum,on barley in Canterbury,New Zealand, 1984–87

Abstract

Spring populations of Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) on barley, at Lincoln and elsewhere in Canterbury, New Zealand, declined markedly during 1984–87. Spring suction-trap catches of M. dirhodum also declined at the same time. Numbers of immigrant M. dirhodum on young barley, and maximum aphid population density on the crop, were positively correlated with suction-trap catches of M. dirhodum during the preceding week or month, respectively. Egg numbers of the most abundant predator, Micromus tasmaniae Walker (Neuroptera: Hemerobiidae), and mummy numbers of the parasitoid, Aphidius rhopalosiphi De Stefani-Perez (Hymenoptera: Aphidiidae), increased relative to aphid numbers during the study. Entomophagous fungus-disease incidence could not be related to monthly rainfall, and the decline in aphid numbers during the study period was apparently not related to disease incidence. It is suggested that aphid population regulation by predators and parasitoids in the wider habitat of M. dirhodum could reduce the rate of immigration, and hence aphid population growth, in a given crop.     

Suitability of greenbugs, cotton aphids, andHeliothis virescens eggs for development and reproduction ofOrius insidiosus

Development, fecundity, and longevity of the predator,Orius insidiosus (Say) (Heteroptera: Anthocoridae), when reared on greenbug [Schizaphis graminum (Rondani)], cotton aphid (Aphis gossypii Glover), or eggs of tobacco budworm [Heliothis virescens (F.)] with green beans or water were examined. Developmental time was shortest when predators were reared onH. virescens eggs and beans and longest when reared on cotton aphids and water. Predators were most fecund when fedH. virescens eggs. The inclusion of beans in the nymphal diet further enhanced fecundity when fed eggs. Longevity of both females and males was significantly shorter when reared on aphids than on eggs. Beans in the nymphal diet enhanced longevity of female predators only in combination with budworm eggs. FemaleO. insidiosus were largest when reared onH. virescens eggs. Addition of green beans in aphid treatments resulted in increased size ofO. insidiosus when compared to aphids and free water.     

Comportement naturel des Coccinelles aphidiphages du Sud-Est de la France: Leur type de spécificité, leur action prédatrice surAphis fabae L.

Summary The use ofCoccinellidae in the Biological control of aphid pests must be based on a thorough knowledge of their host specificity. The present work is a discussion of this problem in the case of aphidophagous predators in the South eastern part of France. Adult coccinellids are able to disperse widely and to discover many aphid pullulations, some of these are unsuitable for egg production and other-ones do not allow larval nutrition, moreover a little number have a toxic effect (at least when they feed on certain host-plants). The incidence of spatial distribution of adults has been discussed in a previous paper (Iperti, 1965). The different consequences of these facts on the natural control ofAphis fabae L. are shown and some general conclusions drawn on the possibility of improving biological control by coccinellids.

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Communication présentée au ?Symposium Ecology of Aphidophagous insects? Praha-Liblice, 27 IX-1 X 1965.     

Impact of predation on establishment of the soybean aphid, <Emphasis Type="Italic">Aphis glycines</Emphasis> in soybean, <Emphasis Type="Italic">Glycine max</Emphasis>

The soybean aphid, Aphis glycines Matsumura is a new invasive pest of soybean in North America. We studied the ability of the existing predator community in soybean to reduce A. glycines establishment in field studies using either predator exclusion, open, or leaky cages that allowed aphid emigration but limited predation. Cages were infested with uniform initial densities of A. glycines adults and subsequent populations of aphids and predators were monitored over 24 h. The most abundant predators in these trials included the carabid beetles Elaphropus anceps (Le Conte), Clavina impressefrons Le Conte, Bembidion quadrimaculatum Say and spiders (Salticidae and Lycosidae). Foliar predators were less abundant and included; Harmonia axyridis Pallas, Coccinella septempunctata (L.), and Orius insidious (Say). Over the 2-year study, we found statistically significant predation on adult A. glycines in one out of six trials at 15 h and two out of six trials at 24 h. There was never significant evidence for predation of nymphs in any trial, however overall survival (adults + nymphs) was significantly reduced in one out of six trials at 15 h and three out of six trials at 24 h. Based on these results we suggest that generalist predators can be a significant but variable factor influencing the establishment of A. glycines populations in soybean. The impact of existing predator communities should be further investigated as a means of managing A.␣glycines populations in North American soybean production systems.     

Combining lacewings and parasitoids for biological control of foxglove aphids in sweet pepper

The role of natural enemy diversity in biological pest control has been debated in many studies, and understanding how interactions amongst predators and parasitoids affect herbivore populations is crucial for pest management. In this study, we assessed the individual and combined use of two species of natural enemies, the parasitoid Aphidius ervi Haliday, and the predatory brown lacewing Micromus variegatus (Fabricius), on their shared prey, the foxglove aphid, Aulacorthum solani (Kaltenbach), on sweet pepper. We hypothesized that the presence of intraguild predation (IGP) and predator facilitation (through induced aphid dropping behaviour) might have both negative and positive effects on aphid control, respectively. Our greenhouse trial showed that overall, the greatest suppression of aphids occurred in the treatment with both the parasitoid and the lacewing. While the combination of lacewings and parasitoids significantly increased aphid control compared to the use of parasitoids alone, the effect was not significantly different to the treatment with only predators, although there was a clear trend of enhanced suppression. Thus, the combined effects of both species of natural enemies were between additive and non‐additive, suggesting that the combination is neither positive nor negative for aphid control. High levels of IGP, as proven in the laboratory, were probably compensated for by the strong aphid suppression provided by the lacewings, whether or not supplemented with some level of predator facilitation. For aphid management over a longer time scale, it might still be useful to combine lacewings and parasitoids to ensure stable and resilient aphid control.     

The influence of nitrogen fertiliser on the population development of the cereal aphids Sitobion avenae (F.) and Metopolophium dirhodum (Wlk.) on field grown winter wheat

The effect of nitrogen fertiliser on populations of the cereal aphids Sitobion avenae and Metopolophium dirhodum on winter wheat was investigated in a three year field experiment. Naturally occurring aphid populations were monitored on three nitrogen treatments; none, nitrogen application using Canopy Management guidelines (130–210 kg ha^-1) and conventional practice (190 kg ha^-1). Inoculations of laboratory reared S. avenae were used to enhance field populations on half the plots. Natural populations of M. dirhodum remained below the current UK spray threshold level of two-thirds of shoots infested at the start of flowering, or five aphids per shoot in all years, whilst populations of S. avenae exceeded the threshold in all years. The response of the two species to nitrogen differed. Significantly higher populations of M. dirhodum were recorded in both treatments which received nitrogen in all years, whilst the response of S. avenae varied between years. In 1994 and 1995 when environmental conditions favoured aphid development, higher populations were recorded in the two treatments which received nitrogen. In 1993 when high rainfall created unfavourable conditions, higher populations were recorded in the plots receiving no nitrogen. Differences in peak density and cumulative aphid index of S. avenae resulted from differences in the rate of population increase between ear emergence and peak density on the different treatments. Populations prior to ear emergence were higher in the plots which received nitrogen but the differences were not statistically significant. There was no evidence of a difference in the timing of population decline in the different treatments. In 1993 higher levels of infection by entomopathogenic fungi were observed in all treatments. Significantly higher levels of infection were recorded in the treatments receiving nitrogen, which may have accounted for the lower S. avenae populations recorded. It is possible that the larger canopies recorded in these treatments produced conditions which favoured infection by fungi, thereby limiting aphid population growth. The results indicate that application of nitrogen increases natural populations of M. dirhodum, and under favourable conditions, populations of S. avenae. However, in suboptimal climatic conditions, the application of nitrogen fertiliser can lead to lower populations of 5. avenae. The data also suggest that there is no consistent difference between a conventional and Canopy Managed approach to nitrogen fertiliser use in terms of the risk of infestation by cereal aphids.     

Effects of aphid-tending Argentine ants, nitrogen enrichment and early-season herbivory on insects hosted by a coastal shrub

The presence of the exotic Argentine ant, Linepithema humile Mayr (Hymenoptera: Dolichoderinae), nitrogen enrichment, and early-season herbivory by the specialist beetle Trirhabda bacharidis (Coleoptera: Chrysomelidae) have been shown, through separate experiments, to affect the densities of insect herbivores of the coastal shrub Baccharis halimifolia (Asteraceae), in Florida. Using a fully-factorial field experiment, we examined the relative importance of all three of these factors to the six most common insect herbivore species utilizing this host plant in a West Central Florida coastal habitat. The presence of ants affected more herbivore species than either early-season herbivory by larval T. bacharidis or nitrogen enrichment. Experimental reductions of L. humile resulted in reductions of an aphid, its coccinellid predators, and adult T. bacharidis, and increases of two species of leafminers and one species of stemborer. Due to the strong negative effects of stemborer herbivory on host plant survival, the increase in stemborer abundance led to increased host plant mortality. Early-season herbivory by larval T. bacharidis only affected the abundance of aphids and their predators, both of which were more abundant on trees with reduced early-season herbivory. Nitrogen fertilization had the most limited effects and only T. bacharidis larvae achieved higher densities on fertilized trees. Our results indicate that aphid tending by the exotic L. humile affects other insects on B. halimifolia more so than herbivory by the exploitative competitor T. bacharidis or nitrogen as a limiting nutrient.     

Immigrant inviability in yellow pecan aphid

1. Host‐associated differentiation (HAD) may be an important driver of parasite biodiversity. The sympatric occurrence of host‐associated populations features prominently in studies documenting HAD but this neither confirms nor denies that the lineages arose in sympatry. It does raise the question of what maintains such structure despite the proximity of populations in space and time. 2. We tested for immigrant inviability in yellow pecan aphid Monelliopsis pecanis Bissell, a species previously shown to be host associated, using a laboratory reciprocal transplant experiment on pecan and water hickory. 3. Immigrant inviability on the alternative host is present in yellow pecan aphid. Immigrants to both hosts were significantly less fecund than residents and their nymphs were significantly less viable. Future studies should establish the genetic and non‐genetic (maternal effects) basis of immigrant inviability in yellow pecan aphid in order to be certain about its contribution to pre‐mating reproductive isolation. 4. Since aphids are cyclic parthenogens, inviability of both immigrants and their offspring has the potential to contribute to total pre‐mating reproductive isolation. This is different than the case of obligate sexually reproducing organisms where offspring viability is a post‐mating reproductive isolating mechanism. We discuss the implications of this for estimating the contributions of immigrant and offspring inviability to total reproductive isolation in aphids.