Lack Of Natural Enemies Research Articles

Native and invaded-range surveys provide mixed support for enemy-release of Nymphoides peltata in the invaded range

Natural or anthropogenic movement of plant species outside their historical ranges may result in exposure to new environmental conditions and a lack of natural enemies, thus promoting their establishment and spread into new areas. A biogeographical comparison of native and invasive populations can provide insight into the mechanisms of successful invasions and guide effective management strategies, such as biological control, by targeting plant traits that promote invasiveness. We studied the Eurasian aquatic plant, Nymphoides peltata, in situ in the native (China, Korea) and invaded (USA) ranges to determine whether there were differences in natural enemy attack rates (percent leaf damage, frequency of leaf herbivory or disease), productivity (plant cover, reproduction), and leaf traits (leaf elemental content, leaf toughness, dry matter content, specific leaf area) between areas. We also investigated whether there was evidence of a tradeoff between natural enemy attack and growth or reproduction, as would be predicted by invasion theory. Plant cover (23–29 % higher) and reproductive output (eight times more seeds / m2) were consistently higher in the invaded range. Leaf traits consistently differed between ranges, which we would expect if plants in the invaded range had fewer herbivores or other pests. The amount of leaf damage present was similar between ranges, but the frequency of herbivory was 50 % greater, on average, in the native than invaded range. Although we did not quantify suppression of N. peltata by natural enemies, we found evidence of more frequent herbivory in Asia and reduced reproduction and plant cover compared to the USA, which supports further investigation into viability of biological controls.

Artificial selection for nonreproductive host killing in a native parasitoid on the invasive pest, Drosophila suzukii.

Establishment and spread of invasive species can be facilitated by lack of natural enemies in the invaded area. Host‐range evolution of natural enemies augments their ability to reduce the impact of the invader and could enhance their value for biological control. We assessed the potential of the Drosophila parasitoid, Leptopilina heterotoma (Hymenoptera: Figitidae), to exploit the invasive pest Drosophila suzukii by focusing on three performance indices: (i) attack rate; (ii) host killing, consisting of killing rate and lethal attack rate (killing efficiency); and (iii) successful offspring development (reproductive success). We found significant intraspecific variation in attack rate and killing rate and lethal attack rate among seven European populations, but offspring generally failed to successfully develop from the D. suzukii host. We crossed these European lines to create a genetically variable source population and performed a half‐sib analysis to quantify genetic variation. Using a Bayesian animal model, we found that attack rate and killing rate had a heritability of h2=0.2, lethal attack rate h2=0.4, and offspring development h2=0. We then artificially selected wasps with the highest killing rate of D. suzukii for seven generations to test whether host‐killing could be improved. There was a small and inconsistent response to selection in the three selection lines. Realized heritability (hr2) after four generations of selection was 0.17 but near zero after seven generations of selection. The genetic response might have been masked by an increased D. suzukii fitness resulting from adaptation to laboratory conditions. Our study reveals that native, European, L. heterotoma can attack the invasive pest, D. suzukii and significantly reduce fly survival and that different steps of the parasitization process need to be considered in the evolution of host‐range. It highlights how evolutionary principles can be applied to optimize performance of native species for biological control.

Assessing vulnerability and resistance to plant invasions: a native community perspective

AbstractRisk assessments of biological invasions rarely account for native species performance and community features, but the assessment presented here could provide additional insights for management aimed at decreasing vulnerability or increasing resistance of a plant community to invasions. To gather information on the drivers of native plant communities’ vulnerability and resistance to invasion, we conducted a literature search and meta-analysis. Using the data we collected, we compared native and invasive plant performance between sites with high and low levels of invasion. We then investigated conditions under which native performance increased, decreased, or did not change with respect to invasive plants. We analyzed data from 214 publications summing to 506 observations. There were six main drivers of vulnerability to invasion: disturbance, decrease in resources, increase in resources, lack of biotic resistance, lack of natural enemies, and differences in propagule availability between native and invasive species. The two mechanisms of vulnerability to invasion associated with a strong decline in native plant performance were propagule availability and lack of biotic resistance. Native plants marginally benefited from enemy release and from decreases in resources, while invasive plants strongly benefited from both increased resources and lack of enemies. Fluctuation of resources, decreases and increases, were strongly associated with higher invasive performance, while native plants varied in their responses. These differences were particularly strong in instances of decreasing water or nutrients and of increasing light and nutrients. We found overall neutral to positive responses of native plant communities to disturbance, but natives were outperformed by invasive species when disturbance was caused by human activities. We identified ecosystem features associated with both vulnerability and resistance to invasion, then used our results to inform management aimed at protecting the native community.

Abundance changes in orb-weaver spider communities at the edge of the Argiope bruennichi expansion range.

Expanding range is an inherent feature of any species and may be caused by climate change, destruction or other change of habitat, or lack of natural enemies. In a new habitat the species may be neutral, or as predator it can displace related species through competition. A strong expansion to northern Europe was observed in the thermophilous spider species Argiope bruennichi. The species doubled its range in Poland during the 1990s and its impact on native species was not investigated so far. In this article, the results of studies about A. bruennichi number (density) and its contribution in orb-weaver spider communities are analysed. We also try to determine the impact of newly occurring species on the local araneofauna.

The Freshwater Mollusk Dreissena polymorpha (Zebra Mussel) - A Review: Living, Prospects and Jeopardies

This review paper aims to address the topic of the living, prospects as well as jeopardies of the invasive Zebra Mussel. Broad literature reviews have been taken place to gather as many facts on this species as possible to summon all the most significant information up in one paper to be available for everyone. To get a better understanding of freshwater mussels in general, a short review of freshwater mussel diversity in America has been written also. The review paper focuses especially on living, prospects, and jeopardies in North America. The review revealed, freshwater mussels to be of practical importance and of great primeness for scientific research due to their unique life cycle. Especially the freshwater mollusk Dreissena polymorpha (Dreissenidae), colloquial known as the zebra mussel draws interest due to the mollusks’ invasive character and their tremendous filtering capacity. Once originated from the Ponto-Caspian region and currently present in 33 European Countries and 33 states of North America the freshwater bivalve draws interest all over the world. Their larval stage allows broad invasions of aquatic systems. Even though they can both harm their habitat as well as other organisms living in it, Dreissena polymorpha enormous filtering capacity can be utilized effectively if managed right. However, a lack of natural enemies and environmentally friendly pesticides makes control of Dreissena polymorpha in open water impossible.

Biogeographical comparison of terrestrial invertebrates and trophic feeding guilds in the native and invasive ranges of Carpobrotus edulis

Plant invasions impact on biodiversity by altering the composition of native communities by disrupting taxonomic and functional diversity. Non-native plants are often released from their natural enemies, which might result in a reduction of the attack of primary consumers. However, they can also be exposed to the attack of new herbivores that they might not be able to tolerate. Hence, invertebrate communities can be influenced by invasive non-native plants, which in turn modify interactions and change environmental conditions. In this study, we examined the compositional and trophic diversity of invertebrate species, comparing ecosystems with and without the plant species Carpobrotus edulis in coastal areas in its native (South Africa) and introduced (Iberian Peninsula) ranges. Results show that C. edulis has a clear impact on invertebrate communities in its non-native range, reducing their abundance in invaded areas, and particularly affecting certain trophic groups. Invasive C. edulis also alters the invertebrate diversity by not only reducing abundance but also by altering species composition. Overall, the physical dominance of C. edulis modifies the co-occurrence of invertebrate assemblages, reducing the number of trophic groups and leading to substantial effects on primary consumers. Results suggest that the lack of natural enemies might be an important driver of the expansion of C. edulis in its introduced range. Further work is needed to examine long-term changes caused by non-native plants on invertebrate assemblages and the subsequent modification of biological interactions.

Rapid growth and defence evolution following multiple introductions.

Rapid adaptation can aid invasive populations in their competitive success. Resource allocation trade‐off hypotheses predict higher resource availability or the lack of natural enemies in introduced ranges allow for increased growth and reproduction, thus contributing to invasive success. Evidence for such hypotheses is however equivocal and tests among multiple ranges over productivity gradients are required to provide a better understanding of the general applicability of these theories.Using common gardens, we investigated the adaptive divergence of various constitutive and inducible defence‐related traits between the native North American and introduced European and Australian ranges, while controlling for divergence due to latitudinal trait clines, individual resource budgets, and population differentiation, using >11,000 SNPs.Rapid, repeated clinal adaptation in defence‐related traits was apparent despite distinct demographic histories. We also identified divergence among ranges in some defence‐related traits, although differences in energy budgets among ranges may explain some, but not all, defence‐related trait divergence. We do not identify a general reduction in defence in concert with an increase in growth among the multiple introduced ranges as predicted trade‐off hypotheses. Synthesis: The rapid spread of invasive species is affected by a multitude of factors, likely including adaptation to climate and escape from natural enemies. Unravelling the mechanisms underlying invasives' success enhances understanding of eco‐evolutionary theory and is essential to inform management strategies in the face of ongoing climate change. OPEN RESEARCH BADGES This article has been awarded Open Materials, Open Data, Preregistered Research Designs Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.6084/m9.figshare.8028875.v1, https://github.com/lotteanna/defence_adaptation,https://doi.org/10.1101/435271.

Eastern chestnuts nuts — dangerous chestnut pest

Goal. Determine the main ways of entry into the country of the dangerous quarantine type of eastern chestnut walnut-fruit (Asian chestnut gall Trach).
 Methods. Analytical informational messages of the European and Mediterranean Plant Protection Organization (EPPO), as well as data of EU phytosanitary services, literature and Internet resources.
 Results. Information is given on the new economically important species of Dryocosmus kuriphilus Yas., which is rapidly spreading in the countries of Europe. In Ukraine, the eastern chestnut nut walnut has a quarantine status. The threat of entering the species Dryocosmus kuriphilus with planting material (seedlings) of certain types of chestnuts from European countries in Ukraine exists. The absence of natural enemies contributes to the active resettlement of the pest. The focus should be on the study of the entomophages of the walnutworm. It is known that in Japan it is found in 26 species of parasitoids that control reproduction, in China — 11, in Korea and Italy — 15 species. The rapid spread of the pest in the EU is of great concern to scientists. The lack of natural enemies, as well as effective drugs approved for use, threaten the disappearance of chestnut plantations. The experience of some European countries shows that it effectively regulates the number of its specialized Torymus sinensis parasitoids, which was introduced into Europe from China and Japan.
 Conclusions There is a danger of entering into Ukraine the species Dryocosmus kuriphilus, which has quarantine status and is a threat to chestnut plantations.

Native or nonnative host plants: What is better for a specialist moth?

The enemy release hypothesis (ERH) predicts that the lack of natural enemies, such as herbivores, contributes to the success of nonnative plants as colonizers. Larvae of the Neotropical specialist moth Utetheisa ornatrix (Erebidae: Arctiinae) can feed on unripe seeds and leaves of both native and nonnative Crotalaria species (Fabaceae). Despite some species being able to eat nonnative plants, such behavior can impair the herbivore, as they are not adapted to the alien plant, and still contribute to the success of the nonnative species via enemy release. We tested the performance of the moth from hatching to adulthood fed on two native (C. micans and C. paulina) and two nonnative (C. pallida, C. juncea) host plants. Utetheisa ornatrix performed better (lower development time, heavier pupae and more eggs) on the native host plants than in the nonnative. However, larva performance in nonnative C. pallida was similar to that in the native host plants. Using the larval weight 7 days after hatching from the eggs as a proxy for performance in twelve Crotalaria species (five Neotropical natives, four nonnatives from Afrotropical region, and three nonnatives from India), we found similar results. Crotalaria nutritional compounds, the defensive pyrrolizidine alkaloids and Crotalaria phylogeny did not explain moth performance. Our results give some support to the ERH. The good moth performance in nonnative C. pallida may be related to its high availability as host plant for U. ornatrix, and its longer time since their introduction in Neotropics which would provide opportunity for the moth to adapt.

Germplasm, chemical constituents, biological activities, utilization, and control of Chinese tallow (Triadica sebifera (L.) Small)

Chinese tallow (Triadica sebifera, also known as Sapium sebiferum) is an important oil seed crop in its native China but has become a noxious invasive species in USA. This review covers 601 publications since 1946. To date, grape (var. conferticarpa), chicken claw (var. laxiarpa), long claw, and mixed types are recognized within the species with 77 compounds isolated from various tissues. Some of the known 13 tetracyclic diterpenoids or phorbols showed pro-inflammatory and skin irritant activities or antibacterial and antiviral activities. In contrast, leaf extracts of T. sebifera showed analgesic and anti-inflammatory activities in animal models. Tallow leaves have been used to treated bacterial infections in birds and fish in China and also showed molluscicidal activities. Seed fats and oils have a long history of uses in China. The seed aril has been used to produce Chinese tallow cocoa butter equivalent since the late 1980s, but product quality and cost need improvement. The kernel oil showed promising potential for biodiesel production. In USA, the plantings were well developed before the mid-1900s since it introduced in 1772. The tree spread quickly in the southeastern USA due to its high fertility, fast growth and broad adaptability, lack of natural enemies, increasing habitat fragmentation, disturbance events like hurricanes, and selection and development of superior high-seed yield cultivars/clones. It is now considered as one of the worst invasive species in the area. Its conventional herbicide, biological, and mechanical control methods are costly, and either largely ineffective or not environmental friendly.

Biological control reduces herbivore’s host range

The enemy release hypothesis predicts that alien populations are more vigorous and grow faster in introduced areas than within their natural range due to lack of natural enemies. An extension of this theory would be that the lack of natural enemies on an herbivore species will indirectly increase the number of host plant species that it can infest. The eucalyptus gall wasp Ophelimus maskelli is a recent introduction to the Mediterranean basin. Its arrival was followed after about two years by its parasitoid, Closterocerus chamaeleon. We evaluated the range of hosts used by the gall wasp by surveying 50 Eucalyptus species, 37 Eucalyptus hybrids and 18 E. globulus half-sibling families, before and after the establishment of the parasitoid. We found a consistent reduction in the observed host range of the gall wasp after the establishment of the parasitoid with a reduction of 83% on Eucalyptus species, 84% on hybrids, and 94% on E. globulus half-siblings. Host range narrowed down from 18 to three closely-related host species, all in the section Exsertaria, series Exsertae, E. camaldulensis, E. tereticornis and E. rudis, which are probably the standard hosts of O. maskelli. In the remaining affected species the intensity of attack decreased significantly in all studied regions. These findings show that, besides the direct gains from classical biological control, there may be indirect benefits through a natural-enemy-induced reduction in the range of host plants used by herbivorous insect pests.

A novel plant volatile attractant scheme to protect corn in China from the white-spotted flower chafer (Coleoptera: Scarabaeidae: Cetoniinae)

Adult white-spotted flower chafers (WSFC), Protaetia brevitarsis, feed on developing sweet corn ears in China causing extensive losses in quality and quantity. Considering these losses, the lack of natural enemies caused by excessive past chemical pesticide use and a complete lack of present insecticides for acceptable use against WSFC on developing sweet corn, it is important to find novel measures for controlling this serious pest. The attractiveness of eight compounds, mainly phenolic compounds, fruit esters, acetoin, and some aliphatic alcohols, was established in a specially designed olfactometer. Subsequently, rubber dispensers containing 40-mg of those attractive compounds were hung in field plots on 12 sweet corn ears on the borders and down the center of 11 × 9.6-m plots comprised of 615 plants. One day later, the same ears were sprayed with Malathion. These attract-and-kill plots were maintained in 2006, 2007, and 2008, and provided substantial protection of non-treated ears in the same field. The mean damage value (DV from 0: none to 3: >5% eaten ears) and percent of damaged ears in the treated fields (DV: 0.8 ± 0.3, 0.8 ± 0.3, and 0.7 ± 0.3; % damage: 19.4, 19.4, and 19.0%) was significantly lower than that in control plots (DV: 1.8 ± 0.2, 2.0 ± 0.1, and 1.7 ± 0.1; % damage: 92.1, 89.7, and 95.0%) for 2006–2008. This attract-and-kill method gives sweet corn growers a method to protect their crops.

CRIAÇÃO DA VESPA-DA-AROEIRA EM CATIVEIRO PARA O CONTROLE BIOLÓGICO DA AROEIRA-MANSA

A aroeira-mansa, Schinus terebinthifolius Raddi (Anacardiaceae), originária da América do Sul, foi introduzida no estado da Flórida (EUA) como planta ornamental. A sua plasticidade e a ausência de inimigos naturais possibilitou-lhe tornar-se uma ameaça à flora nativa do Parque Nacional de Everglades. Para seu controle, várias alternativas, mecânicas e químicas, foram experimentadas, porém nenhuma se mostrou eficiente e satisfatória. A utilização de inimigos naturais específicos pode ser uma alternativa de sucesso, a exemplo do que já ocorreu com outras plantas invasoras. A vespa Heteroperreyia hubrichi Malaise, 1956 (Hymenoptera: Pergidae) é um potencial candidato que está sendo estudado há mais de uma década no Brasil e na Flórida. Os resultados parciais indicam que a sua criação massiva em cativeiro é viável, mas depende de fatores especiais, tais como: tecido vegetal e folhas jovens para postura, incubação e alimentação de larvas neonatas. No restante do período larval, a vespa se alimenta de folhas maduras. O empupamento ocorre no solo. Para evitar contaminação e facilitar manutenção e transporte durante a exportação de pupas, foi desenvolvida uma técnica para a fase de empupamento. A dinâmica populacional foi estudada entre 2003 e 2004 e comparada com resultados anteriores. Areia natural e vermiculita expandida foram selecionadas para a escolha do substrato ideal. Vermiculita expandida demonstrou ser o material adequado dentro do objetivo deste trabalho.

Canada goldenrod (Solidago Canadensis): An invasive alien weed rapidly spreading in China

Invasive species pose a serious threat to native ecosystems and their biodiversity, and cause considerable economic loss to the regions they invade. In the case of Canada goldenrod (Solidago canadensis L.) (Compositae), a long-lived perennial plant native to North America, it was initially introduced as an ornamental plant to Shanghai in 1935, it then escaped into the wild and it is now spreading rapidly in China, especially in eastern China. We here describe briefly this species in relation to invasion biology. S. canadensis is actually a Canada-goldenrod complex that consists of at least six subspecies and varieties. S. canadensis has great reproductive capacity (through both seed production and clonal growth) and high genetic variation, both of which contribute to its great invasiveness. S. canadensis may outcompete or allelopathically exclude native plant species, resulting in monospecific stands with concomitant loss of plant and insect diversity, and ultimately alteration in ecosystem functioning. Lack of natural enemies in the invaded ecosystems makes this species highly invasive. Abiotic factors such as niche opportunities created by habitat disturbance and human activities, and nitrogen deposition, can promote S. canadensis’ establishment and spread through seed dispersal and vegetative structures. In addition, the species’ capacity for early season emergence and growth, rapid clonal growth, wide physiological tolerance, and high architectural plasticity make the species highly aggressive under a wide range of ecological conditions. Although commonly used control methods of weeds may also be suitable for S. canadensis, minimising its seed production seems to be critical to its effective control, which requires that all the control measures be taken during its vegetative growth.

EVALUATION OF EXOTIC SOLANUM SPP. (SOLANALES: SOLANACEAE) IN FLORIDA AS HOST PLANTS FOR THE LEAF BEETLES LEPTINOTARSA DEFECTA AND L. TEXANA (Coleoptera: Chrysomelidae)

Tropical soda apple, Solanum viarum Dunal, wetland nightshade, S. tampicense Dunal, and turkey berry, S. torvum Swartz, are considered three of Florida’s most invasive plant species. These nonnative perennial broadleaf weeds are disrupting native plant communities in agricultural areas and natural ecosystems. The lack of natural enemies in Florida is thought to be an important factor contributing to their invasiveness. The North American leaf beetles Leptinotarsa defecta (Stål) and L. texana (Schaeffer) that attack silverleaf nightshade, Solanum elaeagnifolium Cav., a native congener of the three nonnative solanums, were evaluated for their potential as biological control agents. The suitability of tropical soda apple, wetland nightshade and turkey berry as host plants for the native Leptinotarsa beetles was studied in a quarantine laboratory using single plant and paired plant tests. Neonate larvae of L. defecta developed to the pupal stage only on their natural host plant silverleaf nightshade. Feeding damage on turkey berry and wetland nightshade was negligible and no feeding occurred on tropical soda apple. In contrast, development and reproduction of L. texana on the nonnative turkey berry were comparable with silverleaf nightshade. These results suggest the nonnative turkey berry may be included in the potential host range of the native silverleaf nightshade beetle L. texana.

Differences in susceptibility of introduced and native white grub species to entomopathogenic nematodes from various geographic localities

Abstract Invasive, non-native, white grubs (Coleoptera: Scarabaeidae) cause significant damage in urban landscapes. Although the lack of natural enemies in their new home is often suggested as an important factor in the establishment and spread of invasive species, the potential of incumbent generalist parasites and pathogens to delay their establishment and spread has not been explored. We compared the susceptibility of the introduced Popillia japonica and the native Cyclocephala borealis to 16 species and strains of entomopathogenic nematodes isolated from within or outside the geographic ranges of the two scarabs. We found large variation in the virulence of the species/strains of nematodes with over 50% mortality of P. japonica produced by Heterorhabditis zealandica strain X1 and H. bacteriophora strain GPS11 and of C. borealis by H. zealandica and H. bacteriophora strains KMD10 and NC1. Heterorhabditis indica and H. marelatus caused less than 20% mortality of both scarab species. When considered as a group the nematode species and strains from within and outside the geographic ranges of either P. japonica or C. borealis did not differ in virulence towards either scarab species. Dose response studies with selected nematode species and strains against P. japonica and two additional non-native species Anomala (Exomala) orientalis and Rhizotrogus majalis and the native C. borealis indicated that R. majalis was the least susceptible and P. japonica and A. orientalis were as susceptible as the native C. borealis. Heterorhabditis zealandica was significantly more virulent than any other species or strain against P. japonica with a LC50 of 272 IJs/grub. The LC30 and LC50 values for H. zealandica were also the lowest among the four nematode species/strains tested against A. orientalis and C. borealis. The LC50 values for H. zealandica and H. megidis (UK strain) were significantly lower for the native C. borealis than the introduced A. orientalis. H. zealandica also showed the highest penetration efficiency and the lowest encapsulation in P. japonica and C. borealis grubs. Results suggest that the introduction of the exotic H. zealandica into the front-line states with respect to the movement of P. japonica and A. orientalis should be explored as a tactic to delay their establishment and spread. The results also suggest that the manipulation of the indigenous H. bacteriophora populations may help in delaying spread and mitigating losses caused by the invasive grub species.

Classical Biological Control of Nodding and Plumeless Thistles

Nodding (musk) thistle (Carduus thoermeri Weinmann in the Carduus nutans L. group) and plumeless thistle (Carduus acanthoides L.) are introduced noxious weeds of Eurasian origin. Both weeds are problematic in pastures, rangelands, and croplands and along state highways in many parts of the United States. The success of both species of thistles is largely due to their prolific seed production, seed longevity, competitive ability, and lack of natural enemies. Classical biological control of nodding thistle in Virginia has been achieved with three exotic thistle herbivores, Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). T. horridus also effectively controls plumeless thistle. These insect herbivores complement each other. Nodding thistle biological control is achieved in about 5–6 years in Virginia, Missouri, and Montana. In addition, a rust fungus (Puccinia carduorum Jacky) (Uredinales: Pucciniaceae) has been introduced and established for control of nodding thistle in Virginia. Development and reproduction of the three thistle herbivores are not adversely affected by the rust. The rust hastens plant senescence and reduces seed production. Control of plumeless thistle with R. conicus and T. horridus takes approximately twice as long as control of nodding thistle.

Norway spruce and spruce shoot aphid as indicators of traffic pollution

Two-year-old Norway spruce ( Picea abies (L.) Karst) seedlings were exposed to traffic emissions along roadsides with three different traffic densities and speed limits; highway, street and a quiet local road. The responses of the exposed seedlings as a host plant and those of spruce shoot aphid ( Cinara pilicornis Hartig) were studied. The concentrations of soluble N and free amino acids, defence chemicals (total phenolics, monoterpenes) were analysed, and aphid growth and reproduction were studied. Along the highway, street and at the local road control site, the atmospheric concentrations of black carbon (BC) and oxides of N (NO x) were measured for 1 week during the experiment. The BC data indicate deposition of organic particulate compounds along the highway and street. The NO x concentrations along the highway and street showed great diurnal variation, but the average NO x concentrations were relatively low. Thus, no changes in N metabolism or growth of the exposed Norway spruce seedlings were found. Along the street, the concentrations of many individual free amino acids, such as proline, as well as total amino acid concentrations, were lower than at the associated control site. Correspondingly, there was also no increase in spruce shoot aphid mean relative growth rate. The aphid reproduction, however, increased along the highway and is suggested to be due to more conducive microclimatic conditions at the exposure site or lack of natural enemies. No changes in defence chemicals (total phenolics, monoterpenes) in relation to the traffic exposure were found. Instead, the microclimatic conditions (temperature, solar irradiation) seemed to affect the concentration of total phenolics.

Azolla filiculoides Lamarck (Pteridophyta : Azollaceae), its status in South Africa and control

Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) (red water fern) is native to South America and was first recorded in South Africa in 1948. The lack of natural enemies and the presence of enriched waters have contributed to its establishment throughout the country where dense infestations cause severe degradation of aquatic ecosystems and limit their utilisation. The biology of A. filiculoides precludes it from being controlled manually or mechanically and herbicidal control in the aquatic environment is undesirable. Biological control is suggested as the only sustainable control method for this weed and a natural enemy, Stenopelmus rufinasus Gyllenhal (Coleptera: Curculionidae), was released for this purpose in late 1997. The weevil has established well and has caused a significant decline in the Azolla mat at a number of sites, suggesting that the weevil will be an efficient tool in the management of A. filiculoides in South Africa.

Caribou Overgrazing in the Absence of Predators

Overgrazing in a nonmigrating caribou population, in the absence of predators, is simulated using the Penna life-history model. Despite the lack of natural enemies, the population size may not increase permanently. A balanced ecosystem with moderate predation gives better results.