No abstract

Competitive displacement is the most severe outcome of interspecific competition. For the purposes of this review, we define this type of displacement as the removal of a formerly established species from a habitat as a result of direct or indirect competitive interactions with another species. We reviewed the literature for recent putative cases of competitive displacement among insects and arachnids and assessed the evidence for the role of interspecific competition in these displacements. We found evidence for mechanisms of both exploitation and interference competition operating in these cases of competitive displacement. Many of the cases that we identified involve the operation of more than one competitive mechanism, and many cases were mediated by other noncompetitive factors. Most, but not all, of these displacements occurred between closely related species. In the majority of cases, exotic species displaced native species or previously established exotic species, often in anthropogenically-altered habitats. The cases that we identified have occurred across a broad range of taxa and environments. Therefore we suggest that competitive displacement has the potential to be a widespread phenomenon, and the frequency of these displacement events may increase, given the ever-increasing degree of anthropogenic changes to the environment. A greater awareness of competitive displacement events should lead to more studies documenting the relative importance of key factors and developing hypotheses that explain observed patterns.

A new huanglongbing (HLB) "Candidatus Liberibacter" species is genetically characterized, and the bacterium is designated "Candidatus Liberibacter psyllaurous." This bacterium infects the psyllid Bactericera cockerelli and its solanaceous host plants potato and tomato, potentially resulting in "psyllid yellowing." Host plant-dependent HLB transmission and variation in psyllid infection frequencies are found.

In the enriched carbon dioxide atmosphere expected in the next century, many species of herbivorous insects will confront less nutritious host plants that will induce both lengthened larval developmental times and greater mortality. The limited data currently available suggest that the effect of increased atmospheric CO2 on herbivory will be not only highly species‐specific but also specific to each insect‐plant system. Several scenarios can be predicted, however: (1) local extinctions will occur; (2) the endangered species status as well as the pest status of some insect species will change; (3) geographic distributions for some insect species will shift with host‐plant ranges; and (4) changes in the population dynamics of affected insect species will influence their interactions with other insects and plants. For insect conservation purposes, it is critical to begin long‐term studies on the effects of enhanced CO2 levels on insect populations. An analysis of the available literature indicates that many orders containing insect species important for ecosystem conservation, and even those important as agricultural or medical pests, have not been examined. Without a major increase in research on this topic, we will be unprepared for the species changes that will occur, we will lose the opportunity to document just how some insects adapt to elevated CO2 levels, and we will lack the information necessary for effective conservation efforts.

Abstract. Enemy‐free space (EFS) was defined by Jeffries & Lawton (1984) as ‘ways of living that reduce or eliminate a species’ vulnerability to one or more species of natural enemies’. EFS has emerged in the literature as a significant niche‐moulding factor. However, the lack of consistency among the empirical studies as to how EFS should be defined, and what hypotheses should be tested in order to evaluate its relative importance, prompted us to review the literature and to propose a working definition that results in a general set of testable hypotheses. To test the relative importance of EFS in structuring the communities of organisms, we propose a set of three falsifiable null hypotheses that must be tested sequentially and rejected. Ho1: The fitness of the organism in an original habit (e.g. on an original host plant) in the presence of natural enemies is equal to the fitness of the organism in that habit in the absence of natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the presence of natural enemies is less than in the absence of natural enemies is necessary to demonstrate the importance of natural enemies. Ho2: The fitness of the organism in an alternative habit with natural enemies is equal to the fitness of the organism in the original habit with natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the alternative habit with natural enemies is greater than that in the original habit with natural enemies is necessary to demonstrate that the alternative habit provides EFS. Ho3: The fitness of the organism in an alternative habit without natural enemies equals the fitness of the organism in the original habit without natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in an alternative habit without natural enemies is less than in the original habit without natural enemies is necessary to demonstrate the relative importance of EFS compared with other co‐occurring niche‐moulding factors such as competition or host nutritional quality. We searched the literature and evaluated fifty‐three references (nineteen references to seventeen different terrestrial systems and thirty‐four references to twenty‐four different freshwater systems) to test our hypotheses. Of the forty‐one systems examined, nineteen (46%) tested only for differences in vulnerability of the prey or host species between EFS and non‐EFS options (our Ho2); sixteen (39%) tested for the importance of natural enemies and the effectiveness of the alternative habit in providing EFS (our Ho1 and Ho2); and only ten systems (24%) tested for Ho1, Ho2 and the relative importance of EFS in the system as measured by fitness (our Ho3). Of the systems that tested for EFS, sixteen of nineteen (84%), thirteen of sixteen (81%) and seven of ten (70%) showed evidence in support of the existence of EFS according to hypothesis Ho2 only, hypotheses Ho1 and Ho2, and our three working hypotheses, respectively. These results indicate that very few studies have actually tested for the existence of EFS. Nevertheless, results from this limited number of natural systems suggest that EFS may be important in moulding the niches of arthropods. Because of the large number of claims for EFS in systems where none of the basic hypotheses were investigated, we suggest that authors test for EFS experimentally, be judicious in selecting articles to cite in support of EFS, and exert care in attributing it as a selective force in the evolution of arthropods in specific systems.

Plant allocation to defensive compounds in response to growth in elevated atmospheric CO(2) in combination with two levels of nitrogen was examined. The aim was to discover if allocation patterns of transgenic plants containing genes for defensive chemicals which had not evolved in the species would respond as predicted by the Carbon Nutrient Balance (CNB) hypothesis. Cotton plants (Gossypium hirsutum L.) were sown inside 12 environmental chambers. Six of them were maintained at an elevated CO(2) level of 900 micromol mol(-1) and the other six at the current level of approximately 370 micromol mol(-1). Half the plants in each chamber were from a transgenic line producing Bacillus thuringiensis (Bt) toxin and the others were from a near isogenic line without the Bt gene. The allocation to total phenolics, condensed tannins, and gossypol and related terpenoid aldehydes was measured. All the treatments were bioassayed against a non-target insect herbivore found on cotton, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae). Plants had lower N concentrations and higher C:N ratios when grown in elevated CO(2). Carbon defensive compounds increased in elevated CO(2), low N availability or both. The increase in these compounds in elevated CO(2) and low N, adversely affected growth and survival of S. exigua. The production of the nitrogen-based toxin was affected by an interaction between CO(2) and N; elevated CO(2) decreased N allocation to Bt, but the reduction was largely alleviated by the addition of nitrogen. The CNB hypothesis accurately predicted only some of the results, and may require revision. These data indicate that for the future expected elevated CO(2) concentrations, plant allocation to defensive compounds will be affected enough to impact plant-herbivore interactions.

The potato/tomato psyllid,Bactericera cockerelli(Sulc) (Hemiptera: Triozidae) has been a major pest of solanaceous crops for decades. This pest can cause damage to crop plants by direct feeding and, as has been recently discovered, by transmitting the bacterial pathogenCandidatusLiberibacter psyllaurous (a.k.a.Ca.L. solanacearum). Many studies have been conducted to determine the relationship of this pest to plant injury and to develop management strategies to alleviate the damage caused by this pest in a wide variety of solanaceous plants. Studies in the past decade have documented substantial genetic variability in this invasive species, enhanced our rapidly-evolving understanding of the interactions between the insect and the pathogen it carries, and improved our appreciation of the invasive potential of the pest. This review seeks to provide a comprehensive update toB. cockerellilife history, relationship to plant diseases, and the current state of management strategies againstB. cockerelli.

Several major chemicals in the glandular heads of type VI trichomes ofLycopersicon species were identified and quantified by gas chromatography and mass spectrometry. Two normal odd-chained ketones, 2-undecanone (47 ng) and 2-tridecanone (146 ng), and one unknown sesquiterpene (5 ng), comprised approximately 95% of the contents of a gland ofL.hirsutum f.glabratum Mull. In a closely related plant,L.hirsutum Humb. & Bonpl. (LA 361), two unknown insecticidal sesquiterpenes accounted for 6% of the gland contents. Additionally, small amounts of one unknown monoterpene and another unknown sesquiterpene were found in type VI glands of a commercial tomato variety,L.esculentum Mill. Bioassays comparing the gland exudate (by direct contact) and isooctane extracts of glands to neonate larvae ofKeiferia lycopersicella (Walsingham) (Lepidoptera: Gelechiidae) andSpodoptera exigua (Hübner) (Lepidoptera: Noctuidae) indicated that: (1) 2-tridecanone and 2-undecanone were the major insecticidal compounds inL.hirsutum f.glabratum, (2) the two unknown sesquiterpenes inL.hirsutum were acutely toxic to both species, and (3) gland contents in the commercial tomato variety provided only a physical barrier toK.lycopersicella, and were not detrimental toS.exigua. In topical bioassay trials, synthetic mixtures of 2-tridecanone and 2-undecanone (3 ∶1) demonstrated potentiation. Concentrations of these chemicals decreased as trichomes aged. Quantities of insecticidal chemicals and density of type VI trichomes varied with plant age and location within plants.