Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid–R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations. Julia Ferrari and Claire L. Scarborough contributed equally to the work.     

Altered behavior and distribution of pea aphids, Acyrthosiphon pisum (Homoptera: Aphididae), infected with Pandora neoaphidis (Zygomycetes: Entomophthorales)

The distribution and mobility of infected aphid hosts can have a great effect on the ability of a pathogen to spread throughout a population. The distribution of dead and living pea aphids (Acyrthosiphon pisum) infected with Pandora neoaphidis was compared with that of their healthy conspecifics. Infected aphids were significantly more likely to be found on the undersides of alfalfa leaves and off of the plants than were healthy aphids. These two shifts in microhabitat location have potential costs and benefits for both the host and the pathogen.     

Fitness effects of two facultative endosymbiotic bacteria on the pea aphid,Acyrthosiphon pisum,and the blue alfalfa aphid,A. kondoi

The effects of two bacterial endosymbionts, designated PASS and PAR, were evaluated on the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera:Aphididae), in which they occur facultatively, and on the blue alfalfa aphid, A. kondoi Shinji, in which these bacteria have not been found in natural populations. Subclones of pea aphids and blue alfalfa aphids, derived from parent aphid clones that did not contain PASS or PAR, were infected with one or both bacteria, generating PASS- and/or PAR-positive subclones with minimal genetic differences from the parent clones. Under laboratory conditions at 20 °C, PAR consistently reduced the fecundity (by between 19 and 60%) of subclones derived from three different parent pea aphid clones on bur clover, Medicago hispida Gaertn. PAR had intermediate effects on pea aphids reared on sweet pea, Lathyrus odoratus L., and had no significant effect on pea aphids on alfalfa, Medicago sativa L. The effect of PASS was either neutral or negative, depending on parent clone as well as host plant. Also at 20 °C, PASS reduced fecundity (70–77%) and longevity (40–48%), and increased the age of first reproduction (by up to 1.5 days) of blue alfalfa aphid reared on alfalfa and clover. PAR had a less dramatic effect (e.g., 30–39% reduction in fecundity) on these traits of blue alfalfa aphid. In contrast, PAR and PASS increased the fitness of pea aphid subclones of one parent clone reared for three generations at 25 °C on each of the three test plants. Without facultative bacteria, fecundity of the parent clone was reduced to a mean total of < 6 offspring per adult at this elevated temperature, but with PASS or PAR, mean total fecundity of its subclones was > 35. However, this ameliorative effect of facultative bacteria at 25 °C was not found for two other sets of parent clones and their derived subclones. Alate production in pea aphids was significantly increased in large populations of two PASS- and PAR-positive subclones relative to their parent clones. Attempts to transmit PASS or PAR horizontally, i.e., from aphid to aphid via feeding on host plants (bur clover), were unsuccessful.     

Role of symbiotic and non-symbiotic bacteria in carbon dioxide production from hosts infected with Steinernema riobrave

Entomopathogenic nematodes of the family Steinernematidae and their mutualistic bacteria (Xenorhabdus spp.) are lethal endoparasites of insects. We hypothesized that growth of the nematode’s mutualistic bacteria in the insect host may contribute to the production of cues used by the infective juveniles (IJs) in responding to potential hosts for infection. Specifically, we tested if patterns of bacterial growth could explain differences in CO₂ production over the course of host infection. Growth of Xenorhabdus cabanillasii isolated from Steinernema riobrave exhibited the characteristic exponential and stationary growth phases. Other non-nematode symbiotic bacteria were also found in infected hosts and exhibited similar growth patterns to X. cabanillasii. Galleria mellonella larvae infected with S. riobrave produced two distinct peaks of CO₂ occurring at 25.6–36 h and 105–161 h post-infection, whereas larvae injected with X. cabanillasii alone showed only one peak of CO₂, occurring at 22.8–36.2 h post-injection. Tenebrio molitor larvae infected with S. riobrave or injected with bacteria alone exhibited only one peak of CO₂ production, which occurred later during S. riobrave infection (41.4–64.4 h post-infection compared to 20.4–35.9 h post-injection). These results indicate a relationship between bacterial growth and the first peak of CO₂ in both host species, but not for the second peak exhibited in G. mellonella.     

Effects of elevated CO2 on five plant-aphid interactions

We investigated interactions between five species of phloem-feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO₂; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO₂ generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO₂, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO₂ were species-specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO₂ treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO₂. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO₂ and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO₂ in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem-feeders may not be negatively affected by increased CO₂ concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO₂.     

Survival, growth, and reproduction of two aphid species on sucrose solutions containing host or non-host honeydews

Aphids, like most phloem-feeding insects, commonly exhibit a high degree of host specificity. Plant-specific chemical compounds are likely to serve as important host selection cues for monophagous aphids and such substances could be present in aphid honeydew. Apterous virginoparae ofMyzus persicae (Sulzer) andPhorodon humuli (Schrank) were reared on a buffered sucrose solution containing various aphid honeydews or a mixture of amino acids. In two separate experiments, the host-specificP. humuli (hop aphid) could grow and reproduce only on diets containing honeydew collected from hop (Humulus lupulus L.).M. persicae (the green peach aphid, GPA) did not perform well on diets containing hop honeydew, perhaps because hop is a poor GPA host. Honeydew collected from preferred GPA host plants rape,Brassica napus L., and jimsonweed,Datura stramonium L., allowed GPA growth and reproduction. Hop aphids, however, performed poorly on rape and jimsonweed honeydew diets. Bell pepper,Capsicum annuum L., honeydew supported neither the hop aphid nor GPA. The study of aphid honeydew components may contribute towards a more complete understanding of host preference and selection phenomena in aphids.     

Local and spatial dynamics of a host–parasitoid system in a field experiment

Local extinction and colonisation rates are key factors in host–parasitoid metapopulation theory, but experimental evidence from the field is scarce. We studied the host–parasitoid system consisting of the aphid Metopeurum fuscoviride and its specialist parasitoid Lysiphlebus hirticornis. This system is characterised by a patchy distribution of the host plants (Tanacetum vulgare) and by frequent extinctions of local aphid populations. In a first field experiment, we found that the presence of the parasitoid increases the likelihood of extinction of local host populations (=all aphids living on one plant). In a second field experiment, we manipulated the distance between local host populations. Parasitoid colonisation rate strongly decreased with increasing distance between local host populations. Thus, our results show the importance of parasitoids for local host populations extinction and of distance between local host populations for parasitoid colonisation rate, suggesting the importance of spatial processes for host–parasitoid systems in the field.     

Infection of Sentinel Cotton Aphids (Homoptera: Aphididae) by Aerial Conidia ofNeozygites fresenii(Entomophthorales: Neozygitaceae)

Uninfected adultAphis gossypii(Homoptera: Aphididae) apterae (sentinel aphids) on cotton leaves were exposed for 8 h to the air over a commercial cotton field in Louisiana during the night of 1–2 July 1995. At 0015 h there were 90,437 primary conidia/m³air of the fungusNeozygites fresenii(Entomophthorales: Neozygitaceae) at the midfield position as determined from Rotorod samples. Forty-eight percent (n = 106) of the sentinel aphids exposed for 8 h at midfield were infected by aerial conidia ofN. fresenii.Exposure of sentinel aphids outside the cotton field, at 10 and 100 m downwind and 10 m upwind, resulted in 34.8% (n = 131), 24.0% (n = 129), and 17.4% (n = 146) infected aphids, respectively. These data demonstrate that wind-dispersed aerial conidia ofN. freseniiare infective and rapidly and efficiently disperse the pathogen throughout aphid populations within and between fields.     

Influence of endophyte infection, plant age and harvest interval on Rhopalosiphum padi survival and its relation to quantity of N-formyl and N-acetyl loline in tall fescue

Rhopalosiphum padi L. (Homoptera: Aphididae) is sensitive to loline alkaloids present in tall fescue, Festuca arundinacea Shreb., infected with the endophytic fungus, Acremonium coenophialum Morgan-Jones & Gams. Aphid survival was higher on endophyte-free plants regardless of plant age after germination or age of regrowth tissue after clipping. Survival of aphids on endophyte-infected grass was lower on young tissue but increased as plants aged, although it never reached the same level on endophyte-free plants. Both N-formyl and N-acetyl loline increased as uncut or regrowth tissue aged; however, this was influenced by the age of the plant at the initial cut and the clipping frequency. Although even small amounts of loline cause high aphid mortality, the aphids are able to survive on endophyte-infected plants if the tillers have senescing leaves which contain lower amounts of loline. Preference for senescing leaves may help R. padi avoid plant parts containing high amounts of toxic allelochemicals, thus contributing to higher numbers of aphids on older, endophyte-infected plants.     

The elimination of intracellular microorganisms from insects: an analysis of antibiotic-treatment in the pea aphid (Acyrthosiphon pisum)

Antibiotics are routinely used to eliminate intracellular prokaryotic microorganisms from a wide range of insect species, but concerns about deleterious effects of antibiotic therapy on the insect host are seldom addressed. Here, the impact of antibiotic therapy in the symbiosis between the pea aphid Acyrthosiphon pisum and bacteria of the genus Buchnera is reviewed. Antibiotic-treatment produces aposymbiotic (i.e. symbiont-free) aphids, but does not depress the mitochondrial complement, the assimilation of dietary amino acids or the incorporation of amino acids into protein in these insects and does not impair osmoregulation, feeding rate and the capacity to penetrate plant tissues. It is concluded that the general malaise associated with aposymbiotic aphids is not attributable to a direct effect of the antibiotic. However, an important implication of this study is that aposymbiotic insects exhibit substantial metabolic adjustments to loss of the symbiosis; they are not simply aphids from which the symbiotic bacteria have been removed.     

Aphelinus albipodus(Hymenoptera: Aphelinidae) andDiaeretiella rapae(Hymenoptera: Braconidae) Parasitism onDiuraphis noxia(Homoptera: Aphididae) Infesting Barley Plants Differing in Plant Resistance to Aphids

Russian wheat aphid,Diuraphis noxia(Mordvilko), as a pest of small grains, has prompted research into biological control and host plant resistance. In the presence of Russian wheat aphid, leaves of a susceptible barley (Morex) are curled and chlorotic and sustain large densities of this aphid, while leaves of a resistant barley (STARS-9301B) remain flat and green and sustain fewer aphids. Might parasitism of Russian wheat aphid byAphelinus albipodusHayat & Fatima andDiaeretiella rapaeMcIntosh be affected differently by these plant types? When presented the plants separately and based on parasitism rate relative to aphid density, the largerD. rapaewas more effective in parasitizing relatively high densities of aphids within curled leaves of Morex than relatively low densities of aphids on uncurled leaves of STARS-9301B. Parasitism byA. albipodusdid not significantly differ among the plants. When given a choice of plants, approximately equal rates of parasitism occurred on the two plant lines for both parasitoid species, and parasitism byD. rapaewas greater thanA. albipodus.These data indicate that using parasitoid size as an indicator of success in a physically restricted environment may be misleading, when considered in a plant environment responsive in several manners to aphids (chlorosis, curling, and ability to sustain Russian wheat aphid). We expect that use of resistant barley will result in decreased parasitoid abundance as aphid densities decrease. However, parasitism rates are expected to be approximately equal on resistant and susceptible barley. In this system, plant resistance and biocontrol are compatible management strategies.     

Xylem ingestion by winged aphids

When aphids and their host plant are incorporated in a DC electrical circuit, phloem and xylem ingestion register as separate waveforms of the electrical penetration graph (EPG) signal. Aphids are primarily phloem feeders; xylem ingestion is seldom reported but can be induced experimentally by fasting the insects in desiccating conditions. In experiments with the black bean aphid, Aphis fabae Scop., young winged (alate) and unwinged (apterous) virginoparous adults were collected from their natal host plants (broad bean, Vicia faba L.) and allowed 3-h continuous EPG-recorded access to V. faba seedlings. Several aphids (47% of both morphs) showed ingestion from phloem sieve elements. Alate aphids also showed frequent xylem ingestion (60% of individuals), but no apterous aphids exhibited this activity. The EPG technique involves attachment of a fine gold wire electrode to each insect, a process that may affect normal behaviour at the plant surface. However, when the technique was modified to monitor the stylet activities of freely-settled aphids, high levels of xylem ingestion by alates were also recorded. The results suggest that the developmental physiology of winged aphids somehow predisposes them to xylem ingestion, possibly as a result of dehydration during the teneral period. Alate aphids may reduce their weight by fasting before take-off, giving aerodynamic benefits, but making rehydration, via xylem uptake, a priority following plant contact.     

Cry1Ab protein levels in phytophagous insects feeding on transgenic corn: implications for secondary exposure risk assessment

Enzyme-linked immunosorbent assays (ELISA) and bioassays were used to estimate levels of Cry1Ab protein in four species of phytophagous insects after feeding on transgenic Bt-corn plants expressing Cry1Ab protein or artificial diets containing Cry1Ab protein. The level of Cry1Ab in insects feeding on sources containing the Cry1Ab protein was uniformly low but varied with insect species as well as food source. For the corn leaf aphid, Rhopalosiphum maidis (Fitch), feeding on diet solutions containing Cry1Ab protein, the level of the protein in the aphid was 250–500 times less than the original levels in the diet, whereas no Cry1Ab was detected by ELISA in aphids feeding on transgenic Bt-Corn plants. For the lepidopteran insects, Ostrinia nubilalis (Hübner), Helicoverpa zea (Boddie), and Agrotis ipsilon (Hufnagel), levels of Cry1Ab in larvae varied significantly with feeding treatment. When feeding for 24 h on artificial diets containing 20 and 100 ppm of Cry1Ab, the level of Cry1Ab in the larvae was about 57 and 142 times lower, respectively, than the original protein level in the diet for O. nubilalis, 20 and 34 times lower for H. zea, and 10 to 14 times lower for A. ipsilon. Diet incorporation bioassays with a susceptible insect (first instar O. nubilalis) showed significant Cry1Ab bioactivity present within whole body tissues of R. maidis and O. nubilalis that had fed on diet containing a minimum of 20 ppm or higher concentrations (100 or 200 ppm) of Cry1Ab, but no significant bioactivity within the tissues of these insects after feeding on transgenic Bt-corn plants. The relevance of these findings to secondary exposure risk assessment for transgenic Bt crops is discussed.     

Transcriptional profile and differential fitness in a specialist milkweed insect across host plants varying in toxicity

Interactions between plants and herbivorous insects have been models for theories of specialization and co‐evolution for over a century. Phytochemicals govern many aspects of these interactions and have fostered the evolution of adaptations by insects to tolerate or even specialize on plant defensive chemistry. While genomic approaches are providing new insights into the genes and mechanisms insect specialists employ to tolerate plant secondary metabolites, open questions remain about the evolution and conservation of insect counterdefences, how insects respond to the diversity defences mounted by their host plants, and the costs and benefits of resistance and tolerance to plant defences in natural ecological communities. Using a milkweed‐specialist aphid (Aphis nerii) model, we test the effects of host plant species with increased toxicity, likely driven primarily by increased secondary metabolites, on aphid life history traits and whole‐body gene expression. We show that more toxic plant species have a negative effect on aphid development and lifetime fecundity. When feeding on more toxic host plants with higher levels of secondary metabolites, aphids regulate a narrow, targeted set of genes, including those involved in canonical detoxification processes (e.g., cytochrome P450s, hydrolases, UDP‐glucuronosyltransferases and ABC transporters). These results indicate that A. nerii marshal a variety of metabolic detoxification mechanisms to circumvent milkweed toxicity and facilitate host plant specialization, yet, despite these detoxification mechanisms, aphids experience reduced fitness when feeding on more toxic host plants. Disentangling how specialist insects respond to challenging host plants is a pivotal step in understanding the evolution of specialized diet breadths.     

Classical biological control of Aphis gossypii (Homoptera: Aphididae) with Neozygites fresenii (Entomophthorales: Neozygitaceae) in California cotton

In August 1994 and 1995 classical biological control releases were made in cotton in the San Joaquin Valley, California, with an Arkansas strain of the entomopathogenic fungus, Neozygites fresenii, a pathogen of the cotton aphid, Aphis gossypii. Pre-release samples in both years indicated that N. fresenii was not naturally present in A. gossypii populations in the San Joaquin Valley. Two release methods were compared: dried N. fresenii-infected cotton aphid “cadavers” and chamber inoculation of A. gossypii. Both methods were successful in introducing N. fresenii to cotton aphids in California; however, higher prevalence of fungal infection resulted with the cadaver treatments. N. fresenii persisted and spread in the aphid population until early October 1994 and late September 1995. The highest mean percentage infection in the cadaver treatment in 1994 reached a level (14%) considered imminent for epizootics (12–15%). The use of predator exclusion cages resulted in higher N. fresenii prevalences.     

3种寄主上桃蚜的选择性及形态分化

桃蚜是一种重要的农业害虫,寄主广泛,种下分化复杂。以采自黄土高原旱作区桃树、烟草、甘蓝上的桃蚜为研究对象,通过叶片选择法、传统比较形态测定法研究了3种寄主上桃蚜的选择性及形态分化。结果表明:在3种寄主同时存在的情况下,烟草上的桃蚜嗜食烟草,表现为63.5%的桃蚜选择烟草叶,13.8%选择甘蓝叶,8.2%选择桃叶,而甘蓝和桃树上的桃蚜对原寄主没有表现出明显的嗜好性;从形态指标来看,3种寄主上的桃蚜在体长、触角末节长度、后足腿节长度、触角与体长的比例方面存在显著差异(P0.05),说明这几个特征可以作为区分这3种寄主上桃蚜的依据。综合分析可以初步认为黄土高原旱作区烟草上的桃蚜可能形成了寄主专化型-烟草型。     

Transformation of Tobacco with Genes Encoding Helianthus Tuberosus Agglutinin (HTA) Confers Resistance to Peach-Potato Aphid (Myzus persicae)

The effects of the hta gene encoding Helianthus tuberosus agglutinin (HTA) on an insect in the order Homoptera were investigated. Homologous cDNAs of hta-a, hta-b, hta-c and hta-d with CaMV35S as promoter were introduced into tobacco via Agrobacterium tumefaciens. Southern blot results showed that the exogenous hta gene was inserted into the genome of host plants, and northern blot analysis confirmed that hta was expressed in transgenic plants. A bioassay with peach-potato aphid (Myzus persicae) demonstrated that transgenic plants had deleterious effects on the insect. The average population of aphids fed on transgenic T₀ plants during an 11-day assay decreased by 70%, compared controls. In transgenic plants of T₁ generation, aphid fecundity inhibitions were 53.0%(hta-b) and 64.6% (hta-c), respectively. The development of aphids was notably retarded. We conclude that hta could be a novel and promising candidate for plant transgenic engineering against homopteran insect pests.     

Effects of leaf and sap feeding insects on photosynthetic rates of goldenrod

Summary Herbivory can alter the balance between sources and sinks within a plant, and changes in the source-sink ratio often lead to changes in plant photosynthetic rates. We investigated how feeding by three insect herbivores affected photosynthetic rates and growth of goldenrod (Solidago altissima). One, a phloem-sap feeding aphid (Uroleucon caligatum), creates an additional sink, and the other two, a leaf-chewing beetle (Trirhabda sp.) and a xylem-sap feeding spittlebug (Philaenus spumarius) both reduce source supply by decreasing leaf area. Plants were grown outside in large pots and insects were placed on them at predetermined densities, with undamaged plants included as controls. All insects were removed after a 12-day feeding period. We measured photosynthetic rates both of damaged leaves and of undamaged leaves that were produced after insect removal. Photosynthetic rates per unit area of damaged leaves were reduced by spittlebug feeding, but not by beetle or aphid feeding. Conductance of spittlebugdamaged leaves did not differ from controls, but internal carbon dioxide concentrations were increased. These results indicate that spittlebug feeding does not cause stomatal closure, but impairs fixation within the leaf. Effects of spittlebug feeding on photosynthetic rates persisted after the insects were removed from the plants. Photosynthetic rates per unit area of leaves produced after insect removal on spittlegug-damaged plants were lower than control levels, even though the measurements were taken 12 days after insect removal. The measurement leaf on spittlebugdamaged plants was reduced in area by 27% relative to the controls, but specific leaf area (leaf area/leaf weight) was increased by 18%. Because of the shift in specific leaf area, photosynthetic rates were also examined per unit leaf weight, and when this was done there were no significant differences between control and spittlebug-damaged plants. Beetle and aphid feeding had no effects on the photosynthetic rate of the leaves produced after insect removal. Plant relative growth rates (in terms of height) were reduced by spittlebugs during the period that the insects were feeding on the plants. Relative growth rates of spittlebug-damaged plants were increased above control levels after insect removal, but these plants were still shorter than controls 17 days after insect removal. Beetles and aphids did not affect plant relative growth rates or plant height. Feeding by both spittlebugs and beetles reduced leaf area, and the effect of the spittlebug was more severe than that of the beetle. These results show that effects of herbivory on photosynthetic rates cannot be predicted simply by considering changes in the source-sink ratio, and that spittlebug feeding is more damaging to the host plant than beetle or aphid feeding.     

Effects of constant and fluctuating temperatures on sporulation and infection by the aphid-pathogenic fungus Pandora neoaphidis

Laboratory studies were performed to assess the importance of temperature on sporulation and infection by the aphid-pathogenic fungus Pandora neoaphidis (Remaudière and Hennebert) Humber. Numbers of primary conidia discharged from mycelium formulated as alginate granules and unformulated mycelial mats were assessed, as well as infection of the potato aphid, Macrosiphum euphorbiae (Thomas) (Homoptera, Hemiptera, Aphididae), using culture plugs as inoculum sources. Sporulation from experiments at constant temperatures indicated the optimum temperature range was 10–20°C for both mycelial preparations and there was no or very little sporulation at 30°C. Infection of aphids kept at 15°C was 34–50%, while infection at 25°C was 11–44%. At 20°C, 77–79% of aphids were infected. Under fluctuating temperature cycles, conidia numbers did not differ when mycelial preparations were maintained at 18–25°C compared with 18–20°C, but fewer conidia were recorded when preparations were exposed continuously to 18–30°C. Infections of inoculated aphids kept for varying numbers of days at 18–25°C varied between 24–47%, but only 3–32% of aphids were infected when exposed to a cycle of 18–30°C for various times. Unformulated mycelial mats of P. neoaphidis appear to be superior to forumlated alginate granules for use in experimental greenhouse and field trials, since temperature stability is similar for both materials but mycelial mats are much easier to produce.     

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.