Response by coccinellids to spatial variation in cereal aphid density

The objectives of this study were to determine if coccinellids adjusted their distribution within spring wheat fields in response to spatial variation cereal aphid density in the fields and to describe the patterns of cereal aphid population growth that resulted. Field experiments were completed in which the physical dimensions of patches infested with cereal aphids, cereal aphid density, and access to patches by coccinellids were varied. Aphid infestations consisted of naturally occurring densities (natural patches) and much greater densities created by supplementing patches with aphids (supplemented patches). Coccinellids were denied access to some supplemented patches (exclusion patches) but allowed unlimited access to others. Densities of adult Hippodamia convergens and Coccinella septempunctata were correlated with aphid density in patches whereas density of Coleomegilla maculata was not. Aggregation by coccinellids was independent of patch area. The realized aphid population growth rate (r) was lower in supplemented than natural patches in all four trials but was significantly lower in only one trial. The lower r in supplemented patches was not exclusively caused by coccinellid predation, and emigration of aphids from patches probably also contributed. r was significantly greater in exclusion patches than supplemented and natural patches, indicating that coccinellids markedly reduced aphid numbers in patches even when aphid density was extremely high. Received: February 17, 1999 / Accepted: February 1, 2000     

Relative consumption of three aphid species by the lacewing, Chrysoperla rufilabris, and effects on its development and survival

Relative consumption of three aphid species, Aphis gossypii Glover, Myzus persicae (Sulzer) and Lipaphis erysimi (Kaltenbach) (Homoptera: Aphididae), by larvae of the lacewing, Chrysoperla rufilabris (Burmeister) (Neuroptera: Chrysopidae), was determined in the laboratory, together with effects on lacewing development and survival. Percentages of survival of C. rufilabris from first instar to adult eclosion were significantly different among lacewing larvae fed different aphid species. When larvae were fed A. gossypii and M. persicae, all larvae developed to adulthood. All larvae died prematurely when they were fed L. erysimi. Developmental duration of C. rufilabris larvae was significantly shorter when larvae were fed A. gossypii (18.0 d) than when larvae were fed M. persicae (19.2 d). The number of fourth instar aphids consumed during development by C. rufilabris larvae differed significantly among individuals fed different aphid species. Chrysoperla rufilabris consumed an average of 168 M. persicae, followed by 141.6 A. gossypii, and only 26.6 L. erysimi. The percentage of these total number of aphids consumed by each larval stadium of C. rufilabris varied significantly among aphid species. The percentage of A. gossypii consumed by each larval stadium was similar to that for M. persicae, 12.1 and 11.4% by the first instar, 15.7 and 13.1% by the second instar, and 72.2 and 75.5% by the third instar, respectively; whereas in the case of L. erysimi, 23.3% of the total number of aphids were consumed by the first instar, 30.1% by the second instar, and 46.6% by the third instar.     

Oviposition avoidance of parasitized aphid colonies by the syrphid predator Episyrphus balteatus mediated by different cues

Oviposition decisions made by members of a guild of natural enemies can have evolved to avoid intraguild predation, potentially avoiding the disruption of the extraguild prey control. We have studied the oviposition preference of the aphidophagous predator Episyrphus balteatus De Geer (Diptera: Syrphidae) within colonies of Myzus persicae Sulzer (Hemiptera: Aphididae) in the presence of two developmental stages of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae). Results from a greenhouse choice experiment showed that E. balteatus females lay significantly fewer eggs in colonies with mummified aphids than in unparasitized colonies. Colonies of parasitized, but not yet mummified did not contain significantly fewer eggs than colonies with unparasitized aphids. In three no-choice experiments, we assessed stimuli coming from aphid honeydew, from the aphids themselves and also from extracts of the aphid bodies, and all of these stimuli mediate the discrimination of mummified aphids from healthy aphids. To a lesser extent these stimuli also contribute to the discrimination against aphids that are parasitized but not yet mummified. These results suggest that the effects of these two species could be complementary for the control of M. persicae, since the species that acts as an intraguild predator, E. balteatus, avoids ovipositing on aphid colonies parasitized by the intraguild prey, A. colemani.     

Biocontrol of Sclerotinia lettuce drop by Coniothyrium minitans and Trichoderma hamatum

Fungal isolates, with known activity against Sclerotinia spp. in laboratory assays, were tested for their ability to control Sclerotinia minor in four field experiments (1998–2000). In the first experiment, eight fungal isolates (Trichoderma hamatum LU595, LU593, LU592, Trichoderma virens LU555 and LU556, Coniothyrium minitans LU112, Clonostachys rosea LU115 and Trichoderma rossicum LU596) were evaluated by incorporating spore suspensions into transplant potting mix and planting lettuce seedlings into a S. minor infested field site. At harvest, Trichoderma hamatum LU595, LU593, T. virens LU555 and C. minitans LU112 reduced disease by 30–50% compared with the untreated control under very high disease pressure (100%). In further field experiments C. minitans LU112 and T. hamatum LU593, applied as maizemeal–perlite soil amendments or incorporated into the potting mix, reduced S. minor disease over a range of disease pressures (29–91%). Disease control was equivalent or greater than that achieved with the standard carbendazim fungicide treatment. Both isolates were shown to effectively colonize the lettuce rhizosphere and surrounding soil and this colonization may have protected the roots from infection by S. minor. Multiple applications of C. minitans LU112 or T. hamatum LU593 formulations gave no added disease control compared with a single application at planting. Commercial formulations of both C. minitans LU112 and T. hamatum LU593 applied as transplant treatments, solid substrate soil amendments or as a spore drench gave consistent disease control and are currently being developed further.     

Life history characteristics of Orius insidiosus (Say) fed diets of soybean aphid, Aphis glycines Matsumura and soybean thrips, Neohydatothrips variabilis (Beach)

Field studies in soybeans have demonstrated that the endemic predator, Orius insidiosus (Say), is an important natural enemy of the soybean aphid, Aphis glycines Matsumura. Soybean thrips, Neohydatothrips variabilis (Beach), serve as an important prey resource for O. insidiosus in soybeans and may be important in sustaining O. insidiosus populations before the arrival of soybean aphid. Because soybean aphid is new to the US soybean system, the effects of a mixed diet of soybean aphid and soybean thrips on O. insidiosus life history is not known. We measured the survival, development, and reproduction of O. insidiosus when fed soybean thrips, and a mixed prey diet of soybean aphids and soybean thrips, and compared these results to a previous study of O. insidiosus life history fed soybean aphid alone. Nymphal development to adulthood (15.9 days) and fecundity (68.8 eggs per female) was improved for O. insidiosus fed ad libitum soybean thrips daily compared to O. insidiosus fed ad libitum soybean aphids daily. The contribution of alternative prey to O. insidiosus life history characteristics can be complex depending on the amount and quality of a particular prey item. At low levels of prey, the addition of prey appears to enhance O. insidiosus survival, development, and fecundity. However, as predators are fed more often, the predator’s response depends on the type of prey that predominates in the mixed prey diet. We discuss soybean thrips impact on O. insidiosus population ecology and soybean aphid dynamics.     

Effects of GNA and other mannose binding lectins on development and fecundity of the peach-potato aphid Myzus persicae

Three mannose-binding lectins were assayed in artificial diets for their toxic and growth-inhibitory effects on nymphal development of the peach-potato aphid Myzus persicae. The snowdrop (Galanthus nivalis) lectin GNA was the most toxic, with an induced nymphal mortality of 42% at 1500 g ml^–1 (30 M) and an IC₅₀ (50% growth inhibition) of 630 g ml^–1 (13 M). The daffodil (Narcissus pseudonarcissus) lectin NPA and a garlic (Allium sativum) lectin ASA induced no significant mortality in the range 10–1500 g ml^–1, but did result in growth inhibition of 59% (NPA) and 26% (ASA) at 1500 g ml^–1 (40 M for NPA, 63 M for ASA). All three lectins were responsible for a slight but significant growth stimulation when ingested at 10 g ml^–1, reaching +26%, +18% and +11% over the control values for the garlic lectin, the daffodil lectin and the snowdrop lectin, respectively. GNA, as well as the glucose/mannose binding lectin Concanavalin A, were also provided at sublethal doses throughout the life cycle of the aphids, and effects on adult performance were monitored. Adult survival was not significantly altered, but both lectins adversely affected total fecundity and the dynamics of reproduction, resulting in significant reduction in calculated r _ms (population intrinsic rate of natural increase) on lectin-containing diets. These effects are discussed in relation to the use of transgenic plants expressing these toxic lectins for potential control of aphid populations.     

Inheritance of resistance to lettuce root aphid in the lettuce cultivars 'Avoncrisp' and 'Lakeland"

The inheritance of resistance in two lettuce cultivars to lettuce root aphid, Pemphigus bursarius, was studied in a series of laboratory and field experiments at Wellesbourne between 1989 and 1992. A source of total resistance in the cv. ‘Avoncrisp’ which is linked to the downy mildew resistance gene Dm6, was shown to be governed by a single dominant gene. There were no maternal effects evident in the inheritance of this resistance. The basis of the high level of resistance which exists in the cv. ‘Lakeland’ (formerly known as ‘Jubilee’) was also shown to be controlled by the same dominant gene. The linkage between Dm6 and root aphid resistance was broken in ‘Lakeland’ as this cultivar does not possess the Dm6 gene. The linkage was presumably broken when the original cross between the parents of cv. ‘Lakeland’, ‘Calmar’ and ‘Avoncrisp’ was made. Under laboratory conditions small numbers of aphids commence development on cv. ‘Lakeland’ but colonies fail to develop and under field conditions the resistance provides adequate Protectión against the pest. The resistance in both ‘Avoncrisp’ and ‘Lakeland’ was effective against a population of lettuce root aphid collected from an endive crop in southern France as well as being effective against the Wellesbourne population of this aphid.     

Biology, life table and predation of Feltiella acarisuga (Diptera: Cecidomyiidae) feeding on Tetranychus cinnabarinus eggs (Acari: Tetranychidae)

Feltiella acarisuga (Vallot) is a common predatory gall midge, which feeds on many species of spider mites. All major life history and life table parameters of F. acarisuga were determined using the carmine spider mite, Tetranychus cinnabarinus (Boisduval) eggs as prey under laboratory conditions [26.7 ± 2 °C, 75 ± 5% RH, and a photoperiod of l4:10 (L:D) h]. Developmental times of F. acarisuga were 2.6, 7.1, and 6.7 d for eggs, larvae, and pupae, respectively, with an average of 16.4 d from egg to adult emergence. Female immatures development took ≈1 d longer than male immatures did. Adult F. acarisuga lived an average of 12.8 d, and the female adults (13.3 d) lived significantly longer than male adults did (11.9 d). After an average of 1.5 d preoviposition period, each female laid an average of 27.3 eggs in its life span with an average of 2.1 eggs per day and an average of 2.8 eggs on each of its oviposition day. The raw data were analyzed using an age-stage, two-sex life table method that takes into consideration of the variable developmental rates among individuals and between sexes. The intrinsic rate of natural population increase (r), net reproductive rates (R_o), gross reproductive rate (∑m_x), generation time (T), and doubling time (DT), and the finite rate of increase (λ) of F. acarisuga were estimated using the age-stage, two-sex (male and female) life table analysis as 0.122 d⁻¹, 16.19 eggs per female, 20.81 eggs per female, 22.81 d, 1.1298 d, and 5.7 d⁻¹, respectively. Each of the first, second, and third instar larvae of F. acarisuga consumed an average of 35.5, 54.0 and 86.9 T. cinnabarinus eggs per day, respectively. Larvae of F. acarisuga could consume an average of 175.4, T. cinnabarinus eggs, and female larvae consumed 14% more spider mite eggs (187.6 eggs) than male larvae (165.1 eggs). The significance of other life table parameters related to the population and the potential of using F. acarisuga as a biological control agent are discussed.     

Temperature effects on egg development of the rosy apple aphid and forecasting of egg hatch

The development of Dysaphis plantaginea (Pass.) (Homoptera: Aphididae) winter eggs was studied at six different constant temperatures ranging from 7.5 to 16.5 °C in order to improve the basis for phenological forecasts in early spring. The mortality was generally low at temperatures below 13.5 °C but increased considerably at 16.5 °C. The effect of temperature on development rates could be described with linear regression within the temperature range under study. The lower temperature threshold for development was estimated to be 4.0 °C and the thermal constant 140 day‐degrees. A time‐varying distributed delay approach was used to establish a temperature driven phenology model for winter egg hatch of D. plantaginea considering the intrinsic variability in development time. The model parameters such as temperature‐dependent development times and corresponding variances were quantified based on the experimental data. When compared with independent observations on egg hatch under semifield conditions, the model gave satisfactory validation results. It can be used as forecasting tool for the optimal timing of monitoring and control measures for D. plantaginea in early spring.     

Geographic range, impact, and parasitism of lepidopteran species associated with the invasive weed Lantana camara in South Africa

Six isolates of Trichoderma were screened for antagonism to Armillaria in tea stem sections buried in the soil. The inability of Armillaria to invade Trichoderma-colonized stem sections and the reduction of its viability in the plant materials following invasion of these by Trichoderma were used as indicators of antagonism. Four isolates of the species Trichoderma harzianum significantly (P<0.001) reduced the incidence of the pathogen in the plant materials. Isolate T4 completely eliminated the pathogen from plant materials in sterile soil and also antagonized two different isolates of the pathogen in nonsterile soil. Application of this T. harzianum isolate to the soil as a wheat bran culture significantly (P<0.001) reduced viability of Armillaria in woody blocks of inoculum. Soil amendment with coffee pulp also reduced the inoculum viability but did not affect the incidence of Trichoderma in the blocks of inoculum. We conclude that the direct application of wheat bran-formulated T. harzianum into soil surrounding woody Armillaria inoculum sources can suppress the pathogen. Further, no organic amendment is needed to enhance development of the antagonist in the soil as a pre-requisite to suppressing the pathogen.     

Trophic impacts of a rolled-rye cover crop on the lettuce aphid (Nasonovia ribisnigri) and associated insect fauna in cultivated Histosols

Cultivated peatland (Histosol) in Southern Québec (Canada) is a rapidly declining non-renewable resource used to grow most Canadian lettuce (Lactuca sativa L., Asteraceae). Rolled-rye (Secale cereale L., Poaceae) cover crop is one of the conservation practices proposed to reach a more sustainable lettuce production, but the overall impact on the agroecosystem remains poorly studied in Histosols. We assessed multiple effects of rolled-rye cover crop on the trophic chain associated with the lettuce aphid, Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae), a major pest of lettuce. During one growing season and through two consecutive lettuce crops, we monitored in situ the impacts of rolled-rye cover crop on insect fauna and lettuce quality. We used visual scouting and yellow pan traps to assess plant colonization by N. ribisnigri, its natural enemies, and alternative prey. Exclusion cage experiments were also conducted to measure aphid fitness and population growth. Under greenhouse conditions, following cover crop removal, we examined potential lingering effects of rye within the soil on lettuce plants and N. ribisnigri. In situ experiments showed that rolled-rye cover crop has the potential to inhibit N. ribisnigri field colonization and recruit natural enemies and alternative prey in the first lettuce crop. Rye also reduced aphid fitness as well as lettuce foliar amino acid concentration and weight. For the second lettuce crop, an almost 50% reduction in N. ribisnigri abundance was observed with the use of rolled-rye cover crop. In greenhouse experiments, no persistent effect of rye was observed on the quality of lettuce grown with soil collected under a cover crop, nor on the fitness of aphids inoculated on these lettuces. This study highlights the diversity of trophic effects rolled-rye cover crop may have on lettuce production in Histosols and the potential of rolled-rye cover crop as a cultural practice to reduce aphid populations. Mechanisms at play while underlining agronomic challenges regarding proper rye termination must be further explored to maintain high-performing lettuce yields.     

Comparison of the predation capacities of two soil-dwelling predatory mites,Gaeolaelaps aculeifer and Stratiolaelaps scimitus (Acari: Laelapidae), on three thrips species

Soil-dwelling predatory mites are natural enemies of various soil pest insects and mites. Both Gaeolaelaps aculeifer (Canestrini) and Stratiolaelaps scimitus (Womersley) are commercialized natural enemies of thrips, but there is little information on the predation rate of these predatory mites on different thrips species. We compared their predation capacities on three thrips species, Frankliniella occidentalis, F. intonsa, and Thrips palmi, which are major pests of various horticultural plants. The predatory rate of G. aculeifer was higher than that of S. scimitus. Both predator species fed on more T. palmi thrips than F. occidentalis or F. intonsa thrips, which may be attributable to the smaller body size of T. palmi than the other thrips. Predation rates of female adults were 2.6–2.8 times higher than those of deutonymphs in both species. Predation rates were not separated according to the various developmental stages (i.e., second instar larva, pupa, or adult) of thrips; however, deutonymphs fed on fewer adults than larvae or pupae of F. occidentalis. Our results suggest that both G. aculeifer and S. scimitus are active predators that can prey during any of their developmental stages and on any species of thrips tested.     

Grazer interactions with four species of Lyngbya in southeast Florida

Blooms of the toxic cyanobacteria Lyngbya spp. have been increasing in frequency and severity in southeast Florida in recent years. Lyngbya produces many active secondary metabolites which often act as feeding deterrents to generalist herbivores, possibly increasing the longevity of these nuisance blooms. Whilst diverse arrays of small invertebrate consumers are often found in association with Lyngbya, little is known of their grazing selectivity among species of Lyngbya. This study examines the feeding preference of grazers for four local Lyngbya species (Lyngbya majuscula, Lyngbya confervoides, Lyngbya polychroa and Lyngbya spp.). Stylocheilus striatus and Haminoea antillarum showed no dietary selectivity between L. polychroa, L. majuscula and Lyngbya spp. in multiple choice feeding assays, whereas Bulla striata showed a distinct preference for L. polychroa (P < 0.001). To determine whether preference might be related to species-specific secondary metabolites, L. majuscula, L. confervoides and L. polychroa non-polar and polar extracts were incorporated into artificial diets and offered to a range of mesograzers. No significant difference was noted in feeding stimulation or deterrence amongst extracts and the controls for any of the grazers. When fed a monospecific diet of L. polychroa, S. striatus consumed more (P < 0.001) and attained a higher daily biomass (P = 0.004) than S. striatus fed L. confervoides. As L. polychroa and L. confervoides often co-exist on local coral reefs and yield dense numbers of S. striatus, host switching to a more palatable species of Lyngbya may have important implications regarding top-down control of local blooms leading to proliferation of one species and decimation of another. S. striatus fed a diet of L. polychroa consumed more (P = 0.003), had a greater increase in body mass (P = 0.020) and higher conversion efficiency (P = 0.005) than those fed L. confervoides regardless of host origin. Possible explanations for host switching between species of Lyngbya related to morphology, toxicity and nutrient requirements for growth are discussed.     

Settling behaviour and reproductive potential of the green peach aphid Myzus persicae on peach varieties and a related wild Prunus

Discovery in the late seventies of resistance to the green peach aphid Myzus persicae (Sulzer) (Homoptera: Aphididae) in Prunus species was based on screening in the field or in greenhouses with natural aphid populations. Here, we assess the impact of these wild and domesticated peach trees on the behaviour, development, reproductive performance and demography of cloned aphids under controlled light and temperature. Four peach varieties, i.e., Rubira, Weeping Flower Peach, Summergrand and Malo konare and the clone P1908 of the related species Prunus davidiana were tested against the highly susceptible cultivar GF305. Besides a variability in the performance of aphids among experiments, our results showed that (i) distinct mechanisms were involved in the sources of resistance studied and (ii) the ranking of the genotypes on their resistance/susceptibility status remained roughly stable throughout the experiments. Observations on the settling behaviour of first instar nymphs demonstrated antixenosis components in the resistance conferred by Rubira and Weeping Flower Peach. Nymphs began to leave the plants after a short exposure (19–21 h) and no aphid was left after 4 days. Nymphal mortality remained rather low (16%) compared to the repellent effect on aphids of both genotypes. Nymphs disappeared from Weeping Flower Peach significantly earlier than from Rubira. Summergrand, Malo konare and P. davidiana clone P1908 were accepted as host plants by aphids. On P. davidiana, decreased fecundity and intrinsic rate of natural increase (rm=0.20, averaged on all experiments) were clear expression of antibiosis. In addition, the mean length of the mature embryos within the gonads of the females on the day of adult moult was negatively correlated with the total number of embryos, providing evidence that aphids on this genotype lacked sufficient ressources to be directed both towards production and growth of embryos. Compared to the most susceptible cultivar GF305 (rm=0.36), Summergrand (rm=0.26) and Malo konare (rm=0.28) had, to a lesser extent, a negative impact on nymph production and rate of increase.     

Relative balance of the cost and benefit associated with carnivory in the tropical Utricularia foliosa

Investment by bladderwort (Utricularia foliosa) in carnivory, in terms of total C and N of bladders per leaf, was estimated in places with different nutrient concentrations from the Yahuarcaca Creek in the Colombian Amazon. The aims were to determine whether nutrient limiting conditions stimulate the investment in carnivory, and the relative balance between C and N invested in carnivory versus C and N obtained from prey. There were no significant differences either for phosphate (PO₄³⁻) concentration or for ammonia (NH₄⁺) concentration among five sampling areas, along approximately 5 km long stretch of the creek, with a pooled mean ± S.D. of 0.19 ± 0.06 and 8.6 ± 3.0 μM, respectively. However, there were significant differences in the nitrate (NO₃⁻) concentration ranging from 0.6 to 2.5 μM. Total C and N of bladders per leaf increased with decreasing NO₃⁻. This corroborates the hypotheses that the carnivorous plant U. foliosa optimises its investment in carnivory according to nutrient availability in the water, and that N is a limiting factor that stimulates the investment in carnivory. The numbers of prey per bladder were also higher under NO₃⁻ limitation, thus enhancing the input of nutrients toward the plant through the bladders. The ratio of total C of prey captured/total C invested in bladders was always lower than 1. However, the efficiency of N was higher since when NO₃⁻ concentration was lower than 1 μM, the ratio of total N of prey captured/total N invested in bladders ranged between 0.97 and 1.67.     

Imbibitional damage in conidia of the entomopathogenic fungi Beauveria bassiana, Metarhizium acridum, and Metarhizium anisopliae

When dried organisms are immersed in water, rapid imbibition may cause severe damage to plasma membranes; in unicellular organisms, such damage is usually lethal. This study investigated effects of water activity (dryness) of organisms and immersion temperature on imbibitional damage in three insect pathogenic fungi. Conidial powders of Beauveria bassiana (Bb), Metarhizium anisopliae (Ma) and Metarhizium acridum (Mac) were dried/hydrated to a broad range of water activities (a_w) (0.023–0.961) prior to immersion in water at 0.5–33 °C. Imbibitional damage in conidia of each fungus occurred rapidly, with no differences in viabilities observed following immersion for 2 vs. 60 min. Damage increased with decreasing water activity of the conidia and decreasing temperature of the immersion water. Dry (a_w  0.333) Metarhizium spp. conidia were highly susceptible to imbibitional damage, with viability declining to 5% after immersion at 0.5 °C and 63% following immersion at 15 °C. Germination of the driest Ma conidia was reduced to 66% after treatment at 25 °C. In contrast, Bb was highly tolerant to damage, with significant reductions in viability (to levels as low as 43–65%) occurring only when dry conidia were immersed at 0.5 °C. Damage was prevented when conidia were slowly rehydrated by humidification prior to immersion and immersion temperature was increased to 33–34 °C; germination of all fungi was 94% under these optimal conditions. However, immersion of the driest Bb, Ma, and Mac powders in warm water (33 °C) also resulted in high viabilities (95%, 89%, and 94%, respectively), and slow-rehydrated conidia also retained high viability (87%, 92%, and 83%, respectively) after immersion in ice-cold water (0.5 °C). Formulation of conidia in pure (non-emulsifiable) paraffinic oil provided considerable protection from imbibitional damage. This study underscores a need for establishing standard protocols for preparing aqueous suspensions of sensitive fungi for both research and commercial applications.     

Assessing the risks associated with the release of a flowerbud weevil, Anthonomus santacruzi, against the invasive tree Solanum mauritianum in South Africa

Biological control of Solanum mauritianum Scopoli, a major environmental weed in the high-rainfall regions of South Africa, is dependent on the establishment of agents that can reduce fruiting and limit seed dispersal. The flowerbud weevil, Anthonomus santacruzi Hustache, is a promising fruit-reducing agent, despite ambiguous results obtained during host-specificity evaluations in quarantine. Adult no-choice tests showed that although feeding is confined to Solanum species, normal feeding and survival occurred on the foliage (devoid of floral material) of cultivated eggplant (aubergine), potato, and several native South African Solanum species. During paired-choice tests, involving floral bouquets in 10-liter containers, A. santacruzi oviposited in the flower buds of 12 of the 17 test species, including potato and eggplant, although significantly more larvae were recovered on S. mauritianum than on eight other species. Larvae survived to adults on all 12 species, with survival significantly lower on only four species than on S. mauritianum. However, during multi-choice tests, involving potted plants in a large walk-in cage, A. santacruzi consistently displayed significant feeding and oviposition preferences for S. mauritianum over all of the 14 Solanum species tested. Analyses of the risk of attack on nontarget Solanum plants suggested that, with the possible exception of two native species, none is likely to be extensively utilized as a host in the field. Also, host records and field surveys in South America have suggested that A. santacruzi has a very narrow host range and that the ambiguous laboratory results are further examples of artificially expanded host ranges. These and other considerations suggest that A. santacruzi should be considered for release against S. mauritianum in South Africa, and an application for permission to release the weevil was submitted in 2003.     

Effect of Paenibacillus alvei, strain K165, on the germination of Verticillium dahliae microsclerotia in planta

Verticillium wilt is a devastating disease of a wide range of herbaceous and woody plant hosts, incited by the soilborne fungus Verticillium dahliae. In the present study, the effect of the potential biocontrol isolate Paenibacillus alvei, strain K165, on the germination of V. dahliae microsclerotia (msc) was investigated. Strain K165 was isolated from tomato root tips and its activity against V. dahliae has been shown in glasshouse and field experiments. In the present study, the application of K165 resulted in the reduction of msc germination of V. dahliae, in the root tips and the zone of elongation, of eggplants by 50% compared to the control treatment; whereas 10 and 12 cm away from root tips and in soil without plants the percentage of msc germination was reduced by 26% and 40%, respectively. However, K165 did not significantly affect the number and length of hyphae per germinated msc. In a split-root system, K165 triggered induced systemic resistance in eggplants against V. dahliae by reducing disease severity and msc germination by 27% and 20%, respectively. In addition, K165 colonised the rhizosphere of eggplants and soil in a population density of 5 and 3 log₁₀ cfu g⁻¹, 7 dpi, respectively. This is the first report of evaluating the direct/indirect effect of a rhizospheric bacterium on msc germination in the rhizosphere of eggplants, indicating that strain K165 reduces msc germination.     

Population growth and predation interference between two species of predatory phytoseiid mites (Acarina: Phytoseiidae) in interactive systems

Summary Populations of two species of phytoseiid mite predators, Phytoseiulus persimilis Athias-Henriot and Amblyseius degenerans (Berlese), feeding on a tetranychid prey, Tetranychus pacificus McGregor, were allowed to grow separately as well as together on bush lima bean (Phaseolus lunatus Var.) arenas in the laboratory. The population plateau attained by P. persimilis was nearly 5 times higher than that for A. degenerans when each species was on separate leaf arenas. When they were on the same arena, P. persimilis was outcompeted by A. degenerans after about 70 days of population growth. When dispersal to other arenas was necessary for the predators to find prey in another experiment, P. persimilis survived well but not A. degenerans. The mechanisms underlying species displacement were explored further. The differential mortality of immature predators at different developmental stages due to interspecific predation was concluded to be responsible for the population decline of P. persimilis, and the decline of A. degenerans in another experiment was attributed to its sedentary tendency regardless of prey distribution and to the lack of alternative food sources in the system. The implications to biological control of mutual predation between predator species is discussed briefly.