Variation in hostplant preference and performance by the milfoil weevil, Euhrychiopsis lecontei Dietz, exposed to native and exotic watermilfoils

Exotic plants often displace native plants and thus alter the availability of native hostplants for specialist herbivorous insects. The submersed aquatic weevil Euhrychiopsis lecontei Dietz is endemic to North America, but there are now source populations on the exotic Eurasian watermilfoil (Myriophyllum spicatum L.) as well as on the weevil's ancestral host, northern watermilfoil (Myriophyllum sibiricum Komarov). This provides an opportunity to examine a host range expansion in progress. To further define the host range of the weevil and to determine how population source and rearing plant influence host plant preference and performance, we conducted a series of preference and rearing experiments with weevils from two source populations reared on northern milfoil, on Eurasian milfoil, switched late in larval development from northern to Eurasian milfoil, and vice versa. We also included two rearing treatments with milfoils on which the weevil has not been documented: the native M. verticillatum L. and the exotic M. aquaticum Verd. Preference by weevils in the switched rearing treatments was similar to preferences exhibited by weevils reared solely on the second (later) milfoil species and an increase in preference for Eurasian milfoil was induced by adult exposure to Eurasian milfoil for 2 weeks. In contrast, sizes and development times of weevils in the switched rearing treatments were similar to sizes and development times exhibited by weevils reared solely on the first (early) milfoil species. These results indicate that preference by the milfoil weevil is determined late in larval development or later and Hopkins' host selection principle is not supported. However, size and development time were most affected by hostplant quality during larval development when larvae must acquire the resources needed for pupation. Oviposition preference in the milfoil weevil was a population attribute, not a fixed individual attribute and there was no significant variation in preference among individuals reared on northern milfoil, but significant variation in preference was detected among weevils reared on Eurasian milfoil. Weevils oviposited on all four milfoil species and completed development on three of them, but did not develop beyond the larval stage on M. aquaticum. Weevils reared on Eurasian milfoil developed faster and reached larger adult sizes than weevils in any other rearing treatment. The smallest sizes and longest development times were for weevils reared on the natives, northern milfoil and M. verticillatum. The milfoil weevil oviposits on an array of milfoil species and is unable to distinguish an unsuitable host (M. aquaticum) within this genus. The influence of rearing plant and adult exposure to Eurasian milfoil on hostplant preference suggests that host range expansion to novel congeners may occur more rapidly than predicted by models which assume that genetic variation is required. Significant variation among individuals in hostplant preference suggests the potential for a host shift to a plant for which E. lecontei appears pre-adapted.     

Oviposition specificity and behavior of the watermilfoil specialist Euhrychiopsis lecontei

Eurasian watermilfoil (Myriophyllum spicatum L.) is a nuisance aquatic weed, exotic to North America. The freshwater weevil Euhrychiopsis lecontei (Dietz) is a potential control agent of Eurasian watermilfoil and is a fully submersed aquatic specialist herbivore. Its presumed original host is the native northern watermilfoil (Myriophyllum sibiricum Komarov). We conducted a set of oviposition experiments to reveal first and second oviposition preference of Euhrychiopsis lecontei when presented with seven macrophytes. We tested differences between source (lake) populations of weevils, differences in behavior between weevils reared on the exotic Eurasian watermilfoil and the native northern watermilfoil and between weevils in the presence and absence of their preferred hostplant. Oviposition assays confirmed that E. lecontei is a watermilfoil specialist. Out of the 207 females that laid eggs, only three oviposited on a non-watermilfoil plant, Megalodonta beckii. The weevils' degree of specificity was influenced by the watermilfoil species on which they were reared. Weevils reared on Eurasian watermilfoil tended to oviposit on Eurasian watermilfoil, spent more time on Eurasian watermilfoil than on other plants, and spent more time off plants and took longer to oviposit when Eurasian watermilfoil was removed. Weevils reared on northern watermilfoil did not exhibit a preference for either watermilfoil species in oviposition or in time allocation, although they oviposited on and spent significantly more time on watermilfoils than on other species. Rearing of the two populations on their complementary watermilfoil hostplant resulted in responses typical of the rearing plant, not the original host. These results show that although both weevil populations are watermilfoil specialists, Eurasian-reared weevils prefer Eurasian watermilfoil in general host attraction and oviposition, whereas northern-reared weevils do not. The results support the contention that E. lecontei may be a good biocontrol agent for Eurasian watermilfoil because of its high specificity. The results also suggest that the current host range expansion of the weevil to Eurasian watermilfoil has the potential to become a host shift due to the increased specificity. Herbivory in freshwater systems is not well studied, and the E. lecontei-M. spicatum relationship is a rare example of submersed freshwater specialist herbivore-host-plant interactions.     

Vision is Important for Plant Location by the Phytophagous Aquatic Specialist Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae)

The aquatic milfoil weevil Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae) is a specialist on Myriophyllum spp. and is used as a biological control agent for Eurasian watermilfoil (Myriophyllum spicatum L.), an invasive aquatic macrophyte. We show evidence that visual cues are important for plant detection by these weevils. Weevils had difficulty locating plants in dark conditions and were highly attracted to plant stems in the light, even when the plant sample was sealed in a vial. However, weevils were equally attracted to both M. spicatum and another aquatic macrophyte, coontail (Ceratophyllum demersum L.) in vials. Turbidity (0–100 NTU) did not significantly influence visual plant detection by the weevils. This work fills a void in the literature regarding visual plant location by aquatic specialists and may help lead to a better understanding of when and where these weevils will find, accept, and damage their target host-plants.     

High phenolic content fails to deter mesograzer consumption of <Emphasis Type="Italic">Myriophyllum spicatum</Emphasis> (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the USA. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of herbivores in determining milfoil invasion success via the potential mechanisms of enemy release and biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured macrophyte phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was more than two times higher than in the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory. Furthermore, amphipod consumption rates in our study were an order of magnitude higher than amphipod consumption rates reported in milfoil’s native range, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis in choice experiments. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed 2.5 times more milfoil than E. canadensis in a choice feeding trial. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance.     

Physical control of Eurasian watermilfoil in an oligotrophic lake

The introduction of Eurasian watermilfoil (Myriophyllum spicatum) into oligotrophic waters of high water clarity in temperate zones of North America has produced growth in excess of 6 m depth and yearly biomass approaching 1000 g m^–2 dry weight. From its initial observation in Lake George, New York, USA in 1985, by 1993 milfoil had spread to 106 discrete locations within the lake. A 7-year study of one site having no management showed milfoil to grow expansively, suppressing native plant species from 20 in 1987 to 6 in 1993 with the average number of species m^–2 quadrat declining from 5.5 in 1987 to less than 2 in 1993. Management of milfoil by means of hand harvesting, suction harvesting and benthic barrier has reduced the number of unmanaged sites from 106 in 1993 to 11. One year post-treatment at sites utilizing suction harvesting, showed a greater number of native species at all sites than pretreatment with a substantial reduction in milfoil biomass. At sites where benthic barrier was removed 1–2 years after installation, milfoil had recolonized 44% of grid squares within 30 days. Ninety days after barrier removal 74% of grid squares contained milfoil and one year later 71% of the grids supported milfoil. During the first year following mat removal, the average number of species m^–2 peaked at 4.7 and stabilized at 4.5 during the second year. Hand harvesting by SCUBA in areas of limited milfoil growth (new sites of infestation and sites of former treatment) was found to reduce the number of milfoil plants present in subsequent years. Hand harvesting did not eliminate milfoil at any of the sites and regrowth/colonization necessitated reharvesting every 3 or more years. Results of evaluations of physical plant management techniques indicate that (1) an integrated program utilizing different techniques based on plant density reduced the growth of milfoil and (2) long term commitment to aquatic plant management is necessary since none of the techniques employed singly were found to eliminate milfoil.     

Mutual benefit interactions between banded pine weevil Pissodes castaneus and blue‐stain fungus Leptographium serpens in maritime pine

• 1 Ecological interactions between banded pine weevil Pissodes castaneus and blue‐stain fungus Leptographium serpens, when simultaneously sharing the same host plant (maritime pine Pinus pinaster) in winter and spring, were investigated. Temporal components of the interaction were taken into account by either introducing the weevils and the pathogen simultaneously or sequentially, with the weevils being introduced 1 month after the fungal inoculation.
• 2 We measured larval mortality, development time, offspring number, sex ratio and body size of P. castaneus. Phloem phosphorus and nitrogen concentrations were also assessed. Furthermore, we tested whether: (i) emerging offspring transported propagules of the fungus; (ii) artificially‐contaminated weevils may transmit the disease to healthy trees; and (iii) field collected P. castaneus carry the fungus.
• 3 The fungus enhanced weevil colonization and brood production in both seasons. During winter and spring, adults from trees where the pathogen was inoculated prior to weevil introduction emerged earlier than weevils from trees where they had been introduced simultaneously with the fungus. During winter, weevils from pre‐inoculated trees were also larger. Sex ratio and larval mortality were not affected. Leptographium serpens did not affect phloem nitrogen content but phosphorus content was greater in plants inoculated with the pathogen, which may explain the findings on weevil growth.
• 4 Sixty‐five percent of the weevils that emerged from inoculated trees carried spores of L. serpens, although no successful isolation was made from field collected weevils. The fungus was recovered from 25% of the trees infested with artificially‐contaminated weevils.
• 5 These results suggest that P. castaneus benefits from the presence of L. serpens and may contribute to its spread.

     

Effect of population density of the Azolla weevil (<Emphasis Type="Italic">Stenopelmus rufinasus</Emphasis>) on the surface cover of the water fern (<Emphasis Type="Italic">Azolla filiculoides</Emphasis>) in the UK

Azolla filiculoides Lamarck (Azollaceae) (water fern), is an invasive floating macrophyte capable of rapid growth leading to the complete coverage of water surfaces. The North American weevil Stenopelmus rufinasus, Gyllenhal (Coleoptera: Curculionidae) is a biological control agent that has become naturalised in the UK. To determine the effects of the weevil on A. filiculoides in ‘semi-controlled’ conditions, weevils were introduced at four stocking densities into outdoor floating tanks containing A. filiculoides. The introduction of weevils at all densities resulted in a significant decline in percentage cover of A. filiculoides compared to the control. Increasing weevil stocking density resulted in a more rapid reduction in the area of green A. filiculoides. The highest stocking density of 400 weevils per m² caused complete kill within a month of introduction. The UK summer provides conditions that are suitable for weevil reproduction and herbivory, and even the lowest stocking density of weevils tested (100 weevils per m²) can have a substantial impact on the surface cover of A. filiculoides. Although the weevil is effective in summer, augmentation is required annually for reliable control.     

Interactions between invasive Eurasian watermilfoil and native water stargrass in Cayuga Lake,NY, USA

Aims Eurasian watermilfoil (Myriophyllum spicatum L.) is a common invasive plant in American lakes and has many negative impacts on invaded ecosystems. Drastic decline of this plant at the northern end of Cayuga Lake in the New York State has occurred since the 1980s, with a much smaller magnitude of decline of Eurasian watermilfoil at the southern end (Johnson et al. 2000) During the same period, increases in the abundance of native plants, particularly water stargrass (Heteranthera dubia (Jacq.) MacMill.), have been observed (Johnson et al. (1998)) We aimed to investigate the mechanisms responsible for the decline of Eurasian watermilfoil and evaluate the responses of co-occurring plants at the two ends of Cayuga Lake over time. We hypothesized that plant interactions might have contributed to the drastic decline of Eurasian watermilfoil, particularly allelopathy by native water stargrass.Methods A lake survey was conducted to assess distribution and abundance of plant communities at the northern end and the southern end of Cayuga Lake. Additionally, two sets of greenhouse experiments were conducted to investigate the interactions between invasive Eurasian watermilfoil and native water stargrass. A competition experiment evaluated intra- versus inter-specific competition among plants grown together; an allelopathy experiment examined the responses of plants to each other's extracts.Important findings The lake survey showed that water stargrass was extremely abundant at the northern end, whereas Eurasian watermilfoil was sparse at the northern end but one of the most common species at the southern end. The survey also revealed that water stargrass was more abundant than Eurasian watermilfoil at sites where the two species coexisted in the lake. Results from greenhouse experiments revealed no effects of Eurasian watermilfoil on water stargrass growth. However, Eurasian watermilfoil biomass was reduced by 46% when treated with high concentration of water stargrass extracts. This is likely due to osmotic effects rather than allelopathic effects of water stargrass. We proposed several possible reasons for the drastic decline of Eurasian watermilfoil and the increase in water stargrass abundance at the northern end of Cayuga Lake, including space competition, nutrients, substrates, wind exposure and water clarity in addition to insect herbivory and mechanic harvesting.     

Enumeration and Characterization of Bacterial Colonists of a Submersed Aquatic Plant, Eurasian Watermilfoil (Myriophyllum spicatum L.)

A simple procedure for enumerating and grouping the bacterial colonists of Eurasian watermilfoil (Myriophyllum spicatum L.) is described. Colony characteristics of bacteria associated with M. spicatum were better defined and more stable on nutrient-poor, diluted nutrient broth agar than on high-nutrient media. Acinetobacter, Cytophaga, Flavobacterium, Moraxella, Pseudomonas and/or Alcaligenes, and Vibrio/Aeromonas spp., as well as two highly fastidious unidentified bacterial groups (gram-negative rods and gram-negative cocci), were associated with cultured watermilfoil during January, February, May, June, July, and August 1988. In Lake Wingra (Madison, Wis.), Micrococcus spp. and enterobacters were also associated with Eurasian watermilfoil during July, August, and October 1987.     

Effect of the stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, on the weed Emex australis

The stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, are two potential biological control agents for the annual weed, Emex australis. Neither pathogen nor weevil affected the development of rosette (5-wk-old) E. australis, but both significantly reduced stem length and number of new fruits in mature plants (10-wk-old) in growth room experiments. Stems grew two-thirds less than controls in plants attacked by weevils, had no net increase when both weevils and fungus were present, and collapsed in the presence of the fungus alone. Attack by weevils elicited a host response that slowed the development of the disease. Phomopsis emicis and Perapion antiquum reduced the ability of E. australis to produce new fruits by 77% and 68%, respectively. Pathogen and weevil together reduced the number of new fruits by 83%. Perapion antiquum did not carry or provide infection sites for P. emicis. The lack of damage by pathogen and weevil to rosettes could compromise their effectiveness as biological control agents.     

Evaluation and implications of Andean potato weevil infestation sources for its management in the Andean region

The Andean potato weevil Premnotrypes suturicallus Kuschel (Coleoptera: Curculionidae) is one of the most damaging potato (Solanum spp.) pests in the Andes. The objective of this study was to get a better understanding of weevil infestation sources and its distribution in potato fields as well as on the effect of potato cropping intensity and farmers’ harvest practices on weevil infestation to provide better clues for its management. For this purpose, a methodology was developed to assess weevil field densities in potato. A Taylor’s power law indicated that 177 and 69 samples are necessary to assess low (0.5 weevils/m²) and high (5 weevils/m²) weevil densities for a moderate reliability or precision level (D = 0.5). In potato fields, overwintering weevil densities were highest at field borders with 3.5 weevils/m² at a distance of <2.5 m compared to 0.7 weevils/m² at distances >10 m from the field border. No effects of time of harvest could be detected on soil overwintering weevil densities. The quantification of the larval density per potato plant after harvest showed that fields after 2‐year potato rotations had eight times more overwintering weevils compared to 1‐year rotations. Weevil infestation sources were mainly potato fields with the highest infestation (89%) followed by olluco (Ullucus tuberosus) and oat (Avena sativa) fields having volunteer potato plants (35%). The results confirm and support previous assumptions about the importance of the cropping systems for Andean potato weevil infestation and management. The confirmation that weevils do not occur or only in neglected numbers on fallow fields supports the use of plastic barriers to effectively exclude migrating flightless adult weevils to potato fields cultivated after fallow. The distribution of overwintering weevils indicates that farmers could concentrate efforts to control adult weevils mainly to the first meters of potato fields.     

Overwintering conditions affect cold hardiness,survival, and post-overwintering fitness of the pea leaf weevil

Overwintering conditions affect the physiological state of ectotherms, and therefore, their cold hardiness and survival. A measure of the lethal and sublethal impacts of overwintering conditions on pest populations is crucial to predict population dynamics and to manage pests the following spring. The impact of winter conditions can be most intense for invasive insects undergoing range expansion. Insect herbivores can display plastic host use behaviours that depend on their body condition following winter. The pea leaf weevil, Sitona lineatus L. (Coleoptera: Curculionidae), is an invasive pest of field peas, Pisum sativum L., and faba bean, Vicia faba L. (Fabaceae). Pea leaf weevil has expanded its range in North America to include the Prairie Provinces of Canada. This study investigated the effects of temperature and microhabitat on overwintering survival and cold hardiness of pea leaf weevil in its expanded range. Further, we investigated the sublethal effect of overwintering temperature and duration on post-overwintering survival, feeding, and oviposition of pea leaf weevil. We also investigated the role of juvenile hormone in modulating body condition of overwintering weevils. The overwintering survival of pea leaf weevil adults increased with soil temperature and varied with region and microhabitat. More weevils survived winters when positioned near tree shelterbelts compared to open alfalfa fields. The supercooling point of pea leaf weevil varied throughout its expanding range but did not differ for weevils held in the two microhabitats. The average threshold lethal temperature of pea leaf weevil at all three sites was −9.4 °C. Weevils that overwintered for a longer duration and at a higher temperature subsequently fed more on faba bean foliage and laid more eggs compared to those which overwintered for a shorter duration at a lower temperature. Our findings highlight that warm winters would increase overwintering survival and post-overwintering fitness, facilitating further pea leaf weevil invasion northward in the Prairie Provinces of Canada.     

Influence of host log age and refuge from natural enemies on colonization and survival of Phoracantha semipunctata

The pine cone weevil, Pissodes validirostris Gyll. (Coleoptera: Curculionidae), attacks seed cones of most Eurasian pine species, except these of Swiss stone pine (Pinus cembra L.). Behavioural responses of adult weevils to cone volatile emissions of Swiss stone pine and to those of a common host, mountain pine (Pinus uncinata Ram.), were compared in an olfactometer. Weevils were significantly attracted by the volatile blend emitted by mountain pine, but Swiss stone pine volatiles elicited an inverse response, with most weevils moving in the opposite direction to the odour source. However, the majority of second instar weevil larvae that were extracted from mountain pine cones and transferred into Swiss stone pine cones were capable of developing to the adult stage. This suggests that Swiss stone pine cones do not contain strong feeding deterrents that could prevent larval development. The possible factors involved in the absence of colonization of Swiss stone pine cones by cone weevils are discussed.     

Influence of release size on establishment and impact of a root weevil for the biocontrol of houndstongue (Cynoglossum officinale)

The root-boring weevil, Mogulones cruciger, was introduced into Canada to control the weed, houndstongue (Cynoglossum officinale). To optimise its use as a biocontrol agent, a 2-year study was performed in British Columbia, Canada to test if the number of M. cruciger released at sites predicted subsequent declines in weed populations. No, 100, 200, 300 or 400 weevils were released in 1999 at field sites (five replicates) corresponding to discrete populations of houndstongue separated by distances of 0.3–3 km. The sites were subsequently monitored for weevil establishment, population change, and host attack, and houndstongue population change. By 2001, M. cruciger had established at all 20 release sites and was present in low numbers in three of five control sites. The year following release, release size was positively correlated with number of adult weevils collected, their damage to host plants, and with subsequent numbers of larvae per plant. In contrast, houndstongue populations were reduced at the same rate and amount, regardless of the experimental release size, within 2 years of release. Significant release treatment×time interactions indicated that factors other than M. cruciger contributed to houndstongue reductions (e.g. drought). However, overall the addition of weevils accelerated the reductions relative to sites with no weevils added. Our study demonstrated that the lowest number within a range of release sizes typically used in weed biocontrol programmes (i.e. 100) was as effective as 200–400 weevils in achieving a consistent amount and rate of houndstongue reduction, and thus, could be implemented to optimise weevil use and achieve predictable biocontrol.     

Effects of pepper (Capsicum chinense) genotypic diversity on insect herbivores

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Field assessment of a frond-feeding weevil, a successful biological control agent of red waterfern, Azolla filiculoides, in southern Africa

Azolla filiculoides (red waterfern) is a small, floating fern native to South America, that has invaded aquatic habitats, predominantly water resevoirs in southern Africa. A frond-feeding weevil, Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae), was imported from Florida, USA, and released as a biological control agent against this weed in South Africa at the end of 1997. To date, 24,700 weevils have been released, which has resulted in local extinction of red waterfern at 81% of the 112 release sites. The weevil has not failed to control a single site. Several sites were, however, lost due to flooding or drainage of dams. The surface area of weed controlled totalled 203.5 ha. On average, A. filiculoides was controlled in infested sites in 6.9 (±4.3) months. The weed recolonized at 22 of the sites (through either spore germination or dispersal by waterfowl), but the weevils subsequently spread to all of these sites and successfully caused local extinction of the weed at 18 of the sites. Five years after the release of the weevil, the weed no longer poses a threat to aquatic systems in southern Africa. In comparison to other biological control programs of aquatic weeds, the program against A. filiculoides in southern Africa ranks among the most successful cases anywhere in the world.     

Monitored releases of Rhinoncomimus latipes (Coleoptera: Curculionidae), a biological control agent of mile-a-minute weed (Persicaria perfoliata), 2004–2008

Mile-a-minute weed, Persicaria perfoliata (L.) H. Gross, is an invasive annual vine of Asian origin that has developed extensive monocultures, especially in disturbed open areas in the Mid-Atlantic region of the United States. A host-specific Asian weevil, Rhinoncomimus latipes Korotyaev, was approved for release in North America in 2004, and weevils have been reared at the New Jersey Department of Agriculture Beneficial Insect Laboratory since then. By the end of 2007 more than 53,000 weevils had been reared and released, mostly in New Jersey, but also in Delaware, Maryland, Pennsylvania, and West Virginia. The beetles established at 63 out of 65 sites (96.9%) where they were released between 2004 and 2007, with successful releases consisting of as few as 200 weevils. Weevils were recorded at 30 additional non-release sites in New Jersey, where they had dispersed at an average rate of 4.3 km/year. Standardized monitoring of fixed quadrats was conducted in paired release and control sites at eight locations. Significant differences in mile-a-minute weed populations in the presence and absence of weevils were found at three locations, with reduction in spring densities to 25% or less of what they had been at the start within 2–3 years at release sites, while weed densities at control sites were largely unchanged. Mile-a-minute weed populations at a fourth site were similarly reduced at the release site, but without control data for comparison due to rapid colonization of the paired control site. At the other four locations, all on islands, mile-a-minute weed populations were reduced at both release and control sites without large weevil populations developing, apparently due to environmental conditions such as late frost and extreme drought.     

Contrasting host: parasitoid synchrony drives differing levels of biocontrol by two introduced Microctonus spp. in northern New Zealand pastures

Two species from the genus Microctonus Wesmael (Hymenoptera: Braconidae) have been introduced into New Zealand as biocontrol agents of pest weevils in pasture. Both parasitoids have similar life cycles and co-exist in pasture along with their respective weevil hosts. However, winter parasitism rates by M. hyperodae Loan are low in comparison to the Irish biotype of M. aethiopoides’ Loan. Population studies at two Waikato sites over three consecutive seasons of parasitoid activity showed that M. aethiopoides recovered from near extinction each spring and built up to effective levels by winter because hosts were available continuously throughout summer and autumn. In contrast, M. hyperodae began each season at higher larval populations and parasitism levels than M. aethiopoides, but populations and parasitism levels declined during late summer and early autumn due to low host availability. The contrast between species is consistent with the high levels of endophyte-conferred pest-resistant grass in the pastures, which impacts strongly on M. hyperodae’s host weevil abundance during summer but has no effect on M. aethiopoides’ host weevils which feed only on clovers. It was accentuated by a warming climate with the now regular occurrence of a third host generation after most M. hyperodae adult activity had ceased.

     

Biological control of Salvinia molesta in some areas of Moremi Game Reserve,Botswana

The Curculionid weevil, Cyrtobagous salviniae Calder & Sands, has been established on Salvinia molesta (salvinia) in Botswana. The study determined the intensity of weevil activity on the weed at the margin and the centre of four selected sites on the Khwai system of the Okavango Delta. Random samples of salvinia were collected from each site at monthly intervals in 1998 to extract weevils and to demonstrate the effect of weevil on the weed. The rate at which the weed was controlled at different sites varied with mat and weevil density. The biological control at Paradise Pools was moderate without much increase in the weed biomass per m², while fluctuations in both weevil and plant populations were noticed in the Khwai streams. Weevil numbers remained low at Dombo for the first nine months of the year, during which time the mat density increased. With the onset of higher temperatures, weevil numbers increased to 155 and 457/kg fresh weight of salvinia at the margin and centre respectively in Dombo Pool in early summer. A significant control in MGR 6 HATAB pool between mat biomass and the weevil number resulted in the disappearance of the weed in three months. This study shows that C. salviniae is an effective biological control agent of S. molesta in semiarid areas.     

Flight behaviour of the cabbage seedpod weevil

The influence of host odour, windspeed, position of the sun, and temperature on flight behaviour of the cabbage seedpod weevil (Ceutorhynchus assimilis Paykull) were studied. This weevil showed a positive anemotaxis (upwind flight) inside the odour plume of a host crop (Brassica napus L.). Outside the odour plume the weevil showed a pronounced phototaxis at windspeeds below 1.5 m/s. At higher windspeeds, the seedpod weevils flew downwind. The cabbage seedpod weevil flies most readily at low windspeeds (less than 0.5 m/s) and at temperatures above 22 °C.