Reduction of black vine weevil larval growth by vesicular-arbuscular mycorrhizal infection

Larvae ofOtiorhynchus sulcatus (black vine weevil) were reared at three densities (zero, two or eight), on plants ofTaraxacum officinale (dandelion) with and without infection by the mycorrhizal fungusGlomus mosseae. On plants without the fungus, survival of larvae to late final instar was 84%, but this was only 43% on infected plants. The differential survival of larvae was evident in their effects on plant biomass. Significant interactions were found between larval density and infection, indicating that mycorrhizal presence mitigated the effects of herbivory at the low larval density. Infection byG. mosseae thus conferred some degree of resistance in roots to this insect and the consequences of this for horticultural and natural situations are discussed.     

Positive effects of an arbuscular mycorrhizal fungus on aphid life history traits

Two generations of two aphid species (Myzus ascalonicus and M. persicae) were reared on Plantago lanceolata plants, with and without root colonization by the arbuscular mycorrhizal fungus, Glomus intraradices. Life history traits of the aphids measured were nymphal development time, teneral adult weight, growth rate, total fecundity, adult longevity and duration of post-reproductive life. For both aphids in both generations, mycorrhizal colonization increased aphid weight and fecundity, while other traits were unaffected. The increases were consistent between generations. In a second experiment, M. persicae was reared on plants with and without the fungus, under varying N and P regimes. The results of N addition were inconclusive because there was high aphid mortality. However, under P supplementation, positive effects of the mycorrhiza on aphid growth were seen at low and medium P levels, while at high P levels these effects disappeared. The positive effects of mycorrhizal colonization reported here are contrary to the majority of previous studies with chewing insects, which have reported negative effects. A number of possible mechanisms for this apparent discrepancy are discussed. Received: 1 February 1999 / Accepted: 22 March 1999     

Mechanisms of seedling mortality by subterranean insect herbivores

Summary The nature of the mortality of germinating seeds of Vicia sativa, the common vetch, was investigated in a pot trial under controlled conditions. Chafer and tipulid larvae were restricted by nylon mesh partitions to enable radicle and/or hypocotyl herbivory to occur. The effects of the two insects were very similar. In control situations, an average of 88% of the viable seed sown recruited successfully. Hypocotyl and radicle herbivory had similar effects on seedling mortality, with recruitment of viable seed sown being reduced to 52%. When both modes of attack occurred together, successful recruitment was only 34%. Feeding on both plant parts resulted in an average post-emergence mortality of 14% of the seed sown, but the effect on pre-emergence mortality was of greater importance, amounting to 41%. The mechanisms by which soil-dwelling herbivores may cause seedling mortality in the field are discussed.     

Ecological cues, gestation length, and birth timing in African buffalo (Syncerus caffer)

We examined annual variation in the timing of conception andparturition in the African buffalo (Syncerus caffer) and thesynchrony of birth timing with resource cues, using 8 yearsof monthly birth, rainfall, and vegetation data, measured asNormalized Difference Vegetation Index (NDVI). Monthly birthshad the strongest significant correlations with NDVI and rainfalllevels 12 and 13 months in the past, respectively. In addition,the synchrony of current year births corresponds most stronglyto the synchrony of the previous year's NDVI distribution. Becausethe gestation period of buffalo has been estimated to be around11 months, these findings suggest that improved protein levels,occurring approximately a month after the first green flushof the wet season, are either a trigger for conception or conceptionhas evolved to be synchronous with correlated environmentalcues that ensure females enter a period of peak body conditionaround the time of conception and/or parturition. With a gestationperiod of approximately 340 days, parturition occurs to takeadvantage of the period when forage has its highest proteincontent. A comparative analysis of gestation periods withinthe subfamily Bovinae indicates that African buffalo have aprotracted gestation for their body size, which we suggest isan adaptation to their seasonal environment. We also found thatinterannual variation in the birth distribution suggests a degreeof plasticity in the date of conception, and variation in thenumber of calves born each year suggest further synchrony ata timescale longer than a single year.