Mayetiola destructor (Diptera: Cecidmyiidae) host preference and survival on small grains with respect to leaf reflectance and phytohormone concentrations

The Hessian fly Mayetiola destructor (Diptera: Cecidmyiidae) is a major pest of wheat, globally. We conducted a series of laboratory choice and no-choice assays to quantify Hessian fly host preference for barley (cv. Champion), oat (cv. Cayuse), susceptible (cv. Alturas), and resistant (cv. Hollis) wheat. In addition, larval survivorship and adult emergence were compared among the evaluated host plants. We then examined whether insect preference for a host can be explained by differences in plant spectral reflectance. Further, larval survivorship and adult emergence were compared among host plants in relation to phytohormone concentrations. Hessian flies laid more eggs on wheat compared to either oat or barley. Spectral reflectance measurements of leaves were similar between susceptible and resistant wheat cultivars but different from those of barley and oat. Our results suggested that higher reflectance in the near-infrared range and lower reflectance in the visible range may be used by females for host selection. Hessian fly larvae were unable to develop into the pupal stage on resistant wheat and oat. No significant difference in larval survivorship was detected between the susceptible wheat and barley. However, adult emergence was significantly higher on barley than the susceptible wheat. Phytohormonal evaluations revealed that salicylic acid (SA) may be an important contributor to plant defense response to larval feeding as relatively higher concentrations of SA were present in oat and resistant wheat. While resistance in the resistant wheat is achieved only through antibiosis, both antibiosis and antixenosis were in effect rendering oat as a non-host for Hessian flies.

Putative Pleiotropic Effects of the Knock-Down Resistance (L1014F) Allele on the Life-History Traits in Anopheles Gambiae.

BackgroundExisting mechanisms of insecticide resistance have been known to help the survival of mosquitoes following contact with chemical compounds, even though they could negatively affect the life-history traits of resistant malaria vectors. In West Africa, the knock-down resistance mechanism, kdrR (L1014F) is the most common. However, little knowledge is available on its effects on mosquito life traits. We investigated the fitness effects associated with this knock-down resistance allele in Anopheles gambiae sensu stricto (s.s.).MethodsTwo laboratory reference strains of An. gambiae s.s., Kisumu (susceptible) and KisKdr (kdr resistant) were used. Female mosquitoes were fed and allowed to lay eggs. Fecundity and fertility were assessed by examining the number of eggs per mosquito and larval hatching rates. Larval survivorship and pupation rates were also measured. Female mosquitoes of both strains were fed through membrane feeding assays, then the blood feeding success, blood volume and adult survivorship were monitored. ResultsAn. gambiae carrying kdrR allele (KisKdr) showed a lower ability to lay eggs. The mean number of larvae in the susceptible strain Kisumu was overall threefold higher than that seen in KisKdr strain with significant difference in the hatching rates (81.89% in Kisumu versus 72.89% in KisKdr, p= 0.003). KisKdr larvae had a significant higher larval survivorship than Kisumu. The blood feeding success was significantly higher (p= 2.2.10-16) in the resistant mosquitoes (84%) than that in the susceptible ones (34.75%). However, the mean blood volume was 1.36 µL/mg, 1.45 µL/mg and 1.68 µL/mg in Kisumu, homozygote and heterozygote KisKdr respectively. After blood feeding, the heterozygote KisKdr displayed highest survivorship when compared to that of Kisumu.ConclusionsOur findings provide novel insights on fitness effects of the kdrR (L1014F) allele in An. gambiae. The presence of this resistance allele tends to have an impact on mosquito life-history traits such as fecundity, fertility, larval survivorship and blood feeding behaviour. These data could help to guide the implementation of more reliable strategies for the control of malaria vectors.

Larval Aggregation of Heortia vitessoides Moore (Lepidoptera: Crambidae) and Evidence of Horizontal Transfer of Avermectin

Aquilaria sinensis (Lour.) Gilg is an economically important tree species that produce the highly prized agarwood. In recent years, agarwood production has been seriously threatened by the outbreak of Heortia vitessoides Moore, a leaf-eating pest that shows gregariousness during the larval stage. However, little attention has been paid to the aggregation behavior of H. vitessoides larvae. In the present study, we collected 102 cohorts of H. vitessoides larvae (13,173 individuals in total) in the wild; 54 cohorts were comprised of the same-instar larvae, and 48 cohorts were comprised of larvae with different developmental stages (instars). In general, young larvae (<third instar) tended to form large aggregations, whereas older-instar larvae were either solitary or formed small aggregations. Laboratory studies showed a strong aggregation tendency in the newly hatched and second-instar larvae of H. vitessoides, whenever the individuals originated from the same or different sibling cohorts. In addition, all newly hatched larvae died within two days after they were isolated. When newly hatched larvae were initially assigned in 10-larvae cohorts (containing sibling individuals) or 20-larvae cohorts (either containing individuals originating from the same or different sibling cohorts), their larval survivorship, duration of larval stage, and adult emergence were not significantly different. Interestingly, combining avermectin-treated larvae (donors) with untreated ones (receptors) significantly decreased larval survivorship and adult emergence of receptors, indicating a horizontal transfer of avermectin among H. vitessoides larvae. This study enhances our understanding of the population ecology of H. vitessoides, and may bring novel insights into the management strategies against this pest.

Larval Survivorship and Settlement of Crown-of-Thorns Starfish (Acanthaster cf. solaris) at Varying Chlorophyll Concentrations

The dispersal potential of crown-of-thorns starfish (CoTS) larvae is important in understanding both the initiation and spread of population outbreaks, and is fundamentally dependent upon how long larvae can persist while still retaining the capacity to settle. This study quantified variation in larval survivorship and settlement rates for CoTS maintained at three different chlorophyll concentrations (0.1, 1.0 or 10.0 &micro;g.L-1), achieved by varying densities of single-celled flagellate phytoplankton, Proteomonas sulcata. Based on the larval starvation hypothesis we expected that low to moderate algal concentrations would significantly constrain both survival and settlement. CoTS larvae were successfully maintained for up to 50 days post-fertilization, but larval survival differed significantly between treatments. Survival was greatest at intermediate (1.0 &micro;g.L-1) chlorophyll concentrations, and lowest at highest (10.0 &micro;g.L-1) chlorophyll concentrations. Rates of settlement were also highest at intermediate (1.0 &micro;g.L-1) chlorophyll concentrations and peaked at 22 days post-fertilization. Peak settlement was delayed at low chlorophyll concentrations, probably reflective of delayed development, but there was no evidence of accelerated development at high chlorophyll concentrations. CoTS larvae were capable of settling 17-43 days post-fertilization, but under optimum conditions with intermediate chlorophyll concentrations, peak settlement occurred at 22 days post-fertilization. Moderate increases in nutrient concentrations and algal densities may increase the number of CoTS that effectively settle, but are unlikely to influence dispersal dynamics.

Aestivation provides flexible mechanisms for survival of stream drying in a larval trichopteran (Leptoceridae)

Some freshwater species aestivate to resist drying; however, little is known about factors affecting post-aestivation survival. Climate change prolongs drying and may make short bursts of flow more frequent in southern Australian streams, thereby affecting aestivation success. The tolerance of larval Lectrides varians (Mosley) to drying was tested by inducing aestivation in dry or moist sediment and then re-immersing larvae and measuring survival and activity. Survival did not differ between individuals that were continually immersed (78%) or aestivating on moist sediment (70.5%) after 16 weeks. Survival was significantly lower on dry sediment (29.3%). Furthermore, some larvae showed delayed responses to re-immersion; 65% of individuals showed activity within 4.5 h, whereas over 30% of larvae did not become active until 72 h after re-immersion. L. varians can survive extended periods (112 days) without surface water, showing a bimodal response to re-immersion that increases the likelihood of population persistence by enabling some larvae to remain aestivating during short-lived bursts of stream flow. L. varians populations will therefore be more robust to prolonged stream drying and short-lived flow events than are some other insect taxa, although as the duration of aestivation increases larval survivorship decreases, suggesting that there are limits to the flexibility of aestivation traits.