Host Age Choice for Oviposition in Pauesia juniperorum (Hymenoptera: Braconidae: Aphidiinae) and its Effect on the Parasitoid's Biology and Host Population Growth

Pauesia juniperorum has been selected as a potential agent for the biological control of Cinara cupressivora, an important introduced pest of conifers in Africa. As part of the pre-introductory assessment studies, selection of different host age categories for oviposition was studied in choice and no choice experiments. The duration of development and adult size of parasitoid progeny developing in different host age categories were compared. The effects of parasitism on survival and reproduction of five categories of the apterous morph of the host ranging in age from 3-15 days was also studied. The age of hosts had a significant influence on the degree with which different categories were parasitized. Host defensive behaviour, which increased with age, influenced the outcome of attempts by the parasitoid to oviposit. The effect was greater in older hosts, but in young hosts their small size was more important in enabling aphids to escape parasitism. The duration of development decreased while adult size increased with age of hosts. This demonstrated that fitness attributes of P. juniperorum were affected by larval ontogeny with older/larger hosts being more optimal. The ability of the parasitoid to exploit hosts of varying age was suggestive of a high degree of phenotypic plasticity which is potentially of adaptive value. Hosts parasitized before they were 9-days-old became mummies without reproducing. Those that were 9 days or older achieved part of their reproductive capacity before dying. Thus, the impact of P. juniperorum is markedly reduced when attacking older hosts.     

Thyroid dose and thyroid cancer incidence after the Chernobyl accident: assessments for the Zhytomyr region (Ukraine)

In the Zhytomyr region, about 52 000 measurements of the ¹³¹I activity in thyroids were performed. On the basis of these measurements, individual doses have been assessed for the people monitored and age-dependent average doses have been estimated for those settlements with more than 11 direct measurements. In order to estimate the pattern of thyroid exposure in the Zhytomyr region, these doses have been interpolated or extrapolated to population groups who were not monitored during May–June 1986. For this purpose, a model has been developed based on a correlation between thyroid dose estimates with the ¹³⁷Cs deposition and the co-ordinates of the settlements relative to Chernobyl. Collective doses of people who were born in the years 1968 to 1986 were calculated. The radiation-induced thyroid cancer incidence in the period 1991 to 1995 was assessed by subtracting the spontaneous incidence from the observed incidence. The result is considerably lower than that observed in longer periods after external exposures. Possible reasons for this difference are discussed. Received: 17 January 1997 / Accepted in revised form: 2 October 1997     

Vegetative and reproductive phenological traits of Rhizophora mucronata Lamk. and Sonneratia alba Sm.

Mangrove phenology is important in understanding the past, present and future response of mangrove species to impacts of climate change. Our study is the first long term direct observation of the phenology of Rhizophora mucronata and Sonneratia alba in Kenya. Objective of the study was to determine, interpret and document the timing of the various phenoevents and phenophases, and to establish relationships between phenology and the climatic variables. Phenological traits were investigated in six monospecific mangrove stands in Gazi Bay, south of Mombasa, for 2 years. Leaf emergence, leaf fall, flower bud, flower and fruit initiation data were recorded every fortnight in 54 shoots of 9 trees at each site. Continuous leaf emergence and loss characterized by multimodal peaks was observed for the two species. Leaf emergence and leaf fall peaked in the wet months and was reduced in the dry months. There was a relationship between the leaf emergence and drop with the reproductive phenology in the two species. Mean leaf longevity for R. mucronata and S. alba was 12.8 ± 1.2 and 4.9 ± 0.5 months respectively. The reproductive cycle took approximately 16–20 months in R. mucronata and 4–5 months in S. alba. Bud initiation in R. mucronata was seasonal and occurred in October and September. Buds were observed for 8–11 months developing slowly on the shoots. Shifts were observed in the timing of flower initiation, and the flowering period lasted in total for 4–5 months. The time period from flower appearance to the developmental start of immature propagules lasted about 1 month at reforested sites and up to 5 months in stands of natural vegetation. In R. mucronata initiation of immature propagules was not seasonal and varied among the sites. Fruiting in S. alba was short and seasonal without overlap, and shifts were observed in the timing of flowering, flowering and fruiting peaks. Abscission of reproductive parts started in July and in June, respectively, in the years 2005 and 2006. Fruiting was observed starting in August in both years, and fruiting peak was reached in October in 2005 and one month earlier in 2006. In R. mucronata vegetative and reproductive phenophases significantly correlated with climatic variables, whereas in S. alba only leaf emergence and leaf fall correlated with temperature. The flowering plasticity in the reproductive phenology of the two species indicates possible sensitivity to certain climatic and environmental triggers. Our results also indicate that R. mucronata trees have a distinctly higher investment in the reproductive cycle than S. alba.