RAPID PROCEDURE FOR DETECTING CIGUATOXINS IN FISH

Flesh and viscera/gill tissues of six amberjacks (Seriola dumerilii), suspected positive for ciguatoxins, were each extracted and the toxins partially purified. Both flesh and viscera/gill of only five fish were toxic to mice exhibiting ciguatoxins (CTX) symptoms. The methanol extracts of the five fish were pooled and concentrated, the volume of flesh extract was 50.0 mL (129.4 mg toxins/mL) and viscera/gill had 25.0 mL (25.5 mg toxins/mL). Pooled extracts exhibited CTX symptoms in mice but only flesh killed mice in 6 h and the LD₅₀ was 1.72 mg toxins. The lethal potencies of the pooled flesh killed mice in 6 h and the LD₅₀ was 1.72 mg toxins. The lethal potencies of the pooled flesh was 198.17 g fish, equivalent to 58.3 mouse unit. An efficient fractionation and purification procedure was developed for the extracts using an HPTLC and silica gel 60 plate with a chromatographic solvent mixture of chloroform:methanol:water (60:35:8, v/v). The system yielded 10 fractions for flesh and 9 for viscera/gill. Scanned plates were subdivided into three equal zones, each scraped, methanol extracted and tested in mice. The 2nd zone (R_f fractions between 0.40 and 0.66) was very toxic to mice compared to 1st or 3rd zones and the mice had CTX symptoms. The scanner for this 2nd zone had a cluster of minor peaks on both sides of the major one with a sum total area of 62.47% indicating multiplicity of CTX in amber-jack fish. The major peak, at retention time of 1.48 s and a single area of 43.28%, is believed to be the main ciguatoxins present. The HPTLC is a rapid and sensitive procedure for ciguatoxins in fish flesh extracts with a detection limit of 40.0 ± 1.9 picogram toxins.     

Factors determining the pest status of the rose-grain aphid, Metopolophium dirhodum (Walker), on winter barley in the United Kingdom

To test the hypothesis that early maturation of barley affords it some resistance to cereal aphids, the colonisation, survival, growth, reproduction and emigration of M. dirhodum on barley was measured and compared with previously published results for this aphid on wheat.
Barley plants were colonised as readily as wheat plants. Survival to maturity was similar on both hosts. Relative growth rate was higher on wheat than on barley for most of the season while reproduction was similar on both. Emigration, measured alate production, was significantly higher on mature wheat than on mature barley.
It is proposed that the increase of M. dirhodum populations on barley is normally terminated early in the year directly plant maturation and consequent drying out of the leaves, resulting in the death of aphids. This is different to the situation on wheat, which matures later, and where emigration following plant induced alate induction has been shown to be the major factor affecting the decline in population growth.
These results support the hypothesis that early maturation of barley provides some resistance to M. dirhodum.     

Natural convection from leaves at realistic Grashof numbers

Abstract. The boundary layer resistance of model leaves was measured in still air, at a range of leaf-to-air temperature differences. The results were compared to those calculated from standard formulae for natural convection. The agreement between observed and calculated was only satisfactory when Grashof numbers exceeded about 105. At the lower Grashof numbers, which often prevail in nature, the observed rates of heat transfer considerably exceeded those calculated.     

Life history and production of the predatory caddisfly Rhyacophila vao Milne in a spring-fed stream

1. The predatory caddisfly Rhyacophila vao Milne (Rhyacophilidae) displayed a 2-year, semivoltine life cycle in a small, spring-fed stream in southern Alberta, Canada. Three overlapping cohorts were identified throughout the 2-year sampling program, with five larval instars recognized. Larvae overwintered in instars I —ELI, developed to instar IV by summer and instar V by autumn. The second winter was spent in instar V, Pupation occurred from late May to August and did not commence until stream temperature exceeded 3°C. Adults were collected from mid-June to early September. 2. Larvae displayed a diphasic growth pattern: Phase I, a positive, non-linear growth rate for instars I-IV; Phase II, a constant growth rate during instar V. Phase I coincided with increasing stream temperatures in late winter and spring, where mean instantaneous growth rates (maximum of 2.78% dry wt day^?1) were significantly correlated with stream temperatures. Although the duration of Phase II spanned a temperature range similar to that in Phase I, the instantaneous growth rate remained temperature-independent at 0.87% dry wt day^?1. 3. Larvae exhibited a type III survivorship curve (i.e. an exponential decrease on an arithmetic scale), with the finite rate of mortality averaging 0.80% larvae day^?1. 4. Cohort 1 (later part of 1983 year class) displayed lower total production compared with the equivalent growth phase in the 1984 year class (Cohort 2). In contrast, production of the non-linear and linear growth phases of Cohort 2 was similar. Periods of similar growth characteristics for Cohorts 1 and 2 had comparable P/B ratios for both the unadjusted and time-adjusted annual estimates, although higher ratios were observed for the non-linear growth phase of Cohort 2. Total cohort production (linear + non-linear growth phases) could only be calculated for Cohort 2, and was 870.2 ± 1011.4 mg dry wt m^?2. The corresponding cohort P/B ratio was 5.01 and the adjusted annual P/B, 3.01. 5. Annual larval production (±SE) for the first and second years of the study was similar (Year 1, 480.0 ± 387.5mg dry wt m^?2; Year 2, 526.9 ± 967.5mg dry wt m^?2) as were unadjusted and annual P/B ratios.     

Frequency and geographical distribution of specific virulence factors in Bremia lactucae populations in England from 1973 to 1975

Samples of lettuces infected with Bremia lactucae were collected from seven areas of England during 1973-5. These were tested under controlled environmental conditions on a range of differentially resistant cultivars to determine the frequency of virulence (V) factors in the pathogen population. In most cases the results could be accommodated by the relationship advanced by Crate & Johnson (1976). A virulence phenotype was determined for each isolate and individual virulences within isolates were identified as present at high or low frequencies. The most common V factors were V3, V4, V6 and V8 which reflected the resistance (R) factor complement of lettuce cultivars currently in commercial production. There were few gross differences in the geographical distribution of virulence factors between the seven areas. In the West Central region, however, which encompasses the Evesham and Lancashire production areas, results indicated that the use of cultivars containing resistance (R) factors 2 and 8 respectively would have provided protection from the disease during 1974. There were differences in virulence combinations at the holding or locality level within each large area, which could be exploited by growers providing testing was done in their particular locality. Universally susceptible cultivars and those with only limited resistance factors were often found to be infected with B. lactucae populations which contained large numbers of V-factors. It might have been expected that these pathogen populations would have been restricted to cultivars with large numbers of R-factors. This finding conflicts with the suggestion that pathogen populations with complex virulence patterns are ‘unfit’ as compared with those with more simple virulence components.     

Seasonal changes in fat content,form, state of gonads and length of adult life in the sycamore aphid,Drepanosiphum platanoides (Schr.)

• 1 Even when reared under similar conditions of crowding and nutrition, first and second generation adult sycamore aphids differ in their morphology and physiology. Second generation adults have a smaller abdomen, a longer gut, greater fat reserves, less developed gonads, a longer reproductive diapause and live longer than first generation aphids.
• 2 Second generation nymphs reared in isolation develop into adults which have less well-developed gonads but a reproductive delay similar to that of first generation adults.
• 3 Crowding of second generation adults prolongs the reproductive diapause.