Ivermectin as a systemic insecticide

Like many broad-spectrum drugs, the antiparasitic agent ivermectin is not equally effective in the treatment of all parasitic infestations. Predicting arthropod susceptibility relies not only on an understanding of insect and acarine biology, but also on an appreciation of the pharmacokinetic properties of the drug which can be profoundly influenced by host differences and pharmaceutical formulation. In this review, Helen Jackson attempts to explain the diverse number of ivermectin-induced effects observed among parasitic arthropods feeding on treated animals and aims to provide an overview of the current and future use of ivermectin in the systemic treatment and control of arthropod pests of livestock.     

The secretion of the systemic insecticides dimethoate and phorate into nectar

In pot experiments, root applications of dimethoate and phorate were made to plants of fuchsia, nasturtium and bean. Twenty-five mg dimethoate applied to a 5-in pot made nectar from fuchsia and nasturtium and nectaries of beans toxic to bees and Drosophila melanogaster. Similar amounts of phorate and disulfoton did not cause toxicity. Gas chromatography showed that 4 days after treatment there was at least 100 times more dimethoate than phorate in the nectar of fuchsia and nasturtium. The dimethoate in nectaries taken from bean plants treated at rates between 0·5 and 50 mg dimethoate per pot was assayed by gas chromatography. The concentration of dimethoate in the nectaries depended on the applied dose: it was greatest 4 days after treatment. Loss was more rapid with larger doses than with smaller doses.     

氨基甲酸酯类杀虫剂灭蚜威对麦蚜的防治效果

田间试验表明 :氨基甲酸酯新杀虫剂灭蚜威是一种对麦蚜杀虫活性高的杀虫剂 ,2 5 %灭蚜威EC使用剂量 9 375～ 1 2 5g (a .i.) 667m2 ,药后 3～ 1 0d校正防效山东河南两地分别为 87 64%～ 98 38%和 89 95 %～ 97 66% ,6 2 5g (a .i.) 667m2 药后 7～ 1 0d防效分别为 87 68%～ 92 0 2 %和 88 91 %～94 4 6% ;5 0 %灭蚜威WP使用剂量 1 8 75～ 2 5g (a.i.) 667m2 ,药后 3～ 1 0d防效两地分别为 89 1 7%～99 4 5 %和 93 60 %～ 99 2 3% ,1 2 5g (a.i.) 667m2 药后 7～ 1 0d防效分别为 90 60 %～ 93 5 9%和 92 37%～ 93 5 0 %。     

Cytotoxic and necrotic responses in human amniotic epithelial (WISH) cells exposed to organophosphate insecticide phorate

The in vitro interaction of the organophosphorous insecticide (OPs) phorate with calf thymus DNA (ctDNA), and its potential to cause changes in cell cycle, membrane damage, and cytotoxicity leading to cell death (necrosis) was investigated in human amnion epithelial (WISH) cells. Fluorescence quenching revealed high binding affinity (K(a)=5.62×10(4)M(-1)) of phorate to ctDNA. Molecular modeling of the phorate-ctDNA interaction suggested the binding of phorate at AT rich regions on minor groove of DNA. The interaction ensued alkylation of the N-6, N-7 of adenine and C-4 carbonyl oxygen of thymine. Binding of phorate was stronger in the presence of the transition metal ion copper II (Cu(2+)), and has accentuated the destabilization of the DNA secondary structure. A discernable change in the voltammetric E(1/2) (E(0')) with lesser cathodic (i(pc)) and anodic (i(pa)) peak currents confirmed the formation of phorate-DNA and phorate-DNA-Cu (II) association complexes. Furthermore, the MTT and NRU assays demonstrated substantial phorate cytotoxicity due to loss of mitochondrial and lysosomal membrane integrity, and reduction in mitochondrial membrane potential (ΔΨm) of treated WISH cells. Cell cycle analysis of WISH cells treated with 1000μM phorate exhibited 13.7-fold (p<0.01) augmentation in the sub-G(1) peak. Annexin V-PE and 7-ADD staining of phorate treated cells reaffirmed the development of late apoptotic or necrotic cell population in a concentration dependent manner. Thus, this study demonstrated the phorate induced DNA structural alterations and cellular damage in cultured human cells.     

The effect of rhizoctonia root disease and applied nitrogen on growth,nitrogen uptake and nutrient concentrations in spring wheat

Root disease caused by Rhizoctonia solani is a common problem of spring wheat in South Australia. There are reports that nitrogen applications can reduce the incidence and severity of the disease. A glasshouse trail in pots examined the effects of disease and of applied nitrogen on wheat growth, and evaluated the utility of the basal stem nitrate concentration in diagnosing deficiency in plants with and without root disease. Plants were harvested at the mid-tillering stage. Shoot growth was increased by applied nitrogen until a maximum yield was attained, after which additional N had no effect on shoot yield. Root growth, however, responded positively only to low levels of applied N, after which it declined, and in the highest N treatment root mass was less than in the plants without applied N. Root disease caused severe reductions in plant growth, and both root and shoot mass were affected similarly. Even though growth of diseased plants responded positively to applied nitrogen the response was less than that of disease-free plants. The critical concentration of basal stem nitrate-N did not appear to be affected by root disease, and was estimated at 1200 mg kg^-1, consistent with other glasshouse data. The basal stem nitrate-N concentration, either in fresh or dried tissue, appeared a better diagnostic tool of N stress than did total shoot N concentration or content, because of sharper definition of critical concentrations. Concentrations of other nutrients in shoot tissue were affected differentially by both applied nitrogen and root disease, but generally did not reach critical levels, although phosphorus and magnesium appeared deficient in very disease-stressed plants.     

The economics of reduced-rate insecticide applications to control aphids in winter wheat

Applications of pirimicarb and fenvalerate at full-rate and at a range of reduced dosages were applied on fields of winter wheat between 1983 and 1988 in West Germany. The relative effects of these treatments on the control of cereal aphids varied according to the growth stage at which spraying took place and the size of the aphid populations. The economic profitability of these treatments depended on the extent of post-treatment recovery of the aphid populations, the cost of control and the crop value. Under certain circumstances the high efficiencies of commonly-used insecticides at full dose-rate are not necessary to gain effective aphid control economically; a better return can be made by reducing the quantity of active ingredient per hectare.     

Influence of systemic insecticides on lipid biosynthesis in seeds of Indian mustard (Brassica juncea L.)

The systemic insecticides namely phorate (Thimet 10 G) oxydemeton methyl (metasystox 25 EC) and dimethoate (Rogor 30 EC) decreased oil content in the developing seeds of Indian mustard (Brassica juncea L.) but showed an increase in the mature seeds. The inhibitory effect in the developing seed was accompanied by an increase in soluble sugars and a corresponding decrease in malate dehydrogenase and G6P dehydrogenase activity. The relative proportion of triacylglycerols and glycolipids decreased significantly while that of phospholipids and free fatty acids increased in the developing seeds. In the mature seeds, the proportion of triacylglycerols did not change appreciably from that in control. The erucic acid synthesis which was less at 10 and 20 days after fertilization (DAF) increased at 30 DAF with oxydemeton methyl and dimethoate; phorate was ineffective. In mature seeds, the proportion of erucic acid increased at the cost of linoleic and linolenic acids. All the insecticides appreciably decreased the rate of [1-¹⁴C]acetate incorporation into lipids both in vivo and in vitro experiments. In the in vivo experiment, the synthesis of polar lipids was enhanced at 10 and 20 DAF, and the higher doses of oxydemeton methyl and dimethoate at 30 DAF. On the other hand, the ¹⁴C-incorporation into triacylglycerols showed an opposite trend to that of polar lipids. In the in vitro experiment, oxydemeton methyl and dimethoate enhanced the synthesis of polar lipids at 10 and 20 DAF while these inhibited it at 30 DAF. The synthesis of triacylglycerols was inhibited by the use of these insecticides.     

Identifying sucrose as a signal for nitrate uptake by wheat roots

Some sugars supplied directly to roots can stimulate nitrate uptake by wheat (Triticum aestivum L.) roots. To identify a signaling molecule, we compared the response of net nitrate influx to sugar supply. A method with a high time resolution (minutes) enabled to make a comparison. A signaling sugar should cause a faster and greater response than other compounds. Among nine sugars and mannitol tested, sucrose alone caused an immediate active stimulation of net nitrate influx. Glucose, fructose, and raffinose caused weak responses with a lag. Other carbohydrates had no effect. Sucrose behaves as a specific signal for nitrate uptake, which has long been supposed but not supported experimentally.     

Studies with phorate,an organophosphate insecticide,on some enzymes of nitrogen metabolism inVigna mungo (L.) Hepper

Effects of phorate (Thimet-10 G) on the activities of enzymes of nitrogen metabolism,viz., glutamine synthetase (GS), glutamate dehydrogenase (GDH), and glutamate synthase (GOGAT) in the primary leaves, nitrogenase (N₂-ase) in detached root nodules, and protein concentration in the primary leaves ofVigna mungo (L.) Hepper have been studied. Phorate stimulated the activities of these enzymes and protein concentration in primary leaves at lower concentrations (50 to 200 mg per pot), but inhibited at higher concentrations (400 to 600 mg per pot). Although, the V_max for each enzyme with different treatments varied, the K_m value of each enzyme in the control and the treated plants remained unaltered.     

Molybdenum efficiency in winter wheat cultivars as related to molybdenum uptake and distribution

The highly Mo efficient winter wheat cultivar 97003 yielded more than 90% and the low Mo efficient winter wheat cultivar 97014 less than 50% under Mo deficient conditions when compared to the Mo fertilizer treatment. The mechanism of Mo efficiency, molybdenum uptake and distribution in plant parts during all growth stages, was studied with these two cultivars when grown in an acid yellow-brown earth with no Mo (CK) and added Mo (+Mo) treatments. The results showed that accumulation of Mo and dry matter in shoots of cultivar 97003 was significantly higher than those of cultivar 97014 under CK through the entire growth period. Most of Mo was found accumulated in shoots after the stem elongation stage. Only low amount of Mo was accumulated during the cold winter until stem elongation stage where severe symptoms occurred in cultivar 97014 without Mo supply, while the Mo concentration of cultivar 97003 was significantly higher, thus improved its cold hardiness. Molybdenum concentrations in spikes and seeds were very low pointing to a low Mo mobility even under Mo sufficiency. However, much more Mo was distributed in the upper leaves at stem elongation stage, in spikes in heading stage, in seeds in maturity in cultivar 97003 than in cultivar 97014 under conditions of Mo deficiency. In the efficient cultivar, the Mo distribution ratios to the upper leaves and spikes were even higher without Mo supply, suggesting that a higher phloem mobility and thus a more efficient use of Mo under Mo deficiency stress. The ability of Mo uptake and phloem-mobility are discussed and it is suggested to be the important physiological basis of Mo efficiency.     

Palatability and toxicity of fipronil as a systemic insecticide in a bromadiolone rodenticide bait for rat and flea control

Vector control in plague-infested areas requires a simultaneous killing of rodents and their fleas. We investigated the efficacy of a combination of a systemic insecticide, fipronil, in a rodenticide bait formulation under laboratory conditions. Four different concentrations of fipronil (0.05%, 0.005%, 0.0005% with acetone as a solvent, and 0.05% with propylene glycol as a solvent) and two controls (solvents only) were combined with the rodenticide bait (crushed organically grown wheat with 0.005% bromadiolone). Each concentration was offered together with an untreated non-poisonous challenge bait to 10 singly caged Rattus rattus L., each with 100 rat fleas Xenopsylla cheopis Rothschild (Siphonaptera: Pulicidae) in the nest. Treated bait consumption was relatively low and an unsatisfactory rat mortality of around 50% only was obtained in all tests. The palatability of the bait, however, was not affected by the fipronil concentration. Even at the lowest fipronil concentration, average flea mortality was still above 95%, and doses of more than I mg fipronil per kg rat body weight gave a nearly complete kill of fleas. Fipronil can be highly effective as a systemic insecticide to for flea control, provided that a more attractive bait base for roof rats is used.     

Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with ¹⁵N in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and ¹⁵N-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N ^-2) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with¹⁵ NH₄ ¹⁵NO³ corresponding to the inorganic N applied with the slurries (6 g N m^-2). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m^-2) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. ¹⁵N in the urine fraction was less available for crop uptake than mineral fertilizer ¹⁵N. The first barley crop on the sandy loam removed 49% of the ¹⁵N applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer ¹⁵N (36%) and urine¹⁵ N (32%) differed less on the coarse sand. Of the¹⁵ N added with the faeces fraction, 12–14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones.About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of¹⁵ N from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop.More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3–5% and 1–3%, respectively, of the ¹⁵N in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.     

The growth and nutrient uptake of winter wheat

Summary Measurements with an ion selective electrode under winter wheat and in adjacent fallow soil, from April to July 1976, showed that nitrate concentrations were high in the 0–25 cm zone and correspondingly lower at 50 cm, because of the extreme drying conditions. Maximum differences in nitrate concentrations between cropped and fallow soil occurred at Feekes' stages 6, 10, and 11.1 indicating periods of maximum uptake by the crop (cf Ref.⁴).Dry matter weight of wheat, sampled biweekly, was maximum 15 days before maturity. The foliage senesced and lost weight from Feekes' stage 10.1 onwards. Nutrient concentrations in the foliage decreased from Feekes' stage 4, but N, P and Mg concentrations in the ears increased during Feekes' stage 11. N, P and Mg accumulated in the ears at the expense of the foliage during stage 11, maximum uptake occurring at stages 11.3, 11.1 and 11.2 respectively. In contrast, K and Ca uptake ceased at stages 10.1 and 10.5 respectively and then both were lost from the foliage in heavy showers.Rates of N uptake and soil nitrate depletion correlated significantly, enabling N uptake to be deduced approximately from thesein situ soil nitrate measurements.     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

The growth, yield and nutrient uptake of winter wheat following root-pruning

The effect of root-pruning on shoot growth was investigated in winter wheat growing in lysimeters. Removal of half of the root axes at the beginning of stem elongation reduced shoot dry matter, determined 1 month after pruning, by 13% and grain yield by 8%. Removal of either the seminal or nodal root system during tillering reduced shoot dry weight, measured during the growing season, by 7% and grain yield by 25%. Root-pruning had negligible, or only transient, effects on the concentration of nitrogen, phosphorus, potassium, magnesium and manganese in the shoots. The harvest index was not affected by root-pruning.