Emamectin is a macrocyclic lactone insecticide with low toxicity to non-target organisms and the environment, and is considered an important component in pest-management programmes for controlling field crop pests. It is a powerful compound for controlling the cotton bollworm Helicoverpa armigera (Hubner). A spray concentration of 25 mg AI litre-1 in a cotton field resulted in over 90% suppression of H armigera larvae up to day 28 after treatment, while similar mortality of the Egyptian cotton leafworm Spodoptera littoralis Boisduval, under the same conditions, was maintained for 3 days only. Emamectin is a potent compound for controlling the western flower thrips Frankliniella occidentalis (Pergande) under both laboratory and field conditions and its activity on adults was over 10-fold greater than that of abamectin. Spray concentrations of 10 and 50 mg AI litre-1 in Ageratum houstonianum Mill flowers resulted in total suppression of adults up to day 11 and of larvae up to day 20 after treatment. Under standard laboratory conditions, emamectin exhibits a considerable activity on the whitefly Bemisia tabaci (Gennadius) and the leafminer Liriomyza huidobrensis (Blanchard). Further studies are required to evaluate its potential activity on the latter pests under field conditions.

Emamectin, a novel insecticide for controlling field crop pests†

Conventional insecticides such as chlorinated hydrocarbons, organophosphates, carbamates and pyrethroids were successful in controlling insect pests during the past five decades, minimizing thereby losses in agricultural yields. Unfortunately, many of these chemicals are harmful to man and beneficial organisms and cause ecological disturbances. Although considerable efforts have been made to minimize the adverse environmental impact of pesticides and to maximize food production and health of the human population and domestic animals, there is today a great demand for safer and more selective insecticides affecting specifically harmful pests, while sparing beneficial insect species and other organisms. Furthermore, the rapidly developing resistance to conventional insecticides provides the impetus to study new alternatives and more ecologically acceptable methods of insect control as part of integrated pest management (IPM) programs.

Insecticides with Novel Modes of Action: An Overview

Avermectins are a family of 16-membered macrocyclic lactone natural product homologues produced by the soil microorganism Streptomyces avermitilis MA-4680 (NRRL 8165). These compounds are closely related to the milbemycins, another family of 16-membered macrocycles, which also have potent anthelmintic and insecticidal/acaricidal activity. Milbemycins were first reported by Mishima et al. (1975) for their acaricidal activity; however, the potent anthelmintic activity of these compounds was not discovered at that time. Avermectins were isolated at the Merck Research Laboratories from a soil sample collected from a golf course in Kawanas, Ito City, Shizuoka Prefecture, Japan, by researchers at the Kitasato Institute. The Anthelmintic activity of the avermectins was discovered in 1976 by Merck researchers (Burg et al. 1979; Egerton et al. 1979; Miller et al. 1979) in an in vivo screen based on the survival of the nematode Nematospiroides dubius in mice. Anthelmintic activity of the milbemycins was later discovered at Merck following testing of the individual members of the milbemycin family (Mrozik et al. 1983).

Avermectins: Biochemical Mode of Action, Biological Activity and Agricultural Importance

Laboratory studies were conducted to determine the effect of the naturally derived compound spinosad on Ceratitis capitata Wied. (Diptera, Tephritidae). The organophosphate fenthion was used as a standard. Direct dose-dependent mortality and reduced fecundity were observed in oral treatment of adults with spinosad. The LC 90 values 14 h and seven days after treatment were 19.50 and 0.49 mg litre -1 respectively. Fenthion was less active (the LC 50 eight days after treatment was 1.17 mg litre -1 ) and did not affect the fecundity of the fly. Adults were also very susceptible to spinosad and fenthion via residual contact. For spinosad, 100% mortality was recorded 48 h after treatment for a dose of 10 mg litre -1 . Spinosad was more effective than fenthion in suppressing larval development when neonate larvae were reared on treated diet supplemented with a range of concentrations from 0.02 to 0.83 mg kg -1 diet. Last-instar larvae were much less susceptible to spinosad or fenthion when exposed via dipping or when they pupated in treated medium and both products had similar performance. A lack of ovicidal activity was observed in direct egg-treatments with spinosad but significant reductions from 1 mg litre -1 onwards were recorded for fenthion.

Laboratory evaluation of the novel naturally derived compound spinosad against Ceratitis capitata

:

Aqueous extract of neem seed kernel (NSKE) was tested for its effect on the fecundity and post-embryonic development of Bactrocera cucurbitae and Bactrocera dorsalis, when fed as water source. Comparisons were made with pure azadirachtin to ascertain its role as a compound responsible for these effects. Three oviposition dates (10, 15 and 20) after adult emergence were chosen. In the 10 day experiment no egg laying was recorded at a concentration of 0.25% NSKE for both the species. By the 20th day, mean number of eggs laid were 93 for B. cucurbitae and 100 for B. dorsalis at the same concentration as compared with 241 and 243 eggs in respective controls. This pattern of recovery was observed for the first time using neem extracts on B. cucurbitae and B. dorsalis. Effect on fertility were evident as the per cent egg-hatch in the 10-day experiment ranged from 56 to 72 and 61 to 75% for B. cucurbitae and B. dorsalis respectively, compared with 99 and 99% in controls. The overall egg hatch ranged between 56 and 85% for both the species, covering the reproductive age of the flies. There was no significant effect on pupation. Per cent pupation of the hatched larvae was >90%, while the adult emergence calculated from the number of larvae that pupated was >62%, for both the species, however, when calculated from the initial number of eggs laid the post-embryonic effects were evident. The effects on fecundity, fertility and adult emergence were confirmed with pure azadirachtin. As with NSKE, pupation was not much affected, however, significant effects on adult emergence and behaviour were recorded. The post-embryonic effects of neem seed kernel extracts on the two species of fruit flies was confirmed for the first time. Being safe, cheap and renewable, neem extract can be effectively used as an excellent alternative to synthetic insecticides.

Effects of aqueous extract of neem seed kernel and azadirachtin on the fecundity, fertility and post-embryonic development of the melonfly, Bactrocera cucurbitae and the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae)

Topical toxicity of avermectin B1 to Mediterranean fruit fly, Ceratitis capitata (Wiedemann); oriental fruit fly, Dacus dorsalis Hendel; and melon fly, Dacus cucurbitae Coquillett, was assessed over a 21-d observation period. Mortality stabilized 5–21 d after treatment. LD50’s for female D. dorsalis , D. cucurbitae , and C. capitata were 0.021 μg/g (5 d), 0.042 μg/g (15 d), and 0.29 μg/g (10 d), respectively; LD50’s for male D. dorsalis , D. cucurbitae , and C. capitata were 0.012 μg/g (21 d), 0.025 μg/g (15 d), and 0.58 μg/g (5 d), respectively. Doses that caused <25% female mortality significantly reduced fecundity in all three species whether or not males were treated before mating. Fertility was significantly reduced in D. dorsalis but seemed dependent upon whether males were treated and paired with treated females. D. cucurbitae showed greatly reduced fertility regardless of whether treated or untreated males were paired with treated females. Avermectin B1 treatment also reduced fertility in C. capitata , but the effect was minimal.

Lethal and Sublethal Effects of Avermectin B1 on Three Fruit Fly Species (Diptera: Tephritidae)

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Lethal and Sublethal Effects of Avermectin B1 on Three Fruit Fly Species (Diptera: Tephritidae)