Glutathione S-transferases (GSTs) have been recognized as important metabolic detoxifying enzymes in many phytophagous insects. However, the contribution of GST to insecticide resistance in Zonocerus variegatus (L.) has not been studied. Therefore, we carried out an initial study on the induction and kinetics of GST in Z. variegatus exposed to pyrethroids (PYRs) and Ocimum gratissimum leaf extract. Fifth-instar nymphs of Z. variegatus collected from cassava farms (with no history of insecticide exposure) on the campus of the University of Ibadan were reared to adult stage. Adult insects were divided into four groups of two replicates, each consisting of 30 insects. The groups were exposed to 20 ml of PYR insecticide and 25 mg/dl and 250 mg/dl of O. gratissimum leaf extract, respectively, while the last group served as the control. GST was extracted from the body tissues of the insects using the Bradford method and the kinetics of the enzyme was evaluated using the Lineweaver–Burk plots. No mortality was recorded in insects exposed to 25 mg/dl of O. gratissimum leaf extract, while mortality rates of 93.3 and 43.3% were recorded in insects exposed to PYR insecticide and 250 mg/dl of O. gratissimum leaf extract, respectively. The activity of GST was higher in insects exposed to the insecticides than in the control insects, while a higher enzyme activity was recorded in insects that died after exposure to the insecticides than in insects that were alive after exposure. The GST extracted from insects that were alive after exposure to PYR insecticide and O. gratissimum leaf extract exhibited a high affinity for the glutathione substrate compared with that extracted from insects that died after exposure. Higher GST activity in insects exposed to insecticides is indicative of the role of this enzyme in the metabolic detoxification of insecticides by Z. variegatus.

The purification and characterization of rhodanese, an enzyme that catalyses the detoxification of cyanide, from the gut of the variegated grasshopper (Zonocerus variegatus L.) were carried out to understand the biochemical basis of the survival of this grasshopper living on cyanogenic plants such as cassava. All experiments, including enzyme assay, were carried out at room temperature and all buffers contained 10 mm sodium thiosulphate to stabilize the enzyme. Grasshoppers were caught alive from a cassava farm within the locality and kept frozen until analysis. Each grasshopper was dissected and the gut was removed quickly. Approximately 102 g of the gut were homogenized in three volumes of 0.1 m acetate glycine buffer (pH 7.8) containing ɛ-amino-n-caproic acid. The supernatant was collected by centrifugation at 12,000 rpm, for 30 min at 40 °C. The enzyme was purified to homogeneity by a combination of procedures such as ammonium sulphate precipitation, ion-exchange chromatography (CM-Sephadex and DEAE-Sephadex), gel filtration (Sephadex G-75) and Agarose-Blue affinity chromatography. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS–PAGE) and non-SDS–PAGE were used to ascertain the purity of the enzyme. The native and subunit molecular weights of the enzyme were determined by gel filtration on a Bio-Gel P-200 column and SDS–PAGE, respectively. Kinetic parameters were determined by using varying concentrations of one of the substrates at a fixed concentration of the other, and vice versa. Furthermore, the effects of temperature, pH and cations on the activity of the enzyme were investigated. The purified enzyme had a specific activity of 51.7 μmol thiocyanate formed/ml/min/mg protein (U/mg protein) with a yield of about 29%. The apparent molecular weight of the enzyme estimated by Sephadex G-75 gel filtration was 35,400 ± 482 Da and its subunit molecular weight determined by SDS–PAGE was 33,000 ± 212 Da. The Km values of KCN and Na2S2O3 were found to be 29.63 ± 02.87 and 26 ± 03.04 mm, respectively. An optimum pH and temperature of 7.0 and 35 °C, respectively, were obtained for the enzyme. The results of enzyme inhibition showed that the activity of the enzyme was not affected by NH4Cl, MgCl2, CoCl2, CaCl2, MnCl2, NiCl2 and SnCl2, but inhibited by ZnCl2 and BaCl2. In conclusion, these results suggest that the survival of Z. variegatus depends on the presence of the enzyme rhodanese, which shows high activity and has suitable kinetic properties in the gut of the grasshopper that feeds mainly on cassava leaves which are cyanogenic.

The activity of five enzymes – amylase, α-glucosidase, cellulase, proteinase and lipase – present in the foregut, midgut and hindgut of the African variegated grasshopper Zonocerus variegatus (L.) was studied in the post-embryonic stages of the insect. The activities of all the enzymes increased with age of the insect, with the adult stage having significantly higher (P < 0.05) activity than the lower instars. The midgut had the highest activity for all the enzymes except for amylase. Proteinase activity was the highest, and cellulase the lowest in the gut regions. These findings indicate that the adult stage of the polyphagous insect has higher digestive capacity than the immature stages.

Microbial isolates from the gut region of larvae and adults of the variegated grasshopper Zonocerus variegatus (Linnaeus) were screened for extracellular production of the enzymes lipase, amylase, esterase, protease, linamarase and cellulase. Using the pour-plate culture technique, 15 bacterial, 4 yeast and 5 mould isolates were tested. None of the bacterial isolates showed lipase activity, but virtually all showed esterase and protease activities. Many bacterial isolates demonstrated linamarase and cellulase activities, and a few of them amylase activity. Of the yeast isolates, a few had amylase activity. None of the mould isolates showed enzyme activity, except Aspergillus sp., which showed lipase activity. Evidently, the gut of Z. variegatus contains micro-organisms that are able to produce digestive enzymes. Of significance is the biosynthesis of linamarase (β-d-glucosidase) by the grasshopper's gut bacteria, which probably contributes to the insect's neutralization of the toxic effects of cyanogenic glycosides present in cassava (Manihot esculenta), one of its major food plants.

Field experiments on reduced application doses of Metarhizium anisopliae var. acridum (I 91 609: Zonocerus variegatus strain) were carried out in 1996–1997 in southeastern Benin and central Ghana. The first experiment, which took place in southeastern Benin, consisted of ULV applications of an oil based formulation of M. anisopliae on 1-ha plots of cassava, at the rates of 1 × 1011, 4.65 × 1011 2.5 × 1012 and 1 × 1013 conidia/ha. In a second experiment carried out in central Ghana, the application doses were reduced by increasing the space between spray tracks. Treatments were applied at 5, 10 and 20m track spaces, corresponding to doses of 2.5 × 1012, 1.25 × 1012 and 0.625 × 1012 conidia/ha, respectively. After application, samples of L4–L6 larvae and adults of Z. variegatus were collected using sweep nets and their mortality monitored in small cages. Zonocerus variegatus population density was monitored at three-day intervals over a 21-day period. In southeastern Benin, analysis of survival time, resulting from the first sampling, did not reveal any significant difference between the various dose treatments. In central Ghana, for treatments at different doses and track spaces, there was no difference between 5 and 10m track spaces, corresponding to 2.5 × 1012 and 1.25 × 1012 conidia/ha. But the 1.25 × 1012 conidia/ha treatment at 10 m-track space reduced the application time by half (17 min instead of 34 min).

The life cycle and development of Zonocerus variegatus (variegated grasshopper) from southern Benin was studied at the International Institute of Tropical Agriculture (IITA) Benin Station, from March 1992 to August 1994. Hoppers of the 1st to the 5th stage were collected from uncultivated fields in the Atlantic Province and placed in cages and their development evaluated by counting the number of insects in each stage every two days until the adult stage. Mating and the number of egg pods were also recorded. The study showed that Z. variegatus has only one generation in southern Benin, running from September/October to July/August. Hatching occurs between September/ October and February/March. Zonocerus variegatus females can oviposit up to five times and the interval between ovipositions varies from 14 to 32 days. The variegated grasshopper has an egg diapause and egg hatching is not synchronised. These observations explain the almost perennial presence of the variegated grasshopper in southern Benin.

A laboratory study of the dietary preferences and duration of food plants in the gut of Zonocerus variegatus was carried out using three plant species, namely Manihot esculenta, Vernonia amygdalina and Chromolaena odorata. Whole or cut up plants were offered to the grasshopper in the experiments. Overall, whole V. amygdalina was the preferred diet of Z. variegatus, while the insect ate cassava pieces better than whole plant. Chromolaena odorata was the least preferred diet of the three. The duration of the food plant within the insects' gut was shortest for V. amygdalina, which was consumed all at once soon after being offered to the insects. The insects consumed cassava in two successive sessions and C. odorata in three. The total time spent eating was similar for V. amydalina and cassava, which were significantly shorter than that spent eating C. odorata. Evidently, V. amygdalina could serve as a good bait plant for Z. variegatus. However, its status as a human food plant precludes its use as a bait followed by chemical control; it may be used with, for example, fungal biocides such as Metarhizium.

The chemical nature of the repellent secretion of the African grasshopper, Zonocerus variegatus (Orthoptera: Pyrgomorphidae) reared on Acalypha wilkesiana L. and Manihot esculenta (Crantz) was analysed. It was found to contain alkaloids, glucose, proteins, free amino acids, trypsin-like proteinase, carbohydrases, lipase and the ions Ca2+, Mg2+ and K+ but not Na+ and (PO4)2-. Alkaloids were present in the secretion whether or not the insect was fed on plants containing alkaloids. Cyanide ions were absent in the secretion, even when Z. variegatus was fed exclusively on the cyanogenic Manihot esculenta. The amino acid and glucose contents were the same in grasshoppers reared on different plants. The protein content in the repellent secretion was constant, despite the fluctuations observed in the protein content of the haemolymph.

The temperature, pH, moisture, carbon and nitrogen contents of soils at oviposition sites of Zonocerus variegatus (L.) (Orthoptera: Pyrgomorphidae) located around cassava farms at Aiyepe, southwestern Nigeria, were compared with those of soils at non-oviposition sites (control) during different months. Although oviposition site soils did not differ in particle composition from control soils, their temperatures were, generally, significantly lower. Soils at oviposition sites lost significantly less organic carbon to oxidation and nitrogen to leaching. They also contained significantly more moisture than soils at control sites during February-April when oviposition occurred and newly deposited eggs, lacking the serosal cuticle, would be prone to desiccation in dry soils. Conditions at oviposition sites are a key factor in the selection of egg deposition by gravid Z. variegatus.

A study on the digestive carbohydrates complex was carried out in adult Zonocerus variegatus (Orthoptera: Pyrgomorphidae) to determine the status of the cyanogenetic glucose ingested by the grasshopper feeding on cassava. Two groups of adult Zonocerus were used. One group had been starved for 48 hours immediately prior to the experiment while the other had been fed continuously. After dissection, 10 to 15 portions of intestinal tracts (i.e. foreguts, caeca, midguts, hindguts) and 20 to 30 salivary glands were respectively crushed, centrifuged and dialysed for 24 hours in distilled water. The distillates obtained from different parts of the digestive system were used for determining the osidasic activities. Twenty-three substrates; i.e. 5 oligosaccharides, 10 synthetic heterosides and 8 soluble polysaccharides were selected for measuring the activities of the enzymes. Results showed that Z. variegatus has a wide range of enzymes whose activities differ according to different portions of the gut. The mid- and hind guts are the most active parts of the digestive tract but amylase activity was higher in the salivary glands. The Zonocerus variegatus osidase complex is characterised by the following proprieties: (a) high oligosaccharides activities on α-glucopyrones (maltose, saccharose) and on β-glucopyrones (cellobiose); (b)varied heteroside activities with α-glucosides and linamarase very active; (c) amylase activity very high, and hemicellulase and cellulase activities very low. In Zonocerus, starvation increases maltose hydrolyse but it does not affect other oligosaccharide activities. On the other hand, starvation reduces most of carbohydrases activities. The origin and consequences of the linamarase activity in the insect gut are also discussed.

The free amino acids of the accessory glands of adult male variegated grasshopper, Zonocerus variegatus (Linnaeus) were investigated at five different ages after emergence of insects maintained oh cassava Manihot esculenta, Cranz using automatic amino acid analyser, Beckman 121 MB. Thirteen amino acid components were consistently detected from days 6 to 28: cystine, aspartic acid, serine, glutamic acid, proline, glycine, alanine, valine, tyrosine phenylalanine, histidine, lysine and arginine. Proline, alanine, glycine, glutamic acid and serine occurred in highest concentrations, comprising 14.27, 11.88, 11.09, 9.66 and 8.15 nmoles respectively throughout the 28-day experimental period. Total free amino acids fluctuated in the course of development of accessory gland with highest load at 28 days of age.

Three neem products—an emulsifiable concentrate of neem oil (NOEC), neem kernel water extract (NKWE) and neem leaf water extract (NLWE)—were applied on field cassava for protection against damage by the grasshopper, Zonocerus variegatus (L.). Three conventional insecticides viz. lindane 0.3%, cypermethrin 0.006% and monocrotophos 0.24% were also compared. All the neem products protected cassava for varying periods. Efficacies of NOEC and NKWE were superior to insecticides. From the results, neem products recommended to check the grasshopper damage were as follows: NOEC 0.5–2% to be applied every 8 days, NOEC 3–4% to be applied every 10 days, NKWE 7–10% to be applied every 12 days, NLWE 50% to be applied every 6 days.

Twenty-one insecticides were tested for acute toxicity by surface film contact method against the grasshopper, Zonocerus variegatus (L.) in the laboratory. Decamethrin, cypermethrin, lindane o, o-diethyl(p-methyl sulfinyl) p′-phosphorothioate, and dieldrin were highly potent. Moderately potent insecticides were propoxur, bioallethrin, telodrin, isodrin, heptachlor and rotenone. A toxic symptom common to all potent insecticides in the female was the opening and closing of ovipositor valves which remained wide open at death. Five insecticides—dieldrin, telodrin, p, p′-DDT, chlorobenzilate, DMC and chlordimeform induced cannibalism among the treated females.

Early instars of Zonocerus variegatus fed on most weeds, but preferred those in the families Euphorbiaceae and Solanaceae. Plants that were accepted initially were lesser consumed than later chosen ones. Although there was no apparent difference between weights of nymphs fed on cassava and other plants in the first 9 weeks; thereafter, cassava fed insects excelled others. Somatic growth was not affected by feed type.

An analysis of the variation of fore-, mid- and hindgut contents in third- to sixth-instar nymphs of the African grasshopper, Zonocerus variegatus, freshly caught every hour in the field, and also left without food for 1, 6 and 12 hr, showed that gut motility and the movement of ingesta through the gut was maximal at 08.00–10.00, 13.00–14.00 and 16.00 hr. Movement of ingesta was very slow at night. A diurnal rhythm therefore governs gut emptying. Nymphs, confined in jars with leaf discs, fed from 07.00 to 19.00 hr over the temperature range 17–34°C. Most food was eaten after 13.00 hr when it is warmer. Field insects feed during the night as well. Midgut α- and β-glucosidase activity was detected at all times of testing (06.00, 12.00, 19.00 and 24.00 hr), but the levels of activity were lowest at 12.00 and 19.00 hr.

Marked individuals of Zonocerus variegatus were released in batches of 100 per developmental cohort at definitive loci in either an open field of mainly giant star grass, Cynodon sp. or a cultivated farm of cassava, Manihot esculenta. The distance and direction of movement of individuals in any given stadium were determined in metres and angular degrees. The mean distances moved by marked Z. variegatus for each day of observation were determined by Clark's root mean square formula.

The results showed that older cohorts covered more distance during a given period than young cohorts. However, while the first instar nymphs moved significantly less than any other cohort, the differences in the mean daily distances moved by nymphs of the second through the sixth developmental instars in the cassava farm were not statistically significant. For all cohorts, Z. variegatus covered significantly more distances in the open field than in the cultivated farm, suggesting that the availability of food inhibited the rate of dispersal of Z. variegatus. There was no significant difference in the number of Z. variegatus found on the different aspects of the compass.

This paper reviews the work which had been done in Nigeria from 1911 to 1981 on several aspects of the biology of Zonocerus variegatus (L.), a variegated grasshopper pest of many food crops. The account covers recent advances in the areas of breeding biology, locomotory and migratory activities, morphometrics, feeding habits, parasitisation, and strategies for effective control of the grasshopper in Nigeria.