Calculated LC50 Values Research Articles

A Laboratory Diet-Overlay Bioassay to Monitor Resistance in Lygus lineolaris (Hemiptera: Miridae) to Insecticides Commonly Used in the Mississippi Delta.

A laboratory, diet-overlay pesticide bioassay was developed using a susceptible population of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), to study its susceptibility to neonicotinoid, sulfoxamine, organophosphate, and pyrethroid insecticides (thiamethoxam, sulfoxaflor, acephate, and permethrin, respectively). The diet-overlay bioassay was compared to the traditional glass-vial surface residue bioassay. We measured LC50 values by feeding tarnished plant bug adults known doses of insecticides dispensed on top of diet in a 10% solution of honey water for thiamethoxam and 10% acetone in water solutions for permethrin, acephate, and sulfoxaflor. Both the diet-overlay and glass-vial bioassays used dose-response (mortality) regression lines to calculate LC50 values for each insecticide at 6-, 24-, 48-, and 72-h post-exposure. Data variability from the glass-vial bioassay was higher for permethrin, sulfoxaflor, and thiamethoxam than the diet-overlay bioassay, for all evaluation times. In contrast, there was lower variability among replicates to acephate in the glass-vial assay compared to the diet-overlay assay. Control mortalities observed on diet-overlay bioassay were lower (0-5%) than those observed on the glass-vial bioassay (4-27%). The use of green beans, floral-foam, rolling glass vials, and insect handling made the existing standard method tedious to manipulate and difficult to handle large numbers of individuals. The nonautoclaved solid diet provides an opportunity to significantly reduce cost and variability associated with procedures of other bioassay methods. In general, the baseline data provide a basis for future comparison to determine changes in resistance over time.

Interference Effect of Two Entomopathogenic Organisms against Cowpea Aphid, Aphis craccivora Koch.

The Cowpea aphid, Aphis craccivora Kock, is a common pest of several important ornamental plants.This work aims to investigate the toxic effect ofBacillus thuringiensis and Beauveria bassiana isolate on 2-3 days old adult A. craccivora,the effect of the bioinsecticide was strong on adult individuals. On the other hand the calculated LC50 values on newly emerged adults of aphid were 6.31 CFU/ml and 73868X106 spores/ml, respectively. This study has shown that, the fungus has a more effective than bacteria on adult aphids.

ROLE OF BACILLUS THURINGIENSIS ISOLATE AS BIOLOGICAL CONTROL AGENT AGAINST WHITEFLY, BEMISIA TABACI (GENN.) AND THE SIDE EFFECT ON THE PREDATOR, EUSEIUS SCUTALIS

The potential of a certain Bacillus thuringiensis isolate against first- instar nymphs and newly emerged adults (i.e., sucking stages) of Bemisia tabaci, could be detected on the basis of the calculated LC50 values which recorded 17.7and18.21CFU/ml respectively, This means that, The effect of the bioinsecticide was stronger on adult individuals. On the other hand the calculated LC50 values on newly emerged adults of Euseius scutalis by indirect and direct exposure were 4.89 and 27.5 CFU/ml respectively; this means that indirect exposure was more effective than direct exposure. Biological control agents should be directed to reduce the pest and preserve the predator (natural enemy).

Ecotypes of Aquatic Plant Vallisneria americana Tolerate Different Salinity Concentrations

Increased salinity caused by saltwater intrusion or runoff from de-icing salts can severely affect freshwater vegetation and deteriorate aquatic ecosystems. These habitats can be restored with freshwater ecotypes (locally adapted populations) that tolerate above-normal salinity. Vallisneria americana is a prominent species in many freshwater ecosystems that responds differently to abiotic conditions such as substrate composition and fertility, so, in this study, we evaluated the effects of salt stress on 24 ecotypes of V. americana. Instant Ocean aquarium salt was used to create saline solutions (0.2 to 20.0 parts per thousand (ppt)), then plants were abruptly exposed to these solutions and maintained in these concentrations for five weeks before being visually assessed for quality and destructively harvested. Analysis of variance and nonlinear regression were used to calculate LC50 values—the lethal concentration of salt that reduced plant biomass and quality by 50% compared to control treatment. Growth rate and visual quality varied significantly among ecotypes, and ecotypes that were most and least sensitive to salt had 50% biomass reductions at 0.47 and 9.10 ppt, respectively. All ecotypes survived 10.0 ppt salinity concentration but none survived at 20.0 ppt, which suggests that the maximum salinity concentration tolerated by these ecotypes is between 15.0 and 20.0 ppt.

A New Method to Test Molluscicides against the Philippine Schistosomiasis Snail Vectors.

To expedite the discovery of novel molluscicides in the laboratory, this study aimed to evaluate the performance of a new molluscicidal assay. This assay is based on Oncomelania hupensis quadrasi snails and is called miniaturized plate test or mpt. To perform this assay, a 12-well plate, 3 snails per well, and 24-h exposure period were used. The performance of mpt was evaluated using niclosamide and Ardisia plant extract (tagpo extract) as test substances while WHO's guidelines for a conventional plate test (cpt) served as standard. One cpt and four mpt independent trials were performed for niclosamide and tagpo extract. Probit analysis of dose–response data was run in R to generate lethal concentrations (LC50 and LC90), while lethal ratio test was performed to detect significant difference between paired LC50s (or LC90s). Using niclosamide, the calculated LC50 values were 0.104, 0.127, 0.136, 0.139, and 0.140 g/m2 for cpt, mpt 1, mpt 2, mpt 3, and mpt 4, respectively, while the LC90 values were 0.266, 0.268, 0.244, 0.251, and 0.261 g/m2, using the same sequence, respectively. For tagpo extract, the LC50 values were 1.467, 1.547, 1.659, 1.797, and 1.659 g/m2, for cpt, mpt 1, mpt 2, mpt 3, and mpt 4, respectively, and the LC90s were 2.188, 2.195, 2.501, 2.358, and 2.501 g/m2, respectively. The lethal ratio test revealed that a significant difference exists between the LC50s of cpt and mpt 1 when using niclosamide with a lethal ratio and confidence limits of 0.820 (0.663, 0.977, p < 0.05) and another significant difference between LC50s of mpt 1 and mpt 3 using tagpo extract with computed lethal ratio and confidence limits of 0.861 (0.782, 0.939, p < 0.05). Taken together, the results point out that mpt generates accurate and reproducible lethal concentration values. Hence, mpt may be used as an alternative method to screen molluscicides that are active against schistosome snail vectors.

The Effects Of 2-Hydroxy Chalcone and its Derivative on The Larvae and Adults of Tribolium Confusum

2-Hydroxy Chalcone and its derivatives (compounds 1and 2) were used in a toxicity test on the larvae and adults of confused flour beetle Tribolium confusum (Du Val). Mortality tests were done by applying different concentrations (0.01, 0.02 and 0.04 ppm) on 2-Hydroxy Chalcone and its derivatives which were exposed by topical application with 5 µL of each compound and the data were recorded after 24 h of treatments. The mortality % of [7-Hydroxy-9-(pyridine-4-yl)-6H-benzo[C]chromen-6-one] in (compound 2) were 26.6, 50 and 76.67% and 16.67, 36.67 and 63.3% and 13.3, 23.3 and 36.6 % to 3rd, 5th instar larvae and adults, respectively. While in [ 3-thinyl-1-(2-hydroxynaphthyl) -1-propene] (compound 1), the mortality % were 23.3, 40 and 66.6% and 13.3, 30 and 56.6% and 6.6, 16.6 and 26.6 % to 3rd, 5th instar larvae and adults, respectively. The results indicated that (comp.2) was the most toxic one and (comp.1) was the least toxic to confused flour beetle when applied singly. The calculated LC50 values to (compound 2) were 0.019, 0.028 and 0.074 ppm to 3rd, 5th instar larvae and adults, respectively. While to (compound 1), the LC50 values were 0.025, 0.034 and 0.106 ppm to 3rd, 5th instar larvae and adults, respectively. The order of toxicity of the chemical compounds was 2 &gt; 1. These chemical compounds can be used to control confused flour beetle.

Determination of LC values to some insecticides and amount of total esterase in Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) populations in cotton in Söke (Aydın, Turkey)

Amaç:Bu çalışma, bazı insektisitler inAydın ili Söke ilçesi pamuk alanlarından toplanan dört farklı Pamuk beyazsineği Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) popülasyonu için öldürücü konsantrasyon (LC) değerlerinin belirlenmesi amacıyla gerçekleştirilmiştir.Materyal ve Yöntem:Yaprak daldırma yöntemi ile yörede en çok kullanılan insektisitler seçilerek yapılan denemelerde, zararlının nimf dönemlerinde pyriproxyfen ve buprofezine, ergin dönemlerinde acetamiprid ve bifenthrin için LC değerleri belirlenmiştir. Ayrıca biyokimyasal çalışmalar da yürütülerek söz konusu aktif maddelerin toksisite değer farklılıklarının enzimlerle ilişkisinin belirlenmesine çalışılmıştır.Bulgular:Dört farklı beyazsinek populasyonundan elde edilen LC50 değerleri sırasıyla, acetamiprid için; 37.51, 28.22, 52.12, 75,61 ppm; bifenthrin için 2.72, 4.39, 1.08, 1.89 ppm, buprofezin için 30.95, 29.05, 46.46, 25.95 ppm ve pyriproxyfen için 38.85, 76.06, 108.91, 34.15 ppm olmuştur.Sonuç:Ayrıca, biyokimyasal analizlerde söz konusu dört populasyonda sadece toplam esteraz değerleri tespit edilmiş ve enzim seviyelerinde önemli bir fark görülmemiştir.

Effects of copper on larvae of the marbled crab Pachygrapsus marmoratus (Decapoda, Grapsidae): Toxicity test and biochemical marker responses

The importance of trace elements in ecotoxicological investigations is a well-known issue when monitoring polluted areas such as commercial harbors. Copper represents one of the most common metal contaminants, often detected in these areas as it is widely employed in various fields and has many sources of inflow in the marine environment. Pachygrapsus marmoratus is a widespread intertidal crab species that has been extensively studied in ecology, ethology and population genetics. Ecotoxicological studies have also been performed, exclusively on the adult stage. In the present study we investigated the mortality and biochemical (oxidative stress and neurotoxicity) responses of P. marmoratus larvae exposure to environmental relevant concentration of copper. Results showed dose-dependent responses in terms of larval mortality, with a calculated LC50 value of 0.5 mg/L of Cu2+. The LC50 concentration was used as the starting point for subsequent biochemical response evaluation. Results also demonstrated dose-dependent activation of antioxidant systems assuming a compensatory antioxidant activity to prevent higher cellular damage when larvae were exposed to the highest concentrations of copper. Moreover, a significant enhancement of neurotransmitter activities was observed, assuming a possible direct interaction of copper with the enzymes or an increase of free copper ion aliquot into the cells.

Chemical compositions of leaf extracts from Conocarpus erectus L. (Combretaceae) and their bioactivities against Tribolium castaneum Herbst (Coleoptera: Tenebrionidae)

Abstract Botanical insecticides have long been considered as alternatives to synthetic chemical insecticides in IPM programs. Effects of aqueous and ethanolic extracts obtained from buttonwood, Conocarpus erectus L. (Combretaceae), leaves on a major kind of stored product pests, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), were evaluated under laboratory conditions. For this purpose, LC50, repellency, antifeedant properties, and some biological effects (including body weight, immature developmental time and survival) of the insect were determined. The aqueous and ethanolic extracts were highly toxic to the larvae and adults. Calculated LC50 values ranged between 2.6 and 193.4 (g/kg). Both extracts had repellent and antifeedant properties against the adults. The extracts adversely affected the larval and pupal weights, developmental time, and survival. Aqueous extracts were more effective only for the LC50 values and only in females. All other measured parameters do not differ between the two extracts. The bioactive properties might be related to high amounts of alkaloid, phenol and tannin. Aqueous extract of the plant leaves may be a useful alternative for chemical insecticides.

Identifying sublethal endpoints for evaluating neurotoxic compounds utilizing the fish embryo toxicity test

Fish embryos are increasingly being utilized in aquatic toxicity testing, as evidenced by the Organisation for Economic Co-operation and Development's approval of the fish embryo acute toxicity (FET) test. However, the FET test only allows for the estimation of acute toxicity, whereas other test methods such as the larval growth and survival (LGS) test allow for the estimation of both acute and chronic toxicity. Additionally, it has been demonstrated that the FET test is less sensitive than other test methods for some neurotoxic compounds. To address these limitations, efforts to identify sublethal endpoints that increase FET test sensitivity and allow for the prediction of sublethal adverse effects have begun. As such, the objectives of the current study were 1) to compare estimated LC50 values from the FET and LGS test for three known neurotoxicants: fluoride (F), nickel (Ni), and cadmium (Cd) and 2) to evaluate the responsiveness of potential sublethal endpoints for the FET test related to growth (i.e., wet weight and snout-vent length), neurological development (i.e., spontaneous contraction frequency and eye size), and cardiovascular function (i.e., heart rate and pericardial area). The calculated LC50 values from the F and Cd FET test were significantly higher than those from the LGS test, demonstrating that the FET test is less sensitive than the LGS test for neurotoxic compounds. Only Cd exposure resulted in alterations in any of the sublethal endpoints investigated. Embryos/eleutheroembryos exposed to Cd displayed alterations in length, eye size, and pericardial area at concentrations five-fold less than the estimated LC50 value, suggesting that for Cd the inclusion of these sublethal endpoints would improve the sensitivity of the FET test. Overall, these results provide evidence that for some neurotoxicants, the inclusion of sublehtal endpoints may improve the utility of the FET test; however, further research utilizing a broader range of neurotoxicants with differing mechanisms of action, is needed to fully establish such endpoints in the context of routine FET test.

Enantioselective toxicity and degradation of chiral herbicide fenoxaprop-ethyl in earthworm Eisenia fetida

The enantioselective degradation of fenoxaprop-ethyl in ecological indicator earthworm was studied and the main metabolites (fenoxaprop, 6-chloro-2,3-dihydrobenzoxazol-2-one, ethyl-2-(4-hydroxyphenoxy)propanoate, 2-(4-hydroxyphenoxy)propanoic acid) were also monitored on an enantiomeric level. The individual enantiomers of fenoxaprop-ethyl and its three chiral metabolites were prepared to study the acute toxicity to earthworm. Chiral analysis methods were set up based on HPLC–MS/MS with chiralpak IC chiral column. Fenoxaprop-ethyl was not found in earthworms, while the primary metabolite fenoxaprop was in relatively high levels indicating a quick hydrolysis degradation. Fenoxaprop was accumulated almost exclusively with R-enantiomer in earthworms and the bio-concentration factors of R-fenoxaprop and S-fenoxaprop were 1.39 and 0.17 respectively with the enantiomer fraction (EF) values about 0.99. The degradation of R-fenoxaprop in earthworms followed first-order kinetics with half-life of 1.82day. The other metabolites could not be detected in earthworms. The calculated LC50 values showed ecological indicator earthworm was more sensitive to the four metabolites than fenoxaprop-ethyl. Furthermore, earthworm was more sensitive to the R-form of the chiral metabolites than the S-form and rac-form. The results suggested metabolites and enantioselectivity should be taken into consideration to better predict the exposure concentration and apply ecological indicators in toxicological studies.

Assessment of insecticides and Cry2AB toxin resistance development in Karnataka population of Plutella xylostella (Linn.)

Insecticidal resistance studies against third instar larvae of DBM (Plutella xylostella L.) were carried out to know the rate of development of resistance from F1 to F3 generations in Karnataka population. The third instar larvae obtained from field were subjected to bioassay studies with tested against acephate, cypermethrin, spinosad, cartap hydrochloride and Cry2Ab toxin using leaf dip method to calculate LC50 values. The LC50 values of the insecticides were further used to quantify the resistance in P. xylostella of parental generation (F0) from Karnataka field population. The survivals from F0 generation were reared to next generation (F1). Resistance development studies was assessed from F1 to F3 generation with third instar larvae in every generation with a concentration that caused 80.00 % mortality for all the test insecticides and Cry2Ab toxin. Results revealed that ((0.17 folds) no resistance was developed against acephate in F3 generation. In case of cypermethrin 2.33 folds. Resistance studies further revealed that 1.50 folds resistance was developed against spinosad, 2.28 folds against cartap hydrochloride and Cry2Ab toxin 1.49 folds resistance was recorded in F3 generation. The rate of development of resistance from F1 to F3 generations increased in all the test insecticides and Cry toxin, except against acephate in Karnataka population. This data will be useful in the development of insecticide resistance management approach for DBM.

Toxicity of Amoxicillin and Erythromycin to Fish and Mosquito

This study characterized the toxicity of Amoxicillin (AM), Erythromycin (ER) and Endosulfan (EN) to fish and mosquito larvae obtained from a certified fish farm and from Wadi Gaza. The acute toxicity tests were determined by calculating mortality percentage of fish and mosquitoes through a gradient concentration of the tested compounds. Mortality percentage, exposure time, LC50 and LT50 were taken as indicators of toxicity. The results showed that AM was the potent compound against fish with LC50 value lower that EN. Toxicity to fish is in the following order: AM &gt; EN &gt; ER whereas the toxicity to mosquitoes is in the following order: ER &gt; EN &gt; AM. Fish are more sensitive to AM than mosquitoes whereas mosquitoes are more sensitive to ER than fish. The interesting outcome of the study is the calculated LC50 values are far below the concentration found in different water systems. Mixture toxicity of the tested antibiotics indicates antagonistic effect on both fish and mosquitoes.

Toxicity of noradrenaline, a novel anti-biofouling component, to two non-target zooplankton species, Daphnia magna and Ceriodaphnia dubia

Noradrenaline (NA) is the active component of novel antifouling agents and acts by preventing attachment of fouling organisms. The goal of this study was to examine the toxicity of NA to the non-target zooplankton D. magna and C. dubia. Neonates were exposed to one of five concentrations of NA and effects on survival, reproduction and molting were determined. Calculated LC50 values were determined to be 46 and 38μM in C. dubia and D. magna, respectively. A 10-day C. dubia study found that reproduction metrics were significantly impacted at non-lethal concentrations. In D. magna, concentrations greater than 40μM significantly impacted molting. A toxicity test was conducted with D. magna using oxidized NA, which yielded similar results. These data indicate that both NA and oxidized NA are toxic to non-target zooplankton. Results obtained from this study can be used to guide future ecological risk assessments of catecholamine-based antifouling agents.

Evaluation of five essential oils as botanical acaricides against the strawberry spider mite Tetranychus turkestani Ugarov and Nikolskii

The strawberry spider mite, Tetranychus turkestani Ugarov and Nikolskii (Acari: Tetranychidae), is a serious pest of agroecosystems in southwestern Iran and some other tropical regions. Records show that it has developed resistance to some chemical acaricides. In this study, the chemical composition of five Lamiaceae plant essential oils was examined, and their activity against T. turkestani was evaluated. The essential oil composition of savory (Satureja hortensis L.), penny royal (Mentha pulegium L.), spearmint (Mentha viridis L.), rosemary (Rosemarinus officinalis L.), and a thyme-like plant (Zataria multiflora Bioss) was analyzed by GC–MS and fumigant bioassays were performed to determine their acaricidal activity. The results showed that all the essential oils have acaricidal activity at all the tested concentrations. The calculated LC50 values were 5.5, 9.4, 14.5, 15.3, and 29.8 μl l−1 air for Z. multiflora, S. hortensis, M. pulegium, M. viridis, and R. officinalis, respectively. These results showed that Z. multiflora and S. hortensis essential oils, with carvacrol, thymol, and γ-terpinene as their main compounds, were the most effective oils. In another experiment designed to find the effects of exposure time on the mortality of T. turkestani females, nearly all females died in 20 μl l−1 of all the essential oils after 24 h, except in the case of R. officinalis oil, which caused 90% mortality in mites. This study demonstrated that all the tested essential oils have the ability to control T. turkestani and could be useful in the development of new agents for mite control.

Novel polycarbo-substituted alkyl (thieno[3,2-c]quinoline)-2-carboxylates: synthesis and cytotoxicity studies.

Direct one-pot base-promoted conjugate addition–elimination of 6,8-dibromo-4-chloroquinoline-3-carbaldehyde with methyl mercaptoacetate and subsequent cyclization afforded methyl [(6,8-dibromothieno[3,2-c]quinoline)]-2-carboxylate. The latter undergoes Suzuki-Miyaura cross-coupling with arylboronic acids to yield exclusively the corresponding alkyl [(6,8-diarylthieno[3,2-c]quinoline)]-2-carboxylates,. The cytotoxicity of the prepared compounds was evaluated against the human breast cancer cell line MCF-7 using the MTT assay. The effects of compounds 2, 3c and 4d on cell kinetics were further determined using the xCELLigence Real Time Cell Analysis (RTCA) system. In both the MTT assay and Real Time Cell Analysis, the compounds inhibited cancer cell growth in a dose- and time-dependent manner. Furthermore, on the basis of the calculated LC50 values, the compounds compared favourably with nocodazole, a well-established anticancer drug.

Cytotoxic, antibacterial and nucleic acid interaction studies of square planar palladium(II) complexes

A series of substituted N, O-donor bidentate ligands(Ln) and their square planar mononuclear palladium(II) complexes of the type [PdCl2(Ln)] were synthesized and characterized by elemental analysis, conductivity measurement, TGA, FT-IR, 1H NMR, 13C NMR, LC–MS and electronic spectroscopy. Antibacterial activities of the complexes were evaluated against five pathogenic bacteria. The antimicrobial test results reveal that all the compounds have good antibacterial activity against both Gram(+ve) and Gram(−ve) bacterial species compared to ligands and metal salt. Interaction of a palladium complex with DNA was investigated by absorption titration and viscosity measurement. Hypochromism and bathochromism were observed as a result of interaction between palladium(II) complexes with DNA. It suggests an intercalative binding mode. Furthermore, viscosity measurements data support the intercalation mode of binding. The palladium(II) complexes were found to promote cleavage of plasmid pUC19 DNA from the supercoiled form to the open circular form. Cytotoxic activity of complexes was carried out to calculate LC50 values.

Toxicity and bioaccumulation of ethofumesate enantiomers in earthworm Eisenia fetida

Earthworms represent an important food source for many vertebrates and as a result, predators may encounter toxic effects via the food chain from consumption of contaminated worms. Therefore, including an assessment of xenobiotic to worms in risk assessment procedures is advisable. Here we studied the acute toxicity, bioaccumulation and elimination of ethofumesate enantiomers in earthworm, Eisenia fetida, in a soil. A slight difference in toxicity to earthworm between two enantiomers was found, and the calculated LC50 values for (+)-, rac- and (−)-ethofumesate were 4.51, 5.93 and 7.98μg/cm2, respectively, indicating that the acute toxicity of ethofumesate enantiomers was enantioselective. Earthworm can uptake ethofumesate but the bioaccumulation curve did not reach the steady state. In the elimination experiment, the concentrations of ethofumesate in earthworm declined following a first-order decay model with a short half life of 1.8d. The bioaccumulation and elimination of ethofumesate in earthworm were both nonenantioselective. In combination with other studies, a linear relationship between Log BSAFs and Log Kow was observed, and the Log BSAFs increased with increasing Log Kow. But the elimination rate did not show any correlation with the Kow value.

Enantioselective acute toxicity effects and bioaccumulation of furalaxyl in the earthworm (Eisenia foetida).

The enantioselectivities of individual enantiomers of furalaxyl in acute toxicity and bioaccumulation in the earthworm (Eisenia foetida) were studied. The acute toxicity was tested by filter paper contact test. After 48 h of exposure, the calculated LC50 values of the R-form, rac-form, and S-form were 2.27, 2.08, and 1.22 µg cm(-2), respectively. After 72 h of exposure, the calculated LC50 values were 1.90, 1.54, and 1.00 µg cm(-2), respectively. Therefore, the acute toxicity of furalaxyl enantiomers was enantioselective. During the bioaccumulation experiment, the enantiomer fraction of furalaxyl in earthworm tissue was observed to deviate from 0.50 and maintained a range of 0.55-0.60; in other words, the bioaccumulation of furalaxyl was enantioselective in earthworm tissue with a preferential accumulation of S-furalaxyl. The uptake kinetic of furalaxyl enantiomers fitted the first-order kinetics well and the calculated kinetic parameters were consistent with the low accumulation efficiency.

Are Helicoverpa armigera and Tetranychus spp. populations still susceptible to pesticides? A Zimbabwean study

The African bollworm, Helicoverpa armigera (Hübner), and the cotton red spider mite, Tetranychus spp., are important pests of cotton in Zimbabwe. A study to assess H. armigera resistance to fenvalerate 20 EC (Cyano (3-phenoxyphenyl) methyl 4-chloro-α-(1-methylethyl)benzene acetate) and Tetranychus spp. resistance to amitraz 20 EC (n’-(2,4-dimethylphenyl)-n-[[(2,4-dimethylphenyl)imino]methyl]-n-) was conducted at the Cotton Research Institute (CRI) during the 2005/06 cotton-growing season. Field populations of H. armigera and Tetranychus spp. were collected from some of Zimbabwe’s major cotton-growing areas of Sanyati, Umguza, Chisumbanje, Chinhoyi and the CRI in Kadoma and exposed to bioassays. The African bollworm leaf disc technique and the red spider mite attached leaf-dipping technique were used to assess responses of the African bollworm and red spider mite to the pesticides. Susceptible laboratory populations served as the standard populations and their responses were compared with those of the field populations. The graphical method and MSTAT-C probit analysis computer program were used to calculate LC50 values. Although the CRI field population, used as a reference population for the registration of H. armigera insecticides, had the highest LC50 value (graphical = 0.000100000; MSTAT-C = 0.000088195) compared with all the other field populations, overall log-dose probit bioassays on all field-collected strains of the bollworm showed no resistance to the pyrethroid (RFs = 0.04–0.54-fold). Tetranychus spp. showed very low levels of resistance (RFs = 1.26–2.00-fold). Continuous monitoring of major cotton pests, especially H. armigera and Tetranychus spp., from all cotton districts of Zimbabwe is vital for early detection of resistance development.