Laboratory selection of chlorpyrifos resistance in an Invasive Pest, Phenacoccus solenopsis (Homoptera: Pseudococcidae): Cross-resistance, stability and fitness cost.

The cotton mealybug, Phenacoccus solenopsis is an important polyphagous sucking pest of ornamentals, horticultural and fiber crops worldwide. Some P. solenopsis populations have developed insecticide resistance. This study evaluated cross-resistance, stability of insecticide resistance and life history traits affected by chlorpyrifos resistance in P. solenopsis. After nine generations selected with chlorpyrifos, P. solenopsis exhibited a 539.76-fold resistance level compared to an unselected population (UNSEL Pop). Chlorpyrifos selected population (Chlor-SEL Pop) displayed moderate cross-resistance to profenofos, nitenpyram and high cross-resistance to lambda-cyhalothrin. Biological parameters of P. Solenopsis were affected by chlorpyrifos resistance. The Chlor-SEL Pop had a significant reduction in fitness (relative fitness=0.10), along with significant decreases in pupal weight, fecundity, egg hatching %, intrinsic rate of natural population increase, biotic potential, and mean relative growth rate. It is concluded that selection with chlorpyrifos had marked effect on resistance development in P. solenopsis and upon removal of selection pressure chlorpyrifos resistance declined significantly indicating unstable resistance. Development of resistance led to high fitness costs for the chlorpyrifos-selected strain. These findings should be helpful for better and more successful resistance management of P. solenopsis.

Genetics, realized heritability and preliminary mechanism of spinosad resistance in Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae): an invasive pest from Pakistan.

The cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) has gained recognition as a key pest due to its invasive nature throughout the world. The P. solenopsis has a wide range of host plants and damages the cotton crop in various parts of the world. In view of the economic importance of this pest, a study on selection, inheritance and mechanism of spinosad resistance was conducted on P. solenopsis. Selection of field collected P. solenopsis for seven generations with spinosad resulted in a high resistance ratio of 282.45-fold. Genetic studies of spinosad resistance in P. solenopsis indicated that maternal effects are not involved in spinosad resistance; and resistance development is an autosomal and incompletely dominant trait. The number of genes involved in spinosad resistance was determined to be more than one, suggesting that resistance is controlled by multiple loci. The realized heritability (h (2)) value for spinosad resistance was 0.94. Synergism bioassays of spinosad with piperonyl butoxide and S,S,S-tributyl phosphorotrithioate showed that spinosad resistance in P. solenopsis could be due to esterase only. The study provides the basic information for implementation of effective resistance management strategies to control P. solenopsis.

Inheritance, realized heritability and biochemical mechanism of acetamiprid resistance in the cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae).

The cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is a serious pest in many countries of the world because of its polyphagous nature and has caused huge losses to the cotton crop. The aim of present study was to explore the mode of inheritance and mechanism of acetamiprid resistance in P. solenopsis. After five rounds of selection with acetamiprid, P. solenopsis developed a 315-fold resistance compared with the laboratory susceptible population. The LC50 values of progenies of both reciprocal crosses (F1 and F1') showed no significant difference and degree of dominance values were 0.56 and 0.93 for F1 and F1', respectively. Monogenic model of inheritance and Lande's method revealed that more than one factors were involved in acetamiprid resistance. Realized heritability (h(2)) value was 0.58 for acetamiprid resistance. A synergism study of piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) with acetamiprid also showed the significant presence of P-450 mono-oxygenase and esterase in the acetamiprid resistance. Hence, acetamiprid resistance in the P. solenopsis was autosomal, incompletely dominant and polygenic. These results are a source of basic information to design and plan fruitful management programmes to control P. solenopsis.

Genetics and preliminary mechanism of chlorpyrifos resistance in Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae).

Cotton mealybug, Phenacoccus solenopsis Tinsley, is a serious pest of cotton and other crops and infestation by this pest results in yield losses that affect the economy of Pakistan. Various groups of insecticides have been used to control this pest but resistance development is a major factor that inhibits its control in the field. Chlorpyrifos is a common insecticide used against many pests including P. solenopsis. The present experiment was designed to assess the genetics and mechanism of chlorpyrifos resistance and to develop a better resistance management strategy and assess the genetics and mechanism of chlorpyrifos resistance. Before selection, the field strain showed 3.1-fold resistance compared to the susceptible strain (CSS). After 8 rounds of selection with chlorpyrifos, a selected population developed a 191.0-fold resistance compared to the CSS. The LC50 values of F1 (CRR ♀ × CSS ♂) and F1(†) (CRR ♂ × CSS ♀) strains were not significantly different and dominance (DLC) values were 0.42 and 0.55. Reciprocal crosses between chlorpyrifos susceptible and resistant strains indicated that resistance was autosomal and incompletely recessive. The monogenic model of fit test and calculation of number of genes segregating in the chlorpyrifos resistant strain demonstrated that resistance is controlled by multiple genes. A value of 0.59 was calculated for realized heritability for chlorpyrifos resistance. Synergism bioassays with piperonyl butoxide and S, S, S-butyl phosphorotrithioate showed that chlorpyrifos resistance was associated with microsomal oxidases and esterases. It was concluded that chlorpyrifos resistance in P. solenopsis was autosomally inherited, incompletely recessive and polygenic. These findings would be helpful to improve the management of P. solenopsis.

Genetics and realized heritability of resistance to imidacloprid in a poultry population of house fly, Musca domestica L. (Diptera: Muscidae) from Pakistan.

Imidacloprid, a post-synaptic, nicotinic insecticide, has been commonly used for the management of different pests including Musca domestica worldwide. Many pests have developed resistance to this insecticide. A 16-fold imidacloprid-resistant population of M. domestica infesting poultry was selected using imidacloprid for 13 continuous generations to study the inheritance and realized heritability of resistance. Toxicological bioassay at G14 showed that the imidacloprid-selected population developed 106-fold resistance when compared to the susceptible population. Reciprocal crosses of susceptible and resistant populations showed an autosomal trait of resistance to imidacloprid in M. domestica. There was incompletely recessive resistance in F1 (Imida-SEL ♂ × Susceptible ♀) and F1(†) (Imida-SEL ♀ × Susceptible ♂) having dominance value 0.53 and 0.31, respectively. Monogenic model of inheritance showed that imidacloprid resistance was controlled by multiple factors. The realized heritability value was 0.09 in the imidacloprid-selected population of M. domestica. It was concluded that imidacloprid resistance in M. domestica was autosomally inherited, incompletely recessive and polygenic. These findings should be helpful for better and more successful management of M. domestica.

First report of resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) to lambda-cyhalothrin from Pakistan: baseline susceptibility, selection, occurrence of cross-resistance, realized heritability, and inheritance mode of resistance.

Fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a global pest causing damage to several crops. However, its management using chemical control is a challenge due to its capacity to evolve resistance to insecticides. After 6 generations of selection with lambda-cyhalothrin, the LC50 for the insecticide-resistant strain (Lambda-Sel) was 486 ppm, higher than that of the field strain (FAW-MUL) (7.5 ppm), susceptible laboratory strain (Lab-PK) (0.46 ppm), and laboratory unselected strain (UNSEL) (5.26 ppm). Laboratory selection with lambda-cyhalothrin increased resistance from 16.3- to 1056.52-fold and 1.43- to 92.4-fold to lambda-cyhalothrin compared to Lab-PK and UNSEL strains, respectively. The selected strain of S. frugiperda (Lambda-Sel) presented low cross-resistance to chlorpyrifos, moderate to deltamethrin and indoxacarb, very low to spinosad, and no cross-resistance to emamectin benzoate. The realized heritability (h2) of lambda-cyhalothrin resistance in the Lambda-Sel strain was very high (0.88). The reciprocal cross progenies of F1 (Lambda-Sel ♀ × Lab-PK ♂), F1' (Lambda-Sel ♂ × Lab-PK ♀), BC1 (F1 ♀ × Lambda-Sel ♂), and BC2 (F1 ♀ × Lab-PK ♂) showed high resistance ratios of 545.64-, 396.52-, 181.18-, and 146.54-fold, respectively compared to Lab-PK. The degree of dominance values for lambda-cyhalothrin in F1 and F1' indicates incompletely dominant resistance. The difference between observed and expected mortality in backcross populations (BC1 and BC2) revealed a polygenic resistance. In conclusion, the resistance to lambda-cyhalothrin was autosomal, incompletely dominant, and polygenic. These findings provide new insights for insect resistance management strategies to mitigate the occurrence of resistance in this global pest.

Cross-resistance, the stability of acetamiprid resistance and its effect on the biological parameters of cotton mealybug, Phenacoccus solenopsis (Homoptera: Pseudococcidae), in Pakistan.

BACKGROUND: Acetamiprid is a neonicotinoid insecticide that is effective against both soil and plant insects, including insects of the orders Lepidoptera, Coleoptera, Homoptera and Thysanoptera. In order to estimate the effects associated with insecticide exposure and devise better pest management tactics, a field population of Phenacoccus solenopsis was exposed to acetamiprid in the laboratory. Subsequently, cross-resistance and the effects of acetamiprid on the biological parameters of P. solenopsis were investigated. RESULTS: Following five rounds of selection with acetamiprid, P. solenopsis developed a 315-fold greater resistance to this chemical compared with an unexposed control population. The selected population also demonstrated very high to moderate cross-resistance to other tested insecticides. Furthermore, acetamiprid resistance remained unstable when the acetamiprid-selected population was not exposed for a further five generations. The acetamiprid-selected population had a relative fitness of 0.22, with significantly lower survival rate, pupal weight, fecundity, percentage hatching, net reproductive rate, intrinsic rate of natural increase, biotic potential and mean relative growth rate, with prolonged male and female nymphal duration, developmental time from egg to female adult and male and female longevity compared with the control population. CONCLUSION: P. solenopsis biological parameters are greatly affected by acetamiprid, and it is of significant cost for the insects to counter these effects. This study will be a valuable source of information for further understanding of acetamiprid resistance and for assisting the development of resistance management programmes.

Post-exposure temperature influence on the toxicity of conventional and new chemistry insecticides to green lacewing Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae).

Chrysoperla carnea (Stephens) is an important biological control agent currently being used in many integrated pest management (IPM) programs to control insect pests. The effect of post-treatment temperature on insecticide toxicity of a spinosyn (spinosad), pyrethroid (lambda cyhalothrin), organophosphate (chlorpyrifos) and new chemistry (acetamiprid) to C. carnea larvae was investigated under laboratory conditions. Temperature coefficients of each insecticide tested were evaluated. From 20 to 40 °C, toxicity of lambda cyhalothrin and spinosad decreased by 2.15- and 1.87-fold while toxicity of acetamiprid and chlorpyrifos increased by 2.00 and 1.79-fold, respectively. The study demonstrates that pesticide effectiveness may vary according to environmental conditions. In cropping systems where multiple insecticide products are used, attention should be given to temperature variation as a key factor in making pest management strategies safer for biological control agents. Insecticides with a negative temperature coefficient may play a constructive role to conserve C. carnea populations.