Muhammad Babar Shahzad Afzal

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.

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.

Deltamethrin resistance in the cotton mealybug, Phenacoccus solenopsis Tinsley: Cross-resistance to other insecticides, fitness cost analysis and realized heritability

The cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is a devastating pest that cause rigorous damage to the number of crops through feeding, managed by using various insecticides. To assess the risk of resistance and design a strategy for resistance management, a field collected population of P. solenopsis was selected with deltamethrin in the laboratory for six generations to investigate the cost to its fitness and to examine cross resistance to different insecticides. Bioassay results at G8 showed that the deltamethrin selected population (Delta-SEL) developed a resistance ratio of 100-fold compared to that of the unselected population (UNSEL). The deltamethrin resistance population exhibited strong cross-resistance to acetamiprid and lambda-cyhalothrin, but no cross-resistance to profenofos when compared to that of the UNSEL. The relative fitness of the Delta-SEL population was 0.37, with considerably lower survival rates from crawler to second instar, fecundity, hatchability, number of next generation nymphs, net reproductive rate and biotic potential compared with that of the UNSEL. The cost of fitness associated with deltamethrin resistance was evident in the Delta-SEL population. The present study provided useful information for management strategies to overcome development of resistance.

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.

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, 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 (h2) 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.

Resistance risk analysis to acetamiprid and other insecticides in Acetamiprid-Selected population of Phenacoccus solenopsis

Acetamiprid is a new chemical insecticide recommended for the control of a number of insect pests, including cotton mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae). We report the risk of resistance evolution to acetamiprid and three other insecticides (imidacloprid, chlorpyrifos and deltamethrin) in P. solenopsis. After 24 generations of selection with acetamiprid, P. solenopsis developed a high level of resistance (10631-fold) compared to the susceptible strain. Realized heritability of resistance for acetamiprid, imidacloprid, chlorpyrifos and deltamethrin was 0.21, 0.12, 0.11 and 0.09, respectively. The projected rate of resistance development indicated that if mortality is 30% at each generation then a ten-fold increase in resistance was to be expected after 24 and 20 generations for acetamiprid (h2 = 0.1, slope = 1.16) and imidacloprid (h2 = 0.12, slope = 1.20), respectively, and after 29 generations for chlorpyrifos (h2 = 0.1, slope = 1.44) and deltamethrin (h2 = 0.08, slope = 1.13). Therefore, P. solenopsis has the ability to develop resistance under continuous selection pressure. This study will contribute to the design of a pest management strategy for P. solenopsis.

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.

Studies on genetics, stability and possible mechanism of deltamethrin resistance in Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) from Pakistan

Cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is a serious worldwide polyphagous pest causing serious damage to the cotton crop. This study was conducted to investigate the mode of inheritance and possible mechanism of deltamethrin resistance in P. solenopsis. After 10 rounds of selection (G3–G12) with deltamethrin, P. solenopsis had a 740-fold level of resistance compared to the laboratory susceptible population (lab pop). The 740-fold resistant strain was reared further for next seven generations (G13–G19) without exposure to deltamethrin and bioassayed at G20 which revealed that the resistance was unstable. There was no significant difference in the LC50 values of progenies of both reciprocal crosses (F1 and F1′) and their degree of dominance values were 0.63 and 0.71, respectively. Monogenic model of inheritance and Lande’s method revealed that more than one factor was involved in deltamethrin resistance. Bioassays, piperonyl butoxide expressed synergism with deltamethrin but S,S,Stributyl phosphorotrithioate did not. Hence, deltamethrin resistance in P. solenopsis is unstable, autosomal, incompletely dominant, polygenic and mono-oxygenases based. These results provide basic information for designing and planning fruitful management programmes to control P. solenopsis. Deltamethrin is a pyrethroid that causes inhibition of activated voltage-sensitive sodium channels in the axon of neurons. This results in prolonged permeability of the nerve to sodium ions and produces a series of repetitive nerve signals in sensory organs, nerves and muscles and ultimately death (Joy 1994). Deltamethrin is broad spectrum insecticide