Effect of different host plants on the fitness of diamond-back moth, Plutella xylostella (Lepidoptera: Plutellidae)

The diamond-back moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) is a destructive cosmopolitan pest of cruciferous crops. The pest is present wherever its host plants exist and is considered to be one of the most widely distributed of all the Lepidoptera. We investigated the effect of various host plants on the fitness of P. xylostella and tested the hypothesis by studying development time, growth, fecundity and survival on cabbage (Brassica oleracea capitata), cauliflower (Brassica oleracea botrytis), radish (Raphanus sativus), turnip (Brassica rapa), mustard (Brassica compestris) and canola (Brassica napus var. canola). The developmental time from eggs to adult eclosion was the shortest (10 days) on canola and the longest (13 days) on turnip. Fecundity was greatest on canola (350) followed by cauliflower (268 eggs) by females eclosed from the pupae reared on canola and cauliflower, respectively, while the minimum numbers of eggs (184) were observed on cabbage. The number of eggs hatched was the highest (80%) when larvae fed on cauliflower. Survival to the adult stage was the highest (94%) on mustard followed by cauliflower and lowest (64%) on turnip. The net replacement rate was lowest for populations reared on cabbage (32.3), which was also reflected by the lowest intrinsic rate of population increase (0.20). The correlation between the intrinsic rate of population increase (r_m) and the mean relative growth rate was significant (t = 20.02 d.f. = 4, P < 0.05). Canola and mustard proved to be the most suitable hosts for P. xylostella because of shorter developmental period, higher percentage of survival and higher number of eggs. The data point to the role of host plants in increasing local P. xylostella populations.     

Deltamethrin resistance in the cotton mealybug, <Emphasis Type="Italic">Phenacoccus solenopsis</Emphasis> 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.     

Water stress effects on biochemical traits and antioxidant activities of wheat (Triticum aestivum L.) under In vitro conditions

Water stress is one of the major environmental stresses that affect agricultural production worldwide, especially in arid and semi-arid regions. This research investigated the effect of water deficit, induced by PEG-6000 on wheat genotypes (GA-2002, Chakwal-97, Uqab-2000, Chakwal-50 and Wafaq-2001) grown in modified MS medium solution. Osmotic stress caused a more pronounced inhibition in leaf relative water content and leaf membrane stability more sensitive (index in Wafaq-2001 and Uqab-2000) genotypes compared with the tolerant (Chakwal-50, GA-2002 and Chakwal-97) genotypes. Upon dehydration, an incline in proline, total soluble sugar, total soluble protein, superoxide dismutase, peroxidase, catalase and malondialdehyde activity content were evident in all genotypes, especially at osmotic stress of ?8 bars. The observed data showed that status of biochemical attributes and antioxidant enzymes could provide a meaningful tool for depicting drought tolerance of wheat genotypes. The present study shows that genotypic differences in drought tolerance could be likely attributed to the ability of wheat plants to induce antioxidant defense under drought conditions. In order to develop genotypes with stable, higher yields in dry farming conditions, it is necessary to characterise genetic resources based on drought adaptation, determine suitable genotypes, and then use them in breeding programmes.     

Resistance in field populations of <Emphasis Type="Italic">Amrasca devastans</Emphasis> (Hemiptera: Cicadellidae) to new insecticides in Southern Punjab,Pakistan

Cotton jassid, Amrasca devastans (Distant), is an economically important pest of various crops (i.e. cotton, okra, brinjal) and has the potential to become resistant against insecticides due to the intense use of insecticides in the region which entails high selection pressure. To monitor the resistance levels against recently used new chemistry insecticides (nitenpyram, spirotetramat, chlorfenapyr, fipronil, emamectin benzoate, sulfoxaflor, and flonicamid), four field populations of A. devastans collected from Khanewal, Multan, Muzaffargarh and Lodhran districts were examined by leaf dip bioassays. The results revealed the 32.95–136.47 fold resistance to nitenpyram, 23.03–56.74 fold to spirotetramat, 10.84–31.33 fold to chlorfenapyr, 1.20–9.43 fold to fipronil, 3.27–43.77 fold to emamectin benzoate, 0.66–2.81 fold to sulfoxaflor and 0.59–1.25 fold to flonicamid in different field populations, compared to the susceptible strain. Based on present study findings, it may be suggested to discontinue the use of nitenpyram, spirotetramat, chlorfenapyr and emamectin benzoate for which the populations were resistant and the rotational use of fipronil, sulfoxaflor and flonicamid for which the populations were susceptible. Proactive resistance management approaches such as judicious use of insecticides with proper dose selection, time of spray on early stages and correct method of application are necessary to prevent the development of resistance.