الفهرس 
INTRODUCTION
Toxicity of insecticides against the cotton AphidOnly 14 pages are availabe for public view
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Abstract
Cotton aphid, Aphis gossypii Glover and cowpea aphid, Aphis craccivora Koch, as the most known aphids are polyphagous pests causing extensive damage to cucurbitaceous and legume crops. Both nymph and adult suck sap from the underside of leaves and shoots resulting in reducing fruit yield. Although, aphid is a secondary pest of economic crops, it is now causing major problems to growers in some regions. Problems associated with aphid in Egyptian cotton include yield reductions due to the large early season infestations and effects of honeydew on fiber quality at late season infestations; moreover, transmission of virus diseases to cotton and other economic plants.
The present investigation aimed to study the following aspects:
Part 1: Morphological and physiological properties associated with pirimicarb resistance in cotton aphid, Aphis gossypii Glover [Homoptera: Aphididae].
Part 2: Differences in the biological aspects of the cotton aphid, Aphis gossypii Glover (Homoptera: Aphididae) in relation to color of its color forms.
Part 3: Impact of the resistance to pirimicarb on the ability of the cotton aphids, Aphis gossypii Glover (Homoptera: Aphididae) in transmitting plant viruses.
Part 4: Relationship between selectivity of Aphis gossypii Glover and Aphis craccivora Koch to sucrose and maltose and their resistance to pirimicarb.
The obtaind results can be summarized as follows:
Part 1: Morphological and physiological properties associated with pirimicarb resistance in cotton aphid, Aphis gossypii Glover [Homoptera: Aphididae].
The toxicity of pirimicarb to Aphis gossypii Glover collected from Assiut and New Valley was build up and the selection was carried out through two years. It was found that Assiut aphid seemed to be more tolerant to pirimicarb than that of the New Valley. The resistance ratios (RR) to the same insecticide were increased in both colonies. Moreover the New Valley aphid population developed resistance faster than that of Assiut one. After two years of selection, the RR of dark colored individuals of Assiut aphids became higher (two folds) than that of the mixed colored populations. The RR of mixed and dark colored individuals of the New Valley aphids, were nearly similar. Slope of the toxicity lines of mixed colored population did not changed through the years of selection, while it increased about 2 folds in dark colored individuals of the same population. The difference in resistance (RR) seemed to be due to aphid population, homogeneity of individuals, and area of study, as well as the recurrent of insecticide applications.
Part 2: Differences in the biological aspects of the cotton aphid, Aphis gossypii Glover (Homoptera: Aphididae) in relation to color of its forms
Assiut culture
Duration of the first instar nymph was shown to be 1.19, 1.06, 1 and 1 days for the base line, mixed colored, light colored and dark colored forms, respectively. These values indicate that the first nymphal instars of the base line lasted for a longer time than those of the pirimicarb selected aphids. After the first molt, nymphs reached the second instar. In this instar, the pirimicarb selected aphids showed reversed evidence. The base line aphids lasted for lesser periods than those of the pirimicarb selected groups. These periods were 1, 1.06, 1.04 and 1.15 days for basic line, mixed colored, light colored and dark colored groups, respectively. The nymphal duration of the four tested groups was ranged between 4.11 and 4.22 days with insignificant differences. The periods through which aphid females were able to produce progenies were varied significantly. These periods were 2, 2.27, 4.61 and 7.65 for the base line, mixed colored, light, and dark colored forms, respectively. Our data demonstrate that the fecundity period of the dark form was longer than that of the base line and mixed colored forms by about 4 folds and it was about two folds longer than that of the light colored form. The longevity periods of the four tested aphid groups (base line; mixed, light, dark colored forms) after one year selection by pirimicarb were 4.48, 6.17, 9.11 and 9 days, respecively.
The production of one female was 4.05, 5.67, 19.96 and 21.57individuals for base line, mixed colored, light colored and dark colored forms, respectively.
New Valley culture
Nymphal period of the four aphid groups was varied from 4 to 4.5 days. Periods through which aphid females were able to produce progenies were varied significantly. These periods were 1.42, 2.88, 2.95, and 6.1 for the base line, mixed colored, light, and dark colored forms, respectively. Longevity period of the one year pirimicarb selected dark forms was about two folds longer than that of the base line aphids. The longevity periods of the four tested aphid groups were 4.65, 6.4, 6.63 and 9.77 days. The production of one female was 3.7, 4.06, 4.86 and 8.2 individuals for base line, mixed colored, light colored and dark colored forms, respectively.
Part 3: Impact of the resistance to pirimicarb on the ability of the cotton aphid, Aphis gossypii Glover (Homoptera: Aphididae) in transmitting plant viruses
The present investigations were undertaken to identify the relationship between pirimicarb resistance in Aphis gossypii Glover and its potency in transmitting cucumber mosaic virus (CMV) to squash plants.
Data show that the percent of plants infected by mosaic virus transmitted by the dark form of cotton aphid, commonly, was higher than that inoculated by the light form. In the base line of Assiut aphid colony, virus infection% transmitted by light and dark forms were 46.3 and 55.5, respectively, whereas, the virus infection percentagestransmitted by light and dark colored forms after one year of pirimicarb selection were 65.1 and 94.3, respectively. Data of light and dark colored forms of New Valley aphid colony, showed similar trend of Assiut aphids. The percent of plants infected with mosaic virus transmitted by the dark form of cotton aphid was higher than that inoculated by the light form. In the base line of Assiut aphid colony, virus inoculation periods in case of light and dark colored forms were 23.0 and 11.0 days respectively and after one year of Pirimicarb selection were 15.0 and 7.5 days respectively. The inoculation period of mosaic virus transmitted by the dark colored form of cotton aphid, commonly, was shorter than that of the light form. In the present study, it is clear that the ability of cotton aphid expressed as % infection of inoculated plants is correlated with resistance as 64% of the differences in the plants infected withthe virus are due to the level of pirimicarb resistance value. This finding may be attributed to the inoculation period which negatively correlated with the pirimicarb resistance level. This negative correlation means that the more of resistance level, the less of inoculation period. The coefficient of determination was found to be 0.7 which indicates that 70% of the variability in inoculation period are due to pirimicarb resistance level. Our studies lead to the fact that the presence of the dark form of cotton aphid under the normal conditions is an alarm telling us that the aphid colony is going towards insecticide resistance subsequently, spread of plant virus diseases. Therefore, farmers must pay attention to the integrated pest management programs, without the use of chemical control methods to avoid the appearance of resistant strains (dark forms).
Part 4: Relationship between selectivity of A. gossypii Glover and Aphis craccivora Koch to sucrose and maltose and their resistance to pirimicarb
When 10 sucrose solutions versus water were tested, 8 counts indicated that aphids did not prefer the sucrose solution. After 6 hours cotton aphid could not discriminate between the sugar solution and water. The discrimination between sugar solutions was enhanced by adding 10% maltose. It was observed that the dark form of cotton aphid, A. gossypii and light forms of cowpea aphid, A. craccivora attracted to the higher sucrose solution than the other forms. The cotton aphid, A. gossypii has a light color as the basic color, transforming under reversible conditions to dark green, whereas, the basic color of cowpea aphid, A. craccivora usually black in color, transforming under stress to light brownish. This reversed evidence explained the differences in response towards sugar of the two aphid species.
It may be concluded that the dark morphs of cotton aphid, A. gossypii, and light morphs of cowpea aphid, A. craccivora which are considered pirimicarb resistant preferred high sugar levels. In other meaning, the present study proved that dark morphs of cotton aphid and light forms of cowpea aphid were resistant to pirimicarb and this resistance was positively correlated with the levels of sugars in the plants.
Therefore, plant breeders are requested to pay efforts in screening process of resistant plants to produce plants with low sugar contents in order to increase the effectiveness of pirimicarb against both aphid species.
Conclusions:
It was found that Assiut aphid seemed to be more tolerant to pirimicarb than that of the New Valley, but the New Valley aphid populations developed resistance faster than those of Assiut ones. After two years of Assiut aphid selection, the RR of dark colored individuals became higher (two folds) than that of the mixed colored populations. The variation in resistance rate (RR) may be due to several factors such as aphid population, homogeneity of individuals and area of study, as well as the recurrent of insecticide applications.
It was observed in the present study that the total nymphal duration of dark forms collected from both locations (Assiut and New Valley) did not significantly vary from the base lines. Data on the reproductive biology of aphids from both locations showed that the fecundity period, longevity and productivity of the dark morphs were significantly higher than those of the base lines.
The data showed also that the percent of plants infected with mosaic virus transmitted by the dark form of cotton aphid commonly was higher than that observed in case of plants infected with the virus transmitted by the light form of aphid. with, the light and dark colored forms of New Valley aphid colony showed similar trend of Assiut aphids. The inoculation period of mosaic virus transmitted by the dark form of cotton aphid commonly was shorter than that of the light form. The present study clearly show that the ability of cotton aphid expressed as % infection of infected plants is correlated with resistance as 64% of the differences in plants infected with virus were due to the level of pirimicarb resistant value. These findings may be attributed to the inoculation period which correlated negatively with the pirimicarb resistance level. This negative correlation means that the more of resistance level, the less of inoculation period. Our studies lead to the fact that the presence of the dark form of cotton aphid under the normal conditions is an alarm telling us that the aphid colony is going towards insecticide resistance subsequently, spread of plant virus diseases.
Moreover, it was observed that the dark forms of cotton aphid, A. gossypii and light forms of cowpea aphid, A. craccivora attracted to the higher sucrose solutions than the other forms. This reversed evidence justifies the differences in the response towards sugar of the two aphid species. It may be concluded that the dark morphs of cotton aphids, A. gossypii, and the light morphs of cowpea aphid, A. craccivora are considered pirimicarb resistant preferring high levels of sugars.
In the present study, the high susceptibility of aphid population collected from New Valley as compared with that of Assiut may be attributed to the absence of the larg scale application of insecticides in the New Valley. This may lead to a shortage of some/all resistance mechanisms within the population. This phenomenon may answer the question of why New Valley aphid population developed resistance faster than that of Assiut?
A negative linear relationship was observed between change in aphid numbers and sugar ratio, therefore population growth was limited by high levels of glucose and fructose in cotton leaves. These facts prove that the sucrose and maltose in cotton plant leaves play an important role in plant resistance to cotton aphids.
Therefore, cotton growers and pest control workers must pay great attentions to the following:
1- To avoid the use of pirimicarb as a curative treatment where dark morphs occur.
2- To rotate curative aphicide sprays, if necessary to be used to minimize the development of pesticide resistance.
3- Attempts to manage pest population by using certain recommended pesticide alternatives in order to avoid the developmemt of the pest resistance strains in addition to avoid the spread of plant virus diseases.
4- To pay efforts in screening process of plant resistance to produce plants with low sugar contents in order to increase the effectiveness of plant resistance against both aphid species.