Population dynamics of wheat aphids Rhopalosiphum padi (Linnaeus) and Sitobion avenae (Fabricius) at District Mardan, Khyber Pakhtunkhwa Pakistan

Wheat (Triticum aestivum L.) is one of the major source of food worldwide. However, it suffers from numerous constraints. The aphids, particularly Rhopalosiphum padi (Linnaeus) and Sitobion avenae (Fabricius) species are among the major causes that severly affect wheat production acrossthe globe. To determine the population dynamics of aphids associated with wheat crop in Khyber Pakhtunkhwa, we examined the mean population of both species in the selected areas (Maho Dheri, Sarband, Kandare, Rag Narai and Khazana Dheri) of district Mardan in 2015. The mean population of R. padi and S. avenae per plant were recorded every week by counting the number of aphids on 5 plants picked up randomly from each selected wheat field using the diagonal methods. The results on current study showed that the infestation of both aphid species was started in the 1 week of February (05 standard weeks). The densities of aphids per plant increased as the vegetative growth proceeded and was highest till the mid of March (10 and 11 standard week). The declined in aphid populations for both species were started after mid of March and ended in April. The mean population of both aphid species was found low in the 1 week of February (05 standard week) that is probably due to the low temperature. The population density of aphids reached to the its peak in the second week of March due to suitable temperature for aphid infestation.


Introduction
Wheat (Triticum aestivum L.) is the primary food source, and use as a staple food contains 13% protein providing 20% world food calories. Wheat is used as a food for nearly 40% of the world's population [1]. The wheat crop is grown on 23% global cultivated land having utmost importance in international trade for worldwide market [2]. During 2017-18 the total yield of wheat grown in Pakistan was 26.7 million metric tons, and their consumption was 25.3 million tons [3], while 755 million metric tons of wheat was produced during 2015-16 worldwide. In Pakistan, Wheat is a major crop and use as a staple food [4]. . The straw and bran are used to feed livestock and also in industries to make several products. Owing to the immense cosumptions, wheat crop is considered to be the 2 nd largest crop having 12.5% contribution in agriculture and 2.6% to the GDP (Government of Pakistan. 2012). However, various factors affect the quality and quantity of wheat production in Pakistan [5]. Wheat production has been severly affected by so many factors such as immproper seedbeds, low aquality seeds and fertilizers, water shortage and most importantly lack of IPM techniques to mnage insect pests. [6,7]. Aphids attack is one of the most crucial factors for wheat disease and low yield. Aphid is an economically significant insect pest damaging a broad variety of crops, fruits, and vegetable plants [8].
The bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus) and grain aphid, Sitobion avenae (Fabricius) are deleterious insect pests of wheat crops in Pakistan [9, 10] and worldwide [11][12][13]. R. padi and S. avenae are polyphagous sucking insects having worldwide distribution [14] and cause 15% reduction in the yield of the wheat crop at flowering stage only [15]. These two wheat aphids usually coexist in the wheat crops and cause damage through direct feeding on the sap from shoots and leave, causing distortion, curling, and chlorosis of leaves growth [16,17]. Besides, these aphid species also act as a vector to transmit numerous fungal and viral diseases that ultimately reducing the yield up to 80% [18,19]. Furthermore, they also act as a vector to transmit barley yellow dwarf virus (BYDV), which cause the indirect destruction of wheat crops [20]. The plants are also damaged indirectly by the excess production of honeydew [21]. The outbreaks of R. padi and S. avenae lead to severe yield losses in wheat crops [22]. Wheat crops sown in the late season may have high chances of aphids outbreak if the weather remains cool till the end of March [23]. Aphid epidemic could be controlled by sowing cereal crops earlier in the season [24], because low aphid infestation has been documented on early sown wheat crops. Early maturing varieties of wheat can avoid aphids outbreak [25] while infestation increases on late sowing, ultimately cause yield reduction [26]. The seasonal fluctuation in their dynamics are determined through the population growth rate and duration at which the population can grow. Aphids growth rate mainly disturbed by the host plant quality, predators, parasitoids and environmental factors [27]. Diverse factors were encouraged in search for determining the cyclic dynamic of aphids including long-term trends in agriculture practices [21], weather alterations [28], natural enemy profusion, and intraspecific competition [29]. We hypothesized that the annual deviation of aphid populations are driven by the temperature fluctuations. Their profusion is predominantly determined by the duration of aphid population growth. Winter and early spring temperatures thus define stages when aphids attack on cereal crops and the duration available to them prior the onset of plant senescence. In this study, we determine the field population trend of R. padi and S. avenae in wheat crops at different locations of District Mardan. This constitutes a step towards exploring the diversity of wheat aphid throughout the wheat growing season to elaborate an appropriate control plan against these pests and thus contributing to increase in the production of cereal crops.

Materials and methods Field site
The current experiment was carried out at randomly selected five different locations of District Mardan in Khyber Pakhtunkhwa during the wheat grwing growing season i.e. February 2015 -April 2015.

Study design
Population dynamics of two aphid species R. padi and S. avenae were estimated following previously described method [30]. Data were recorded at each site by counting aphid population per tiller. At each location, five spots were randomly spaced along a diagonal across the field. The selected spots for samples collecction were not closer than 10 feet from the field edge [30]. The tillers were randomly chosen at each spot and was checked for the R. padi and S. avenae infestation. Considering the population buildup, which was started in February, therefore data collection was started in the first week (05 standard weeks) of February 2015. Data on mean mean population density of aphid species was recoreded at each location till the crop maturity.

Data analysis
To assess the mean (±SE) number of R. padi and S. avenae per plant, raw data were subjected to the SPSS 22.0 (IBM Statistics). The statistical differences among data related to the population dynamics of R. padi and S. avenae were examined using one-way ANOVA followed by Tukey post hoc test (P < 0.05) (IBM, SPSS Statistics). Sigma Plot 12.0 (Systat Software Inc., San Jose, CA) was applied to generate line graphs with error bars.

Results Population dynamic of R. padi in wheat fields
The mean population density of R. padi is reported in (Table 1). The estimated mean values shows the increasing trend of aphids population from February till April, as the plant growth proceeded. 11.00 ± 0.577f 12.50 ± 0.204f 5.00 ± 0.471h 18.66 ± 0.509e 3.33 ± 0.384h Week 10 3.00 ± 0.333g 2.16 ± 0.597g 1.16 ± 0.152i 12.33 ± 0.384g 0.00 ± 0.00 * Within the same column, different letters indicate significant differences at P < 0.05 level (one-way ANOVA followed by Tukey HSD tests). week (SW 10). The population growth of S. avenae declined after week 6 (SW 10), whereas the population of R. padi remain high that was later on declined after 7 th week (SW 11) (Fig. 1).

Population dynamic of R. padi and S. avenae in Kandare
The mean population of both species (R. padi and S. avenae) per plant was observed low as compared to other locations of District Mardan. The highest mean value were 31.50 ± 0.311 and 36.00 ± 0.942 at week 7 (SW 11) and 5 (SW 09) respectively for both species (Fig. 3). The population of S. avenae started to decline after week 5 (SW 09), while population of R. padi went down after week 07 (SW 11).

Population dynamic of R. padi and S. avenae in Rag Narai
The mean highest population of R. padi was observed in Rag Narai (52.50 ± 0.311) as compared to all locations of District Mardan (Table 1, Fig. 4) while mean highest value (43.33 ± 0.192) for S. avenae was also recorded at week 6 (SW 10). Both species populations were declined after week 6 (SW 10) (Fig. 4).

Population dynamic of R. padi and S. avenae in Khazana Dheri
The highest population growth for S. avenae and R. padi was 33.66 ± 0.304, and 42.50 ± 0.311 observed at week 6 (SW 10) respectively, after which population of aphids was rapidly decreased and ended at week 10 (SW 14) (Fig. 5).  A gradual increasing trend in population desnity of both species was noticed at vegetative growth stage of wheat crops in all areas. However, some variations occurred in aphid density tiller -1 , which might be due to the resistance response of wheat varieties against aphids. [8] reported that the aphid population decreased when the temperature reached to the maximum and minimum limit (24°C and 9°C). We found similar results, in which the population growth of both species was lower at the start of February and end of April because in February temperature is less than 10ºC, while in April it is above 24ºC. The population growth of R. padi and S. avenae was estimated maximum in March when the temperature was in a favorable range of aphid. We also found fluctuation in the mean population for both aphids at different locations, which may be due to various abiotic factors such as temperature, humidity, and rainfall. The rapid increase and decrease in the aphid population were affected by numerous factors [8]. Moreover, it has also been found that alterations in the densities of the aphid population were due to the various physical and biological factors [32]. According to [8,33] temperature have a crucial role in the fluctuation of the aphid population. Similarly, the abundance of aphid population was also affected owing to the heavy rainfall [34].

Conclusion
Based on current study, it is concluded that the population density R. padi and S. avenae could be controlled by early sowing of wheat crops. Additionally, the indiscriminate use of insecticide influence the population growth of natural enemies, and due to the decrease population of natural enemies in wheat crops, aphids' outbreak occurs and cause severe destruction to the wheat crops.