A Study of the Population Density of Aphids and Coccinella septempunctata on Different Wheat Cultivars

Wheat (Triticum aestivum) is considered one of the primary and strategic grain crops in the world in terms of production and importance. It holds the first rank globally, with China, India, the United States and Russia accounting for 25% of the world’s production. In the Arab world, Syria leads the production, followed by Algeria according to FAO, (2011). The wheat production quantity in Iraq reached 3,088.6 thousand tons, with an average yield of 474.8 kilograms per donum. This production is significantly lower compared to the global average production rate of 1.5 - 2 tons per donum. Currently, Iraq imports 4.5 million tons annually. The Northern provinces of Iraq cultivate wheat using rain-fed methods, while in central and southern regions; wheat requires irrigation during most of its growth and development stages. The planting season extends from mid-October to mid-December (Iraqi Ministry of Agriculture, 2011); (Abebe et al., 2020).

In a study by Jadou et al. (2017)  it was noted that the variety ‘IPA 99’ and ‘Abu Ghraib’ exhibited superior performance in all studied traits, indicating a competitive relationship between the main stem and branches. Cheyed et al. (2020) indicated that the Abu Ghraib and IPA 99 varieties gave the highest average number of spikes and the lowest average percentage of broken spikes and fallen grain stored upon storagein the warehouses.

Observations by Rekani et al. (2017) emphasized the significance of phosphate fertilization on wheat germination and its impact on grain weight per spike and the number of grains per spike. Furthermore, Fakhri et al. (2023); Meena et al., (2019) utilized modern techniques, including high-yield improved cultivars, foliar fertilization with micronutrients and Atlant® pesticide, to enhance both the quantity and quality of wheat production. Wheat crops are susceptible to various pests and diseases, including both insects and non-insect pests. Among these pests is the aphid insect, Hussein et al., (2014) and Al-Obaidy  (2019) pointed out that there are three species of aphids affecting wheat in Iraq: Schizaphis graminum (Rondani), Rhopalosiphum padi (L) and R. maidis (Fitch). The grain aphid, Schizaphis graminum (Rondani), is a species that inflicts significant damage to wheat plants.

Large numbers of natural enemies are associated with aphid species, often maintaining a balance within the pest community. Numerous species of predators have been observed to coexist with aphids, such as ladybugs (Coccinellidae), green lacewings (Chrysoperlasp) and some hemipteran insects (Khan, 2005); (Alhomndawi et al., 2017); (Omran and Abdel Moaty, 2017). The predator Coccinella undecimpunctata, for instance, can be utilized to control aphid species like Aphis fabae and A. gossyp (Alhamawndy et al., 2021). Khidr (2018) noted the presence of predatory insects, Coccinella septempunctata and Syrphus corolla, in wheat fields in Erbil province. Ahmad et al., (2022) found that volatile organic compounds released by healthy and aphid-infested Brassica oleracea plants attract natural enemies. Furthermore, Al-Khafaji and Qassim (2023) mentioned three parasitoids, L. fabarum, B. acalephae and A. asychis, which could be used in biological control as an alternative to chemical pesticides for managing A. fabae., Chhangani et al., (2022)  mentioned the amount of predation on aphids by ladybugs and the importance of their use in biological control of aphids.

As a result of the widespread and large spread of these species in the irrigated wheat fields in central Iraq and the great damage they cause. The study aims tostudy the extent of infection of the IPA 99, IPA 22 and Abu Ghraib wheat varieties with oat aphid, cereal aphid and corn aphid.

Infection of different varieties of wheat with oats and cereals aphids
 
Three varieties of wheat, name “IPA 99, IPA 22 and Abu Ghraib, were tested for their susceptibility to different species of the aphid insect from the seedling stage to the mature stage. The varieties “IPA 99” and “Abu Ghraib” were obtained from the General Commission for Seed Testing and Certification / Ministry of Agriculture, while “IPA 22” was obtained from the Seed Technology Center / Ministry of Science and Technology.

The experiment was conducted in the fields of the College of Agriculture / University of Baghdad in Abu Ghraib in during 2021-2022. All recommended pre-planting practices were followed, including plowing and leveling. The experimental field was divided into plots with dimensions of 5x3 meters, following a randomized complete block design with four replications. The seeds were sown in rows, with three rowsfor each variety, at a rate of 160 grams per variety. The distance between rows was 25 cm and the distance between varieties was 50 cm. Planting was done on the first of December and recommended agricultural practices were followed throughout the cultivation period for optimal production. No pest control measures were implemented during the experiment and the trial was left for natural infestation. Samples were collected every two weeks, with a total of 10 plants sampled for each variety within each replication. The samples were collected in polyethylene bags, transferred to the laboratory and examined under a magnifying lens. The types and numbers of aphids present were recorded, along with observations on the plant’s health status. The results were analyzed according to a randomized completely block design (RCBD) (Al-Rawi and Khalafallah, 2000).

Types of aphids in wheat fields
 
The results of the field survey indicated the presence of three species of aphids belonging to the order Hemiptera and the family Aphididae: the Oat aphid (Rhopalosiphum padi L.), the Corn aphid (Rhopalosiphum maidis) and the Wheat aphid [Schizaphis graminum (Rondani)].

These species were identified by the researcher and through field observations throughout the growing season; it was found that the oat aphid, wheat aphid and corn aphid are the dominant and most damaging species. Therefore, they were the focus of the study.
 
Population density of Rhopalosiphum padi
 
The results of Table 1 showed the presence of oat aphid activity in the end of December in Baghdad province on different wheat varieties when the plants were in the seedling stage for the varieties IPA 22, IPA 99 and Abu Ghraib. The number of aphids continued to increase gradually until the end of February. The IPA 99 variety had the highest population density and was significantly different from the rest of the varieties  Abu Ghraib and IPA 22, with the numbers being 1.593, 0.926 and 0.815 respectively. The number of aphids did not visibly affect the phenotypic traits of these varieties. In general, there is a similarity in the seasonal presence of aphid species and their natural enemies. Aphids are rarely found when temperatures are below 12°C and their population density peaks between 15-20°C. Temperatures of 15°C are the point at which natural enemies begin to increase in abundance, when aphid populations reach their peak. The population density of oat aphids begins to increase when temperatures approach an average of 15°C. In Baghdad, the relative humidity in February was 64.1%, which means that the relative humidity and temperatures are suitable for the growth and development of aphids.  


 
Population density of (Rhopalosiphum maidis)
 
The corn aphid was present from the beginning of the wheat plant’s germination for all the cultivated varieties. It was observed that in the second third of December, the average population density of the corn aphid was 1.481, 0.926 and 0.778 insects per plant for the varieties IPA 22, Abu Ghraib and IPA 99, respectively were as shown in Table 2. Then, it began to increase with rising temperatures, reaching its highest density of 17.11, 9.630 and 8 insects per plant in the first third of February when the temperature was 14°C and the relative humidity was 58.95%. In March, it decreased gradually to reach its lowest density at the end of March, reaching the lowest population density of corn aphid, which was 0.852, 0.148 and 0 for the varieties IPA 99, Abu Ghraib and IPA 22, respectively. Ladybugs began to appear on the plants infected with insects of both the IPA 99 and Abu Ghraib varieties in early December with a density of 0.185 and 0.111 insects per plant, respectively. This was observed when the temperature was 10.8°C and the relative humidity was 68.01% (Fig 1). Then, their numbers gradually increased to reach their highest peak at the end of February with a density of 1.111 and 1.148 individuals plant^-1 on the IPA 99 and Abu Ghraib varieties, respectively. Then, 5 their numbers began to decrease during the months of February and March to reach their lowest density at the beginning of April, with 0 insect per plant  for ladybugs at a temperature of 24.4°C. As for the number of ladybugs, they were not found on wheat plants infected with corn aphids of the IPA 22 variety throughout the study period, despite the presence of aphids in different numbers.


 
Population density of Schizaphis graminum
 
The results showed the presence of a few wheat aphids in the second third of December on all varieties and at the end of the month (Table 3). Their numbers continued to increase until they reached their highest density on February 10, with rates of 2.889, 2.481 and 2.370 on the varieties IPA 99, Abu Ghraib and IPA 22, respectively, when the temperatures were 11.8°C and the relative humidity was 48.8%. Then, their numbers decreased at the end of February and beginning of March and then gradually increased until March 5, reaching their highest density of 0.296, 0.222 and 0.196 at an average temperature of 15.5°C and a relative humidity of 45.8% (Fig 1) for the varieties IPA 22, IPA 99 and Abu Ghraib, respectively. Then, their numbers decreased again suddenly and quickly to reach their lowest population density at the beginning of April at an average temperature of 24.4°C and a relative humidity of 35.9% and the numbers were 0 for all varieties.

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The results of the field survey indicated the presence of three species of aphids, the wheat aphid [Schizaphis graminum (Rondani)], the oat aphid [Rhopalosiphum padi (L.)] and the corn aphid (Rhopalosiphum maidis). It was observed that the wheat and oat aphids are the most commonly found on the crop. The presence of insects on the plants was affected by the predominant environmental conditions like temperature, humidity, the chemical components of the host plants and the interactions between these factors. Feng (1991) confirmed that a humidity of 35-53% in the spring can cause a significant increase in aphid populations. In a previous study by Ali et al., (2006) on the seasonal occurrence of oat aphids in Baghdad province, the study was conducted at two sites, one in Fadhiliyah and the other in Abu Ghraib, for three growing seasons. The results showed that oat aphids began to appear in the field in December, but their numbers were very few. They then began to increase in February and continued to increase as the season progressed and temperatures rose at both sites for the three years. The highest numbers of this type of aphid were reached in March of each year. In April, the number of insects began to decrease gradually and completely disappeared from the plant due to changes in environmental conditions and the plant reaching maturity. As for the natural enemies, a sharp decrease in the population density is observed in the first three months of crop growth, especially the lady bugs. This may be due to the influence of environmental conditions, especially the low temperatures during these months. It may also be due to the impact of pest control operations and the increasing use of pesticides in the control of aphids in the study areas, which are repeated annually. These results show that oat aphid can infect all varieties, but with different degrees of response. The reason for this may be due to environmental conditions that affect the nature of plant growth, the nutritional components in plant tissue, or the plant’s own defensive chemicals. Studies have shown that secondary compounds produced in resistant and susceptible plants can help the plant defend itself against pests. For example, Velozo et al. (1999) found that the increase in the substance (Gramine) may play an important role in enhancing the resistance of cereal crops to aphid infestation in various plants. Its role lies in determining the type of interaction that occurs between the host plant and the pest. Feng (1991) indicated that a humidity rate of 35-53% can cause a significant increase in the population density of wheat aphids. When comparing the population density rates of the pest on different varieties throughout the growing season based on the least significant difference, the highest population density of the pest was found on the IPA 99 variety, which differed significantly from the IPA 22 and Abu Ghraib varieties. The attraction of the pest to one variety over another is determined by the chemicals emitted from the plant. When the phenotypic characteristics and the chemical content of the tissue of the variety are not negatively affecting the life of the pest, it adapts to start developing and reproducing (Webster and Inayatuallah, 1988). Wheat aphids have enzymes in their saliva that are effective in stopping or inhibiting the function of the cell wall of chloroplasts in sensitive plants. This is clearly evident at the beginning of the pest’s feeding on the plant, where the leaves turn yellow or red spots appear on the leaves. Continued feeding leads to a general yellowing of the plant, followed by the drying of the leaves and roots and then the death of the plant. If the plant survives, it affects the size of the plant and the amount of yield in sensitive varieties (Al-Mousawi et al., 1983). However, this condition was not observed on the plants in the field, perhaps because the population density did not reach the level that caused that damage.

The population density of aphids on wheat crop in Iraq can vary depending on the aphid species, the wheat variety, the sowing date and the weather conditions.
Aphids are a major pest of wheat in Iraq and they can cause significant yield losses if their population is not controlled.

There are a number of different ways to control aphids on wheat in Iraq, including:
• Cultural controls: These include practices such as crop rotation, planting resistant varieties and destroying volunteer wheat plants.
•  Chemical controls: These include insecticides that can be applied to the wheat crop.
•  Biological controls: These include using natural enemies of aphids, such as ladybugs and lacewings.

It is important to monitor aphid populations on wheat crops in Iraq regularly and to take control measures as soon as the population density reaches the economic threshold. By taking these measures, farmers can help to protect their wheat crops from aphid damage and ensure a good yield.

All authors declare that they have no conflicts of interest.