The linear sequences of organisms through which nutrients and energy circulates when an organism is eaten by another are called Food Chains.
By Danyal Haider Khan1, Muhammad Awais Ahmad1, Muhammad Nauman Sadiq1
Food chain and Trophic relations:
The linear sequences of organisms through which nutrients and energy circulates when an organism is eaten by another are called Food Chains. Tritrophic interaction can be defined as the impact of three trophic levels and their effect on one another. The plant, the herbivore and the enemy or predator of that herbivore.
In wheat the Tritrophic relation will be like following:
Wheat is a plant that is the producer in the present food chain and acquires the first trophic level while aphid is the primary consumer while the predator Lady bird beetle are secondary consumer so they will automatically acquire the 3rd trophic level.
Wheat (Triticum aestivum) is a grass specie which is cultivated worldwide for its seed which is staple food. Wheat is the most harvested crop in the world . It is cultivated on more than 220.4 million hectares. In Pakistan it is cultivated from November to April-May. It takes almost 110-130 days between sowing and harvesting if climatic conditions and soil features are optimum. Usage of spring fertilizers , herbicide , fungicide and growth regulators at a specific stage is a common practice. Understanding of each stage helps in preventing crop from hazards. For example , pollen formation from mother cell , and the stages between anthesis and maturity are susceptible to high temperature and water stress that makes the condition of crop even worst.
The duration for the opening of wheat florets ranges from 8-60 minutes depending on environmental conditions and genotypes of plant. Once the anthers dehisce 5-7 % pollen shed on the stigma . 9-12% remain in anther and the rest of them scatter. The first spikelet to the middle third of the spike and usually near the upper part of this section ,the flowering proceeds more in upward direction than in downward direction.
Aphid and its effect on wheat crop:
Aphid are small sap-sucking insects of superfamily Aphidoidea . It varies greatly in colour from yellow to green to black. Almost 4000 species of aphid have been discovered that feed on 250 agricultural crops. Usually eggs mature inside female body and female gave birth to nymph that have no wings but as population becomes crowded they develop wings for dispersal. They are mostly found at developing parts at high density. Aphid use stylets to probe epidermis beyond parenchyma and mesophyll cells. Stylets are thin appendages formed by mandibles and maxillae, they penetrate the plant cell and route phloem .
Aphid is the main and sometimes only pest of wheat crop in Pakistan. It is the major pest in almost every major crop due to its unparallel reproductive capacity and their ability to manipulate host plant physiology. Rapid growth of aphid population and its effect on plant fitness is directly related to its relation with other organisms like pathogens, predator, parastoid , ants , endophytes aphid endosymbionts. Aphids can cause damage to the wheat plant either by directly sucking the cell sap or by indirectly transmitting viral and fungal diseases. Sometimes the population density of aphid increases in spring season due to favorable environment but the population of aphid predator Coccinelids also increases in spring to control aphid population. Following are the signs of aphid attack: leaf chlorosis, leaf folding, leaf rolling, plant stunting.
Aphid decreases the vitality of wheat plant during growth stages of plant because both the adult and nymph of aphid vigorously suck the cell sap of leaves. The infected leaves turn pale ,wilt and wear a silky appearance. Toxins in the saliva of some species or severe infection may damage the roots. Honey dew secretions also cause severe damage to plant because it causes ease in the development of souty mould that effect the rate of photosynthesis in plants. The main reason of low yield of wheat is because of its low resistance to aphid infestation.
The infestation cause severe distortion in leaves and inflorescence. The deduction in the yield of wheat crop is also caused by many abiotic factors like traditional methods of cultivation, low yielding varieties, lack of proper irrigation facilities in most of the areas, relatively low level of soil fertility and a higher incidence of insect pests and diseases.. The present study was conducted to evaluate the yield losses in wheat crop due to the infestation of aphids. The investigations were projected to manage this serious pest and to boost up the wheat production keeping in view the quality and quantity of the production ..
Coccinellids and their Effect on Aphid:( Predator-Pest relation)
Ladybird beetle are generally predators of many insects with soft bodies that include thrips , coccids and most famously aphids that shows its economic importance on many commercial crops. There are 5,000 species of coccinelids observed in entire world . Out of them 265 species are found in subcontinent. Aphid population can be easily suppressed by Ladybird beetle by the ratio of 1:100 however, this ratio can vary depending upon the aphid specie and plant physiology. The most common specie that feeds on aphid are Coccinella septempunctata, C. transversalis, Menochilucoms sexmaculatus. These species have been tested to reduce aphid population in both wild and in Laboratory conditions. Density of aphid population and environmental factors greatly affect coccinellids population.
In a research, it was reported that more coccinellids are present on insecticide free crops than on treated ones. 30’ C is most favourable for the growth of coccinellid population. The development of coccinellid species vary with the variation in the prey density , prey species and also with variation in agro-climatic conditions stated that the incubation period of coccinellid ranged from 2-13 days, larval period 6-31 days and pupal period 3-13 days . The developmental stages (eggs, larvae and pupae) of lady bird beetle shows high mortality at 14’C .The number of females adults dwindle with the rise in temperature. Female beetle of C. septempunctata laid twice as many eggs when it is fed on thirteen aphid species.
It was observed that C. transversalis preferred A. gossypii in terms of survival, development and reproduction, whereas A. nerii was the least preferred species among the ladybird beetle for prey. It was also disclosed in another observation that A. craccivora and Rhopalosiphum madis were the most preferred prey for the development of both the ladybird species. It was also discovered that different species of Ladybird beetle have different preference of aphid for prey A. nerii was least preferred by other but in case of larval mortality. It was recorded again and again that an adult beetle can kill more than 56-80 aphids a day.
Wheat resistance against Aphid:
At first insecticides were used against aphid in cereal crops but then resistance in aphid against insecticide developed that was first reported in 1950s and now it’s a global problem. Resistance breeding against cereal aphids began in 1970s when resistance was first reported in the species of Triticum turgidum, Aegilops squarosa, Triticum tauschii against greenbug Schizaphis graminum. Since than resistance varieties are developed again and again to cope with aphid problem. The most important thing is to study the defence mechanism of wheat against aphid. The selection pressure to evolve a biotype which can incorporate antixenotic plants and is able to break host plant resistance which is very high because there are several overlapping generations of aphid a year that results in population boom.
Trimorphic relationship in Wheat:
It has been concluded from above discussion that there is a complex trophic relationship among wheat ,aphid and coccinellid . All have somehow direct or indirect relation with one another. Sudden decrease in the the population of any one of them causes severe effects on other. If wheat crop is not sown than the population of both aphid and coccinellid will be eliminated. And if the aphid population decline than coccinellid will have no food to eat so they will be eradicated from a field . And if the population of coccinellids decreases than the population of aphid will flourish that will cause severe damage to the wheat crop.
Authors: Danyal Haider Khan1, Muhammad Awais Ahmad1, Muhammad Nauman Sadiq
1 Institute of Plant Protection , MNS University of Agriculture , Multan