Drought: A constraint in the efficacy of systemic pesticides

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Rise in the global temperature, erratic fluctuations in rainfall and severe and prolonged drought periods due to climate change seems to have a dramatic impact on agricultural production systems. Insect pest populations in agricultural and horticultural systems are also expected to be impacted, both in terms of their spatial and temporal distributions and in their status as pest species. Drought conditions associated with climate change not only alters the physiology of crop plants but also affect the performance of certain systemic pesticides against field crop pests. To get effective results from systemic pesticides they should first enter plant cells and become a part of the host plants. Systemic pesticides after entering living plant cells usually translocate in the whole plant with the aid of available water stream in xylem cells. The water stream helps pesticide to move relatively long distances in plant tissues to reach the growing points in the roots and shoots. Adequate water availability and pesticide solubility usually work in an integrated fashion for the successful pesticide movement in the plant cells. These systemic pesticides depend on efficient water and nutrient uptake through the root system and on vascular flow. Both these mechanisms may be partially compromised under drought conditions because this leads to lower uptake and non-uniform distribution of systemic pesticides in plant tissues and therefore higher risk of pests not acquiring a lethal dosage occurs.

Moreover, scientist believed that leaf surfaces are one of the more complex biological structures known today. Plant surfaces are usually involved in various activities like harvest light, breath, exchange water and provide natural defenses and resistance against physical forces. The binding, spreading and penetration of a pesticide droplet in the plant cell depends largely on the composition of leaf surface. Plants usually avoid low humidity, high temperature, and prolonged water stress by producing thicker coatings of surface wax on leaf tissues. This process is known as hardening off and it helps plants to cope with water scarce conditions and early drying out. Hardening off makes it even more difficult for systemic pesticides in a water carrying solution to adhere to, spread over and penetrate a plant’s surface. Plants may also alter the thickness, molecular arrangement and composition of the wax in response to environmental cues, age and even leaf position on the plant. When foliar-applied systemic pesticide applications fail to control the target during droughts, a thicker layer of wax on the plant’s cuticle is commonly the culprit.

Additionally, sufficient amount of water is required for the better translocation of systemic pesticides throughout the plant, resulting in a more even distribution of the pesticide within the plant. However non-uniform distribution of systemic pesticides in plant tissues may also responsible for behavioral resistance in target pest populations. More pronounced and prolonged drought will decrease uptake and increase the magnitude of non-uniform translocation of systemic pesticides within treated crop plants and that may have two concurrent consequences: 1) reduced pesticide performance; 2) increased probability of insect pests evolving behavioral resistance. Under this scenario, pests that can sense and avoid acquisition of lethal dosages of systemic pesticides within crop plants will have a selective advantage. This may lead to selection for insect behavioral avoidance so that insects predominantly feed and oviposit on portions of crop plants with low concentration of systemic insecticide.

Another significant factor which may lead to an increase in behavioral resistance within a pest population is the unequal acquisition of pesticide dosage among the neighboring plants occupying the same field. This happens when irrigated water level differs in certain areas of the same field, a situation likely to occur when field is not evenly leveled. An adequate amount of soil moisture favor water uptake, and consequently insecticide translocation but when the field is not leveled certain plants do not receive required amount of water. Hence pesticide fails to translocate evenly in the whole plant tissues because of limited available water stream in xylem cells.

Hence it is strongly recommended that application of systemic pesticides must be preferred after irrigating the field crops to enable better and uniform distribution of pesticide within the whole plant. Moreover, laser land leveling is also very important so that all plants in the field acquire equal amount of water and chances of non-uniform distribution of pesticides among the neighboring plants may reduce.

This article is collectively authored by Asim Abbasi, Dr. Muhammad Sufyan, Iqra and Hafiza Javaria Ashraf- Department of Entomology, University of  Agriculture Faisalabad, Pakistan.


Published in: Volume 08 Issue 37

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