166
impact of climate-induced pest outbreaks on pesticide use and exposure. Farm
workers and local communities are at increased risk of pesticide exposure through
pesticide drift irrigation channels (Wilson et al. 2004) as they use water that flows
through irrigation canals for bathing and household chores.
3.2 Pesticide Toxicity
Higher temperatures affect the toxicity of pesticides on their target pests although
these effects vary with different pesticide-pest combinations (Fishel 2015; Noyes
et al. 2009; Donahoe 2014). Pesticides may be particularly affected by temperature
extremes if they are not stored correctly; higher temperatures may cause pesticides
to expand and volatilize and to spill out of their containers when opened. Farmers
lacking proper chemical storage systems may store pesticides within their homes
and are at greater risk of pesticide exposure. Sadly, this is the case in many developing countries (Mengistie et al. 2015; Stadlinger et al. 2010; Kasambala Donga and
Eklo 2018). Pesticides containing phosphoric acid (a.k.a. organophosphates) tend to
be more toxic to insect and mite pests at 26–28 °C than at 20 °C, while pesticides
derived from naturally occurring pyrethrin extracted from Chrysanthemum plants
are more toxic at lower temperatures (Jegede et al. 2017; Noyes et al. 2009).
Maximum temperatures in the sugarcane-growing areas of Malawi range between
27 °C and 37 °C (Phiri and Saka 2009), which is higher than the temperatures used
in pesticide toxicity studies (Jegede et al. 2017; Noyes et al. 2009). Since cypermethrin is widely used in Malawi to control a range of insect pests infesting sugarcane,
a reduction in efficacy is likely to result in either increased frequency or amount of
pesticide application.
3.3 Pesticide Degradation
As shown in Fig. 1, temperature strongly influences the degradation of pesticides,
and several reports exist on the effects of temperature on some of the pesticides
examined in this study (de Beeck et al. 2017; Jegede et al. 2017). The rate of degradation of atrazine increased with increasing temperature (Dong and Sun 2016).
Higher temperature also enhances the activities of microorganisms that degrade
pesticides. At 30 °C and pH 7, bacteria degraded 90% of chlorpyrifos and profenofos within 8 days (John et al. 2016). Acetamiprid degradation was rapid in soils with
higher temperatures (Vela et al. 2017). The sugarcane growing districts of Malawi
experience high temperatures (above 30 °C) during most of the year. Hence, we
expect the estimate of the risk of pesticide exposure to be significantly lower under
rising temperatures, assuming all other degradation factors remain constant.
Soil moisture is also an important factor in pesticide degradation (Chai et al.
2013; Sebaï et al. 2010). With the exception of rainfed sugarcane (less than 20%),
T. K. Donga et al.
impact of climate-induced pest outbreaks on pesticide use and exposure. Farm
workers and local communities are at increased risk of pesticide exposure through
pesticide drift irrigation channels (Wilson et al. 2004) as they use water that flows
through irrigation canals for bathing and household chores.
3.2 Pesticide Toxicity
Higher temperatures affect the toxicity of pesticides on their target pests although
these effects vary with different pesticide-pest combinations (Fishel 2015; Noyes
et al. 2009; Donahoe 2014). Pesticides may be particularly affected by temperature
extremes if they are not stored correctly; higher temperatures may cause pesticides
to expand and volatilize and to spill out of their containers when opened. Farmers
lacking proper chemical storage systems may store pesticides within their homes
and are at greater risk of pesticide exposure. Sadly, this is the case in many developing countries (Mengistie et al. 2015; Stadlinger et al. 2010; Kasambala Donga and
Eklo 2018). Pesticides containing phosphoric acid (a.k.a. organophosphates) tend to
be more toxic to insect and mite pests at 26–28 °C than at 20 °C, while pesticides
derived from naturally occurring pyrethrin extracted from Chrysanthemum plants
are more toxic at lower temperatures (Jegede et al. 2017; Noyes et al. 2009).
Maximum temperatures in the sugarcane-growing areas of Malawi range between
27 °C and 37 °C (Phiri and Saka 2009), which is higher than the temperatures used
in pesticide toxicity studies (Jegede et al. 2017; Noyes et al. 2009). Since cypermethrin is widely used in Malawi to control a range of insect pests infesting sugarcane,
a reduction in efficacy is likely to result in either increased frequency or amount of
pesticide application.
3.3 Pesticide Degradation
As shown in Fig. 1, temperature strongly influences the degradation of pesticides,
and several reports exist on the effects of temperature on some of the pesticides
examined in this study (de Beeck et al. 2017; Jegede et al. 2017). The rate of degradation of atrazine increased with increasing temperature (Dong and Sun 2016).
Higher temperature also enhances the activities of microorganisms that degrade
pesticides. At 30 °C and pH 7, bacteria degraded 90% of chlorpyrifos and profenofos within 8 days (John et al. 2016). Acetamiprid degradation was rapid in soils with
higher temperatures (Vela et al. 2017). The sugarcane growing districts of Malawi
experience high temperatures (above 30 °C) during most of the year. Hence, we
expect the estimate of the risk of pesticide exposure to be significantly lower under
rising temperatures, assuming all other degradation factors remain constant.
Soil moisture is also an important factor in pesticide degradation (Chai et al.
2013; Sebaï et al. 2010). With the exception of rainfed sugarcane (less than 20%),
T. K. Donga et al.