AchE Inhibition leads to Decrease, Population growth rate

• Wildlife studies including fish and birds have shown that exposure to AchE inhibitors can lead to physiological changes and behavioral alterations, which can make individuals more vulnerable to prey. It is biologically plausible that decreased survival and reproduction could decrease the population trajectory, if the scale of the changes were significant.

Invertebrates

• Study of fourth instar Ailanthus silkworm exposed to malathion for 5 days found increased mortality, decreased AChE, and increases in Acetylcholine as compared to controls. Most moths did not emerge and those that emerged did not oviposit (Pant, 1983). 

• Studies using earthworms (Lumbricus terrestris) found a significant increase in AChE inhibition and concurrent mortality after exposure methiocarb, and a significant reduction in the reproduction rate of adults after 4 weeks of exposure to concentrations recommended for use by the OECD in agricultural pest control (Calisi 2011).

Fish

• In a pond study of bluegill and largemouth bass, chlorpyrifos treatment resulted in cumulative mortalities ranging from 46-55%. The authors presented data where the fish, especially the bluegill, fed heavily on the invertebrates in the first 7 days of the study thereby compounding the exposure that in the laboratory would be only via water (Macek 1972). 

• Brain AChE activity was significantly (10-89%) inhibited in fathead minnows exposed to varying concentrations of Dursban. Increased sensitivity was observed in second generation fish (Pimephales promelas) exposed after hatch to chlorpyrifos (Jarvinen et al., 1983). Biomass of the second generation fish recruitable to the population was estimated using a model that assumed a normal distribution for the population and used total egg production, hatchability, survival, and fish weight as input variables. The model estimated a significant reduction in the recruitment population (Jarvinen et al., 1983).

• In a study of Japanese medaka exposed to acute and subacute levels of chlorpyrifos,  brain reduced swimming speed and reduced AChE activity were correlated. The fish exhibited behavioral alterations, including decreased swimming speed and hypoactivity in the acute test, and social group hyperactivity and long time schooling in the subacute test.  These findings may indicate a subacute effect of chlorpyrifos on social behavior, which can pose a risk at the population level (Khalil 2013).

• Rainbow trout exposed to diazinon at sub-lethal (0.1 and 0.2 mg/L) concentrations showed significantly reduced plasma AChE activity.  Effects of AchE inhibition included the following behavioral alterations, which could lead to decreased population trajectory over time: vertical and downward swimming patterns, swimming near the water surface, lethargic and erratic swimming, loss of schooling behavior, hyperactivity, seizures and convulsion (fish suffering from severe muscular contraction and tremor) and loss of buoyancy (Banaee 2011).

• Larval zebrafish were exposed to chlorpyrifos, diazinon and parathion over the first five days of development to examine survival, AChE activity, and behavior. At non-lethal concentrations, chlorpyrifos was more effective than diazinon and parathion in producing 

• AChE inhibition. At higher concentrations of diazinon and parathion, lethality occurs before they can produce the degree of AChE inhibition observed with chlorpyrifos at 300 nM. Motility at 6 dpf was examined using 300 nM, a concentration that does not cause mortality,  as a standard dose and tracking behavior for 2 minutes. At 300 nM, a concentration that does not affect survival, only chlorpyrifos significantly affected motility, reducing locomotor activity by 35% (Yen 2011).

Birds

• White-crowned sparrows exposed to imidacloprid and chlorpyrifos during spring migration and exposed to low and high doses exhibited declines in fat stores and body mass for those exposed to imidacloprid, and failed orientation for both those exposed to imidacloprid and those exposed to chlorpyrifos. This that wild birds who consume insecticide treated seeds could be in impaired condition to migrate, have delayed migration, or improper direction, resulting in increased mortality or loss of breeding opportunities (Eng 2017).

Mammals

• A review of studies from captive and wild birds and mammals exposed to acute doses of OP and carbamate pesticides found acute effects thermoregulation (hypothermia), food consumption (anorexia, altered forging behavior), and reproduction (altered hormone levels, reduction in clutch and litter size, and alterations in behavior) (Grue 1997).

• In bluegill and largemouth bass, the principal mode of exposure was unclear. The relative uptake from pesticide-treated insects versus direct uptake from water was not quantifiable, although the data suggest that fish more readily metabolize insecticides introduced via diet than via oral exposure (Macek 1972).

• Fish appeared to be more sensitive to exposure at a higher water temperature (Macek 1972).