A far less common practice is to determine the effects of pesticides via oral exposure, in which the study organism is provided with a pesticide-treated food source. This approach has more frequently been used for testing pesticide toxicity among pollinators. Pesticides are added to a diet of sugar water and fed to pollinators, such as honey bees and bumbles bees . This type of bio-assay may be used to determine acute toxicity and/or effects on foraging behavior. With respect to natural enemies, a treated diet may be used to assess oral exposure. For instance, treated honey has been used to assess oral toxicity of several insecticides on parasitoids . Cryptolaemus montrouzieriis a mealybug predator used in integrated pest management programs for citrus. To determine non-target effects of acaricides, C. montrouzieri was provided treated prey, the citrus mealy bug Planococcus citri. Another key natural enemy in IPM programs is Coccinella undecimpunctata , a generalist predator of aphids. In order to evaluate how the neuroactive pesticides pirimicarb and pymetrozine may affect C. undecimpunctata consumption of aphids, the predator was provided a diet of treated Myzus persicae. As some pesticides are specifically designed for ingestion, oral exposure of natural enemies is an important factor to consider when assessing pesticide impacts. Historically, one of the most common toxicological measurements of pesticide exposure is the median lethal dose or the lethal concentration . Though acute toxicity assays are a good method for beginning to understand the effects of pesticides,stacking pots they are limited to the measurement of survivor ship only. Pesticides can induce a much wider range of sublethal effects that impair physiology or behavior .
Studies which consider sublethal effects are viewed as more accurately measuring toxic effects than those which only measure acute effects . For instance, when the parasitoid Aphidius erviwas exposed to lambda-cyhalothrin, the LD20 was found to be 50 times lower than the recommended field rate . Additionally, the study found, that even at such low concentrations, exposure to lambdacyhalothrin decreased the response of A. ervi females to its host aphid, M. persicae. This same parasitoid species also exhibited a change in response to plant-host odor when exposed to triazamate . Although it is worthwhile to determine lethal doses of pesticides for natural enemies relative to applied field rates, this measurement alone could not have predicted the disruptive behavioral effects on A. ervi females. Studies on sublethal effects can be used determine changes in all aspects of the life history performance of natural enemies that result from pesticide exposure. For instance, pesticides have been shown to significantly affect reproduction in the predatory mirid bugs, Nesidiocoris tenuis and M. pygmaeus . For N. tenuis, fresh residues of azadirachtin reduced fertility but those of spinosad did not. Conversely, for M. pygmaeus, spinosad reducedfertility, but azadirachtin did not. Another study found that methoxyfenozide significantly reduced fecundity of Aphidius colemani , while exposure to pure azadirachtin had no significant effect . Interestingly, a pesticide may display chronic toxicity, yet have no significant effects on reproduction. For example, exposure to any one of three insect growth regulators caused an 80% reduction in adult emergence of the lacewing C. externa, but there was no significant impact on the number of eggs laid per female or hatch rates . In this case, solely examining sublethal reproductive effects would have been misleading as a measure of potential demographic effects because the low adult emergence indicates otherwise. Therefore, it is best practice to link several effects of pesticides, lethal and sublethal, to estimate changes in the demography of natural enemy populations.
Demographic toxicology was initially used in 1962 to study the effects of gamma radiation on the intrinsic rate of increase Daphnia pulex . Since then, demographic toxicology has been increasingly employed, as it provides a more complete determination of the effects of a toxicant on the population growth rate of an organism rather than simply the acute effects on individuals . For example, using life table data , critical extinction thresholds were estimated for four economically important parasitoids: Diachasmimorpha longicaudata, Psyttalia fletcheri, Fopius arisanus, and Diaeretiella rapae . Though all species are braconid parasitoids, D. rapae was far less vulnerable to pesticide exposure, suggesting that risk assessments cannot be generalized for an entire guild. Another demographic study found that imidacloprid exposure reduced adult emergence of the egg parasitoid Trichogramma cacoeciae , yet mean longevity of emerged females or the mean number of female offspring per female was not significantly different from controls . In order to more fully understand the effects of pesticides, it is useful to measure both lethal and sublethal effects on natural enemies. Acute toxicity studies can be a useful starting point for determining lethal doses relative to recommended field rates. This is especially necessary when maximum field rates cause 100% mortality, making it difficult to study sublethal effects. Moreover, by determining both lethal and sublethal effects, we can begin to measure how these effects translate to changes in population dynamics. The accuracy of these tests can be increased even further by incorporating multiple routes of exposure, thereby predicting what actually may occur to natural enemies in a field setting. Taken together, the aforementioned bio-assays yield a more complete assessment of pesticide impacts on natural enemies, ultimately informing more effective integrated pest management strategies. The purpose of this dissertation was to assess the effects of reduced-risk pesticides on Hippodamia convergens. This coccinellid is an important predator in high-value tree crops in western North America, namely apples, pears, and walnuts. Hence, the pesticides included in this study are those that are commonly used in these cropping systems. In order to more fully understand pesticide impacts, several different assessment methods were used and are described below.
There were three primary objectives, each one addressed in a different chapter. For more than a decade, regulatory limitations have caused a reduction in the use of organophosphates in the U.S. as successive products have been restricted or can celled according to the Food Quality Protection Act . Additionally, the U.S. EPA’s Reduced-Risk Initiative has created a shift away from OPs toward other classes of insecticides . The goal of this initiative was to encourage the development and registration of insecticides that, compared to OPs, pose lower human and environmental health risks. This included lower health risks to mammals, aquatic organisms, birds, and other non-target organisms, such as beneficial insects . In principle, this initiative would be expected to enhance the potential for biological control of arthropod pests by the natural enemies that had previously been limited by the use of OPs. However, while reduced-risk insecticides may lower human and environmental health risks, they do not necessarily have lower toxicity to natural enemies . It is important to determine the selectivity of insecticide alternatives to OPs, particularly to natural enemies that contribute effectively to pest management . For instance, several reduced-risk insecticides showed acute toxicity to important natural enemies in high-value tree crops in western North America. Spinetoram and spirotetremat were toxic at recommended field concentrations to the predatory mite Galendromus occidentalis. Acetamiprid was toxic to Deraeocoris brevis, a natural enemy of pear psylla, Cacopsylla pyricola. Furthermore,strawberry gutter system novaluron and lambda-cyhalothrin were highly toxic to larvae of two different green lacewing species, Chrysoperla carneaand Chrysoperla johnsoni Henry, Wells and Pupedis . On the other hand, some reduced-risk insecticides appear to be compatible with certain natural enemy species, demonstrating low toxicity. Chlorantraniliprole had no significant effects on survival, parasitism rates, or emergence of seven different species of parasitoid wasps, including two sensitive indicator species, Aphidius rhopalosiphiand Trichogramma dendrolimi. In addition, it had no significant impact on the mortality of the predatory ground beetle Harpalus pennsylvanicus DeGeer when fed treated food or on parasitism rates of the ectoparasitoid, Tiphia vernalis Rohwer when treated directly. Another diamide insecticide, cyantraniliprole, had no significant impact on the mortality of Tamarixia triozaeexposed to residue on either a glass or leaf surface , and caused only 23% mortality of nymphs and no significant mortality of adults of the predatory mirid, D. brevis . For many natural enemies, the selectivity of insecticide chemistries is not yet clear. The primary objective of this study was to determine the acute toxicity of a range of pesticides for Hippodamia convergens, a well-known generalist aphid predator in a wide variety of agricultural systems . H. convergens is especially prevalent in western North America , where it serves as an indigenous biological control agent for aphid pests in high-value tree crops .
As for other natural enemy species, reduced-risk pesticides have also shown acute toxicity to H. convergens. In laboratory and greenhouse bio-assays, the two neonicotinoid insecticides, acetamiprid and imidacloprid, were toxic to H. convergens , and indoxacarb residue was highly toxic even when aged for 14 days . In this study, seven pesticides were tested that are commonly used in high-value tree crops in western North America . These included five insecticides used for management of key pests such as codling moth, Cydia pomonella, plus two pesticides used for management of bacterial or fungal pathogens in these crops. Each pesticide was tested for acute toxicity using laboratory bio-assays for each of three different exposure routes. Natural enemies, such as H. convergens, can experience topical exposure from direct applications of pesticides, residual exposure from a treated plant surface, and/or oral exposure from ingestion of a treated food source . The objectives of this study were 1) to assess the acute toxicity of selected pesticides on two life stages of H. convergensand 2) to determine the difference in acute toxicity through three different exposure routes: oral, residual, and topical. Adult H. convergens were obtained from a commercial source and stored for up to two months in a screened wooden cage at 6 °C in order to maintain them in a state of overwintering hibernation. To prevent desiccation, several 8.5 cm petri dishes lined with cotton wool were included in the cage and sprayed with distilled water on a weekly basis. Prior to use in the laboratory bio-assays, adults were removed from hibernation storage and placed into an incubator maintained at 22 °C, 60-70% RH, and a 16:8 h photoperiod. Males and females were identified and approximately 20 adults were placed into 96.1 ml plastic cups . Cups were lined with cotton wool soaked in a 1:1 honey-water solution and covered with a perforated plastic lid to allow for ventilation. After a period of 24 h under these conditions, adults were then used for bio-assays. To obtain larvae of H. convergens for the laboratory bio-assays, adults were removed from cold storage, sexed, and individual mating pairs were placed into 96.1 ml cups, capped with perforated plastic lids for ventilation, and assigned to the same incubator. These adults were reared on a diet of pea aphids, Acrythosiphum pisum, which are known to provide a high quality diet for H. convergens . 20 adult A. pisum were provided to each mating pair every day. After 3-4 days, males were removed from the cups and the remaining females were fed approximately 20 adult A. pisum per day. When a female oviposited, it was moved to a new plastic cup, and when eggs hatched, the first instar larvae were fed a diet of A. pisum, ad libitum, for a period of 48 h before being used in bio-assays. For each of the seven pesticides tested in comparison to a control, three variables were manipulated 1) life stage , 2) pesticide concentration , and 3) exposure route . For each bio-assay, a minimum of 28 replicates was used. In bio-assays using adults, the gender ratio was 1:1 . For all bio-assays, test insects were placed individually in 15 x 45 mm glass vial arenas, with cotton wool placed over vial openings to allow for ventilation. Insects were kept at 22 ⁰C, 60-70% RH, and 16:8 h photoperiod. Bioassays lasted for a period of 48 h, after which acute effects were recorded. Acute effects were estimated as the number of live and dead or moribund insects , where moribund insects were those that were unable to right themselves. For adult bio-assays, acute effects were recorded separately for each gender.