Neonicotinoid-Induced Mortality of Diaphorina Citri (Hemiptera: Liviidae) is Affected by Route of Exposure.

The use of neonicotinoids in citrus (Rutaceae) has increased substantially to help manage the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), a vector of the devastating citrus disease, huanglongbing (HLB). In citrus pest management programs, neonicotinoids are most often applied to the soil as a drench and move through xylem channels from the roots into the foliage. We developed a novel assay to quantify the dose required to kill D. citri following ingestion and compare it with the dose required to kill by contact. The LC50 of the laboratory strain for ingestion of imidacloprid, thiamethoxam, and clothianidin were each approximately 10-fold greater than the respective LC50 by contact exposure. Four field populations were tested to validate comparative exposure of the laboratory strain to imidacloprid and determine the relative susceptibility of field populations to imidacloprid by exposure through ingestion and contact. The contact assay exhibited low (<10) RR50 values for the Vero Beach and Labelle populations when compared to the ingestion assay method. High (>10) RR50 values were observed for the Lake Placid and Lake Alfred populations using the contact and the ingestion method. This research demonstrates that the ingestion assay method described herein is more sensitive in detection of low-level resistance and should be the standard methodology used in monitoring for resistance to systemic insecticides for this global pest. We found D. citri populations with a lower than expected susceptibility to neonicotinoids in the field, which warrants the implementation of resistance management practices to preserve the utility of soil-applied neonicotinoids in citrus.

Testing Dose-Dependent Effects of the Nectar Alkaloid Anabasine on Trypanosome Parasite Loads in Adult Bumble Bees.

The impact of consuming biologically active compounds is often dose-dependent, where small quantities can be medicinal while larger doses are toxic. The consumption of plant secondary compounds can be toxic to herbivores in large doses, but can also improve survival in parasitized herbivores. In addition, recent studies have found that consuming nectar secondary compounds may decrease parasite loads in pollinators. However, the effect of compound dose on bee survival and parasite loads has not been assessed. To determine how secondary compound consumption affects survival and pathogen load in Bombus impatiens, we manipulated the presence of a common gut parasite, Crithidia bombi, and dietary concentration of anabasine, a nectar alkaloid produced by Nicotiana spp. using four concentrations naturally observed in floral nectar. We hypothesized that increased consumption of secondary compounds at concentrations found in nature would decrease survival of uninfected bees, but improve survival and ameliorate parasite loads in infected bees. We found medicinal effects of anabasine in infected bees; the high-anabasine diet decreased parasite loads and increased the probability of clearing the infection entirely. However, survival time was not affected by any level of anabasine concentration, or by interactive effects of anabasine concentration and infection. Crithidia infection reduced survival time by more than two days, but this effect was not significant. Our results support a medicinal role for anabasine at the highest concentration; moreover, we found no evidence for a survival-related cost of anabasine consumption across the concentration range found in nectar. Our results suggest that consuming anabasine at the higher levels of the natural range could reduce or clear pathogen loads without incurring costs for healthy bees.

Environmental fate of soil applied neonicotinoid insecticides in an irrigated potato agroecosystem.

Since 1995, neonicotinoid insecticides have been a critical component of arthropod management in potato, Solanum tuberosum L. Recent detections of neonicotinoids in groundwater have generated questions about the sources of these contaminants and the relative contribution from commodities in U.S. agriculture. Delivery of neonicotinoids to crops typically occurs as a seed or in-furrow treatment to manage early season insect herbivores. Applied in this way, these insecticides become systemically mobile in the plant and provide control of key pest species. An outcome of this project links these soil insecticide application strategies in crop plants with neonicotinoid contamination of water leaching from the application zone. In 2011 and 2012, our objectives were to document the temporal patterns of neonicotinoid leachate below the planting furrow following common insecticide delivery methods in potato. Leaching loss of thiamethoxam from potato was measured using pan lysimeters from three at-plant treatments and one foliar application treatment. Insecticide concentration in leachate was assessed for six consecutive months using liquid chromatography-tandem mass spectrometry. Findings from this study suggest leaching of neonicotinoids from potato may be greater following crop harvest in comparison to other times during the growing season. Furthermore, this study documented recycling of neonicotinoid insecticides from contaminated groundwater back onto the crop via high capacity irrigation wells. These results document interactions between cultivated potato, different neonicotinoid delivery methods, and the potential for subsurface water contamination via leaching.

Nicotine-like behavioral effects of the minor tobacco alkaloids nornicotine, anabasine, and anatabine in male rodents.

Tobacco use is associated with lethal diseases in an estimated 440,000 persons in the United States each year (Centers for Disease Control and Prevention, 2005). Successful smoking quit-rates are estimated at 5%-8%, even though a quarter of those attempts included use of smoking-cessation aids (Messer et al., 2008; Henningfield et al., 2009). Current projections are that 16% of the U.S. population-35 million people-will still smoke in 2025, thus more effective smoking-cessation aids are urgently needed (Pollock et al., 2009). The minor tobacco alkaloids may be promising candidates, but further research is necessary (Hoffman & Evans, 2013). Accordingly, we systematically evaluated the minor tobacco alkaloids nornicotine, anabasine, and anatabine using assays of behavioral tolerability, nicotine withdrawal, nicotine discrimination, and nicotine self-administration in male rodents. At doses that were well tolerated, all 3 minor alkaloids dose-dependently engendered robust substitution for a nicotine discriminative stimulus in mice (0.32 mg/kg, IP), and anabasine attenuated nicotine withdrawal. When the ED50 dose of each alkaloid was administered in combination with nicotine, the discriminative stimulus effects of nicotine were not enhanced by any of the alkaloids, and anatabine blunted nicotine's effects. In drug self-administration studies, only nornicotine was self-administered by rats that self-administered nicotine intravenously; anabasine and anatabine had no reinforcing effects. Moreover, prior administration of each of the minor tobacco alkaloids dose-dependently decreased nicotine self-administration. Collectively these results suggest that the minor tobacco alkaloids may substitute for the subjective effects of nicotine and attenuate withdrawal and craving without the abuse liability of nicotine.

Anabaseine is a potent agonist on muscle and neuronal alpha-bungarotoxin-sensitive nicotinic receptors.

We assessed the pharmacological activity of anabaseine, a toxin found in certain animal venoms, relative to nicotine and anabasine on a variety of vertebrate nicotinic receptors, using cultured cells, the Xenopus oocyte expression system, contractility assays with skeletal and smooth muscle strips containing nicotinic receptors and in vivo rat prostration assay involving direct injection into the lateral ventricle of the brain. Anabaseine stimulated every subtype of nicotinic receptor that was tested. It was the most potent frog skeletal muscle nicotinic receptor agonist. At higher concentrations it also blocked the BC3H1 (adult mouse) muscle type receptor ion channel. The affinities of the three nicotinoid compounds for rat brain membrane alpha-bungarotoxin binding sites and their potencies for stimulating Xenopus oocyte homomeric alpha7 receptors, expressed in terms of their active monocation concentrations, displayed the same rank order, anabaseine>anabasine> nicotine. Although the maximum currents generated by anabaseine and anabasine at alpha7 receptors were equivalent to that of acetylcholine, the maximum response to nicotine was only about 65% of the acetylcholine response. At alpha4-beta2 receptors the affinities and apparent efficacies of anabaseine and anabasine were much less than that of nicotine. Anabaseine, nicotine and anabasine were nearly equipotent on sympathetic (PC12) receptors, although parasympathetic (myenteric plexus) receptors were much more sensitive to anabaseine and nicotine but less sensitive to anabasine. These differences suggest that there may be different subunit combinations in these two autonomic nicotinic receptors. The preferential interactions of anabaseine, anabasine and nicotine with different receptor subtypes provides molecular clues that should be helpful in the design of selective nicotinic agonists.