## 4 Discussion  
Insects rely on a variety of cues to identify acceptable host plants for mating and reproduction, including light intensity, color patterns, humidity, and nutritional value [22]. Volatile odor components of plants also play a pivotal role in host identification and acceptance in insects. For some insects, odor cues can be exploited to alter how insects perceive their landscape. Acceptable plant host odors, in combination with other environmental cues, can be attractive to insects while non-host odors can either repel insects or have no effect on behavior, allowing insects to navigate a complex landscape to find suitable hosts [23]. By using concentrated, non-host botanical odors, we attempted to compromise the ability of _D. citri_ to find and infest a preferred host (citrus). However, in subsequent field-testing, populations of _D. citri_ were significantly reduced by fir oil treatment as compared with the control plots. This occurred when the initial fir oil release rate per tree was doubled (8 dispensers/tree). Although it is impossible to directly compare between the field deployment rates of fir oil (4/tree vs. 8/tree) between the two years in this investigation, because they were conducted independently during two consecutive years with varying abiotic conditions, the population densities of _D. citri_ were quite similar between years (Figure 3A,B). Therefore, we postulate that the significant reduction in _D. citri_ populations observed during the second field experiment may have been the result of doubling the field deployment rate. However, the cost of the fir oil dispensers used in the field experiments is likely prohibitive (approximately $17/device or $136/tree for the effective treatment) as a stand-alone tool based on the results obtained. Although the higher deployment rate tested in 2014 reduced populations of _D. citri_ for two weeks, the cost of insecticides for similar efficacy per hectare is much lower [7]. To become economically viable, a less expensive and more efficient deployment method for fir oil application must be developed. For instance, slow-release nano-dispenser technology that can be sprayed directly onto trees with conventional equipment has been proven to effectively increase the efficiency of insecticide release for management of _D. citri_ in citrus with much lower (100 fold) rates of active ingredient needed [25]. Additionally, this tool may be more effective as part of a push-pull or stimulo-deterrent diversion strategy by using the repellent along with trap crop attractants [24]. Use of repellents and establishment of effective push-pull management of _D. citri_ will be inherently limited by the fact that _D. citri_ is an effective vector of CLas. Indeed, a single _D. citri_ adult is capable of inoculating this disease-causing pathogen into a tree [26]. Repellent, and push-pull systems may only decrease, but not control, HLB infection in commercial citrus production. In the present investigation, we did not measure the level of CLas infection before and after the treatment applications because it would have required much larger scale deployment and long-term measurements of infection in the field only possible by establishing a new grove without infection. Therefore these results only provide proof of concept of suppression of vector populations. Much more investigation is required with the entire pathosystem to determine the impact on spread of disease. Furthermore, response of _D. citri_ to citrus volatiles is influenced by previous experience with hosts (learning) [27]. It is therefore possible that the effectiveness of olfactory repellents may vary between groves depending on the specific citrus varieties cultivated among different regions. Additionally, it is known that _D. citri_ is more attracted toward CLas-infected citrus trees than uninfected ones [28]. It is therefore possible that repellents may be less effective on CLas-infected trees than on healthy trees.