Plants emit phytochemicals (volatile organic compounds) as a defensive mechanism to protect themselves from insect herbivores and parasites. One of the most common phytochemicals excreted in response to bark beetle infestations is $gamma$-terpinene. The goal of this investigation is to develop a simple, low-cost sensor that could be used to detect these volatile organic compounds and hence early stages of insect infestations. The sensor is composed of a silicon-based substrate with interdigitated electrodes and an active sensing film, poly3-hexylthiophene (P3HT). The effect of varying morphology of the P3HT film on sensor performance is investigated. Morphology of the P3HT thin film is varied by using different solvents and casting methods. The casting methods investigated are spin coating and drop casting. The solvents under investigation are chloroform and toluene. Chloroform is found to be a better solvent while spin coating is the better casting method. This paper also investigated the sensor mechanism of P3HT sensors. Atomic force microscopy images showed structures that are consistent with a model in which the resistance increase of the chemi-resistor sensors is due to adsorption of analyte within crystalline regions. This adsorption reduced the mobility of the charged carriers.