Quantifying Western Flower Thrips (Frankliniella occidentalis Pergande) (Thysanoptera: Thripidae) Damage on Ivy Geranium (Pelargonium peltatum (L.) L'Her ex Ait.) (Geraniaceae Juss.) with Adobe^®Photoshop™ and Scion^®Image Software

Western flower thrips (WFT), Frankliniella occidentalis (Pergande) Thysanoptera: Thripidae, is a major insect pest of many herbaceous greenhouse crops because of damage caused by its feeding and its role as a vector of tospoviruses (Oetting et al., 1993). Visual damage from WPT feeding includes lesions or spots on foliage, leaf distortion, and petal discoloration, all of which reduce the aesthetic quality and marketability of ornamental plants (Lewis, 1973). Symptoms of direct crop feeding by WFT on ivy geranium {Pelargonium peltatum (L.) L'Her ex Ait.) (Geraniaceae), an important spring bedding plant, include round, yellow-tan to dark brown spots on leaves. Based on our experience, ivy geranium is asymptomatic for tospoviruses; therefore, direct feeding damage is the immediate problem for this crop. Accurate damage assessment is necessary for determining economic and action thresholds and evaluating the efficacy of various control tactics in Integrated Pest Management programs. However, it is difficult to objectively quantify feeding damage caused by arthropods such as WTT. Visual damage assessment via subjective ratings can be influenced by many factors besides those that can be solely attributed to pest feeding. Some of these factors include plant size, leaf color, and the ability to thoroughly detect all damage. Furthermore, these subjective ratings are sometimes not precise enough to distinguish between treatment effects. Specific types of aesthetic damage can be quantified in a consistently reproducible way by using readily-available equipment and computer software. For example, chlorophyll meters (e.g., Ma et al. [1998] for aphid on sorghum) and digital image processing software (e.g., Warnock [2000] for thrips on impatiens and Stevens et al. [2000] for root-feeding chironomid midge larvae on rice) can be used to quantify direct damage caused by arthropod feeding. As part of a study to investigate WFT population response to different fertilization regimes in ivy geranium (Chen, 2003), we developed an image analysis method to quantify WFT feeding damage as Percent Damaged Area (PDA). In this paper we describe this method and compare results derived from it with a subjective visual rating of plant quality as an accurate reflection of WTT population levels. We present our method as a step by-step guide to assist others in quantifying arthropod feeding damage.