J Insect Behav (2013) 26:109–119 DOI 10.1007/s10905-012-9343-7 Phototaxis, Host Cues, and Host-Plant Finding in a Monophagous Weevil, Rhinoncomimus latipes Jeffrey R. Smith & Judith Hough-Goldstein Revised: 21 May 2012 /Accepted: 24 May 2012 / Published online: 5 June 2012 # Springer Science+Business Media, LLC 2012 Abstract Rhinoncomimus latipes is a monophagous weevil used as a biological control agent for Persicaria perfoliata in the eastern United States. Density of adult R. latipes and resulting feeding damage has been shown to be higher in the sun than in the shade. This study aimed to determine whether phototaxis, sensitivity to enhanced host cues from healthier sun-grown plants, or a combination is driving this behavior by the weevil. A series of greenhouse choice tests between various combi- nations of plant and light conditions showed that R. latipes is positively phototactic, responsive to host cues, and preferentially attracted to sun-grown plants over shade- grown plants. From our experiments, we hypothesize two phases of dispersal and host finding in R. latipes. The initial stage is controlled primarily by phototaxis, whereas the later stage is controlled jointly by host cues and light conditions. Keywords Host-plant finding . phototaxis . host plant cues . Curculionidae . Rhinoncomimus latipes . Persicaria perfoliata Introduction A major goal in the study of plant-herbivore interactions is to determine the mech- anisms governing insect host-plant finding and selection. It is generally accepted that host-plant selection is a catenary process consisting of a sequence of behavioral phases or “reaction chains” (Tinbergin 1951;Atkins1980; Schoonhoven et al. 2005). For example, Finch and Collier (2000) suggested that host-plant selection in pest insects of cruciferous plants involves three stages, the first governed by volatile host plant chemicals, the second by visual stimuli, and the final stage by chemicals on or in the leaf surface. The specific cues used in host finding are highly variable among insect families, and even among species within a family (Schoonhoven et al. 2005). J. R. Smith : J. Hough-Goldstein (*) Department of Entomology and Wildlife Ecology, University of Delaware, 531 S. College Ave, Newark, DE 19716-2160, USA e-mail: [email protected] 110 J Insect Behav (2013) 26:109–119 Our goal here was to examine the factors influencing the host finding behavior of a monophagous weevil, Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae). Rhinoncomimus latipes is a small black weevil, approximately 2 mm long, that feeds and reproduces exclusively on Persicaria perfoliata (L.) H. Gross (Colpetzer et al. 2004; Korotyaev 2006; Frye et al. 2010). In 2004, R. latipes was introduced in the Mid-Atlantic States as a biological control agent for P. perfoliata (Hough-Goldstein et al. 2008, 2012; Lake et al. 2011). Persicaria perfoliata, or mile-a-minute weed, is an annual, invasive vine introduced from Asia to the United States in the 1930s (Moul 1948). The plant often grows over and outcompetes native vegetation and is now found in the eastern United States from Massachusetts to North Carolina and westward to Ohio and West Virginia (Oliver 1996; Kumar and DiTommaso 2005; EDDMapS 2012). Field observations and experiments have shown that density of adult R. latipes and resulting feeding damage to mile-a-minute weed is higher in full sun compared to shaded areas (Hough-Goldstein and LaCoss 2012). Mile-a-minute grows best in full sun, but will tolerate some shade (Mountain 1989; Hough-Goldstein 2008; Hough- Goldstein et al. 2008). Thus, the observed preference of weevils for full-sun areas may be due to an attraction to high light conditions, higher quality sun-grown host plants, or both. Although they demonstrated that weevil density and feeding damage was higher in full sun areas, Hough-Goldstein and LaCoss (2012) did not segregate light conditions and resultant host plant quality to determine their importance in influencing host-plant finding. Orientation using the sun, through phototaxis or by maintaining straight-line movement at a fixed angle to the sun (the light-compass reaction), is common in insect dispersal flights (Johnson 1963; Atkins 1980; Jermy et al. 1988). Positive phototaxis has been shown for other curculionids (Hollingsworth et al. 1964; Cross et al. 1976; Meyer 1976; Kjaer-Pedersen 1992). However, in these species, positive phototaxis plays a secondary role in host finding. For example, while the boll weevil (Anthonomus grandis grandis [Boheman]) is attracted to light, migration by boll weevils was found to be determined primarily by attraction to plant odors (Mitchell and Taft 1966). Positive phototaxis is also important in host finding by the cabbage seedpod weevil (Ceutorhynchus assimilis [Paykull]), but it plays a secondary role to chemical cues (Kjaer-Pedersen 1992). Similarly, positive phototaxis has been shown in the alfalfa weevil (Hypera postica [Gyllenhal]) in laboratory settings, but time of day was found to be the driving force behind this weevil’s dispersal activity (Prokopy and Gyrisco 1965; Meyer 1976). Enhanced host plant cues also may contribute to increased R. latipes activity on mile-a-minute grown in full sun conditions. Growth in full sun has been shown to increase biomass of mile-a-minute plants (Hough-Goldstein 2008). The plant vigor hypothesis predicts that larger, more vigorously growing plants will be more attrac- tive to insect herbivores than slower growing shaded plants (Price 1991; Cornelissen et al. 2008). Attraction to host plant cues, both chemical and visual, has been shown for many curculionids. For example, attraction to host plant volatiles has been shown for boll weevils (McKibben et al. 1977), pepper weevils, Anthonomus eugenii Cano (Addesso and McAuslane 2009), and cranberry weevils, Anthonomus musculus Say (Szendreietal.2009). Björklund et al. (2005) found that walking pine weevils (Hylobius abietis L.) responded to both odor and visual host stimuli, and traps with J Insect Behav (2013) 26:109–119 111 both types of stimuli caught more weevils than either stimulus alone. Reeves and Lorch (2011) reported that aquatic milfoil weevils, Euhrychiopsis lecontei Dietz, may use chemical cues if plants are not initially seen, but use vision at closer distances to precisely locate their host plants. In order to more fully understand the mechanisms for dispersal and host-plant finding in R. latipes, greenhouse trials were conducted to test weevil preference for combinations of light conditions and host plant quality. Our objectives were to determine if the weevil is positively phototactic; if sun-grown plants are more attractive to weevils than shade-grown plants; and if both are true, what is their relative importance in influencing weevil host finding behavior? Two sets of experi- ments were designed, one to test weevil behavior when they lacked a suitable host plant, and the other to determine whether weevils would abandon one host plant and light condition combination for a different set of conditions. Materials and Methods Eight greenhouse trials were conducted during the summer of 2011. Each trial had five replicates, each consisting of two fine mesh, white, tent-like cages measuring 61×61× 61 cm (BugDorm 3, Bioquip Products, Rancho Dominguez, CA). The two cages were connected by a tube constructed from metal window screening and thin metal wire, supporting overlapping cage sleeves that created a passage approximately 18 cm in diameter and 20 cm long. Four beige ceramic tiles were used to create a floor for each cage. For shade treatments, greenhouse blackout cloth was cut into panels and sewn into fitted covers that blocked the front, rear, top, and side panels of the cages. For each trial, 100 weevils were collected from P. perfoliata plants at White Clay Creek State Park, Newark, DE the morning of the experiment. The weevils were separated into either five petri dishes with 20 weevils each (for trials where weevils were to be released in the central tube) or ten petri dishes with ten weevils each (for trials where weevils were to be released in each of the two linked cages). Weevils were not sexed, but were allocated at random to the replicates and treatments. The potted P. perfoliata plants used for the trials were grown from seed in a greenhouse. The seeds were collected from White Clay Creek State Park, Newark, DE on 23 October, 2010, and kept at room temperature for 3 weeks. On 12 November, 2010, the mile-a-minute seeds were combined with moist peat moss in a self-sealing plastic bag and put in a refrigerator at approximately 4 °C where they remained until they were planted. Seeds were planted 3 to 5 weeks prior to the start of each trial. Seeds were planted in Redi-Earth potting soil (Grace-Sierra, Milpitas, CA) in 7.5 cm square pots, and placed in a mist room. When plants were approximately one and a half weeks old, the seedlings were transplanted to 15 cm diameter round pots with Pro-Mix BX soil (Premier Tech Horticulture, Quakertown, PA) and moved to a green- house room. At this time plants designated as shade-grown plants were placed on a metal greenhouse bench under a frame draped with blackout cloth with an open north face and wire mesh bottom to allow air ventilation and indirect sunlight. Shade-grown plants were grown under these conditions for between 2 and 4 weeks. All other plants were grown in full sun. Each individual trial used plants of the same age. Plants were watered daily at the base of the plants, immediately after weevils were counted. 112 J Insect Behav (2013) 26:109–119 The trials conducted and their conditions are summarized in Table 1. Trials were conducted in two separate rooms of the greenhouse (assigned randomly) to allow two trials to be run simultaneously. Each pair of cages was oriented in an east–west direction and the assignment of Side A and Side B conditions was randomized prior to each trial; however each set of conditions had either two west and three east or three west and two east cages to minimize any possible differences between the east and west cage.