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Early-Season Dynamics of Alligatorweed Biological Control by Agasicles Hygrophila in Louisiana and Mississippi

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Early-Season Dynamics of Alligatorweed Biological Control by Agasicles Hygrophila in Louisiana and Mississippi J. Aquat. Plant Manage. 55: 89–95 Early-season dynamics of alligatorweed biological control by Agasicles hygrophila in Louisiana and Mississippi NATHAN E. HARMS AND JUDY F. SHEARER* ABSTRACT weed flea beetle population dynamics and associated plant impacts in the southern United States. Timing of management can be critical to long-term Key words: aquatic weed, biological control, herbivore– biological control of weeds, but may vary by location with plant interactions, management timing, population dy- arrival (or releases) of agents during times when conditions namics, seasonal ecology. are unsuitable for agent population development. We investigated, during spring and summer, the timing of INTRODUCTION occurrence and intensity of damage (percentage of leaf area consumed) caused by the biological control agent Timing of pest management is important for success in alligatorweed flea beetle (Agasicles hygrophila Selman and both agricultural and natural systems (Paynter 2003, Carisse Vogt) on alligatorweed [Alternanthera philoxeroides (Mart.) and Rolland 2004). In integrated pest management (IPM) Griseb.] at two sites in southern Louisiana and two sites in programs of weeds, application of management (e.g., northern Louisiana/central Mississippi. Alligatorweed flea herbicides, prescribed burns) may be timed to maximize beetle population peaks were documented at southern impact on the target weed population by exploiting sites in May (7.6 6 3.45 insects stemÀ1) and June (3.35 6 particularly vulnerable host phenological stages (McAllister 0.25 insects stemÀ1). Mean leaf damage at southern sites and Haderlie 1985, Pesacreta and Luu 1988, Luu and was 21 6 2% and maximum leaf damage was 76%, which Getsinger 1990, Owens and Madsen 1998) or coincide with coincided with the first peak. At northern sites, where periods of low natural enemy activity, abundance, or annual recolonization must occur because of overwinter- susceptibility (Chi 1990, Newman et al. 1998, Mudge et al. ing limitations, alligatorweed flea beetles first appeared 2013). In biological control, timing of agent releases may later in the season (June) and insect populations never occur during a period thought to be conducive to formed a distinct peak or reached similar abundances to establishment, such as when weather is mild or the chance the southern sites. Mean leaf damage at northern sites was for disturbance is low (Norris et al. 2002, Van Driesche et al. 7 6 3% and maximum leaf damage was 25%. Alligatorweed 2008). Some agents benefit from releases during periods in northern sites, subjected to less herbivory, maintained a when their host is in the appropriate phenological stage, positive relative growth rate (RGR) of 0.012 6 0.016 (mm2 such as during flowering or seed production (e.g., multiple leaf area [m2 water surface area]À1 dayÀ1) during the study agents of yellow starthistle, Centaurea solstitialis L.; Wilson et period, whereas alligatorweed in southern sites, subjected al. 2003). However, to date, most discussion about optimal to early-season damage, had a RGR of À0.027 6 0.035. release methods centers on the number of releases and Overall, insect abundance was positively associated with quantities of released agents rather than timing of releases percentage of leaf damage (R2 ¼ 0.56) and negatively (Memmott et al. 1996, Memmott 1998, Grevstad 1999, Shea associated with RGR (R2 ¼ 0.58). Our data support the and Possingham 2000, Grevstad et al. 2011). notion that an early-season herbivore peak, associated with Although the timing of impacts may be important for near total defoliation, was responsible for the reduced efficacy of biological control, there are few examples in the RGR and suppression of alligatorweed at sites within the literature that discuss the effects of damage timing on the southern range of the alligatorweed flea beetle. Northern host or the importance of release timing for establishment sites experienced delayed and lower impact from the of the agent. Because phenological events are often related insects. Although previous authors have suggested the to seasonal variables (resource availability, temperature, importance of this early-season phenomenon, this study rainfall, photoperiod, etc.), understanding the importance represents the first quantification of early-season alligator- of damage timing may aid biological control practitioners in developing release or monitoring schedules. A prime *First author: Research Biologist, U.S. Army Engineer Research and example in which the importance of seasonal impacts has Development Center, Vicksburg, MS 39180 and Graduate Student, been suggested is that of alligatorweed [Alternanthera Department of Biological Sciences, Louisiana State University, Baton philoxeroides (Mart.) Griseb.] (Amaranthaceae) and its prima- Rouge, LA 70810. Second author: Research Plant Pathologist, U.S. Army ry biological control agent, the alligatorweed flea beetle Engineer Research and Development Center, Vicksburg, MS 39180. Corresponding author’s E-mail: [email protected]. Re- (Agasicles hygrophila Selman and Vogt) (Coleoptera: Chrys- ceived for publication August 17, 2016 and in revised form March 2, omelidae) (Coulson 1977, Spencer and Coulson 1976, Vogt 2017. et al. 1992). J. Aquat. Plant Manage. 55: 2017 89 Alligatorweed is a semiaquatic perennial weed species data obtained from an online database (see Weather present throughout the southeastern United States (USDA Underground 2016), then compared to published estimates NRCS 2015). A biological control program for alligatorweed of climatic limits of the alligatorweed flea beetle. Northern was initiated in the 1950s and ‘60s and produced three host- sites had a mean daily winter temperature of 8.6 C whereas specific agents, of which the alligatorweed flea beetle southern sites had a mean daily winter temperature of 11.6 received the most attention because of the severe and rapid C during winter 2014 to 2015. This temperature informa- defoliation caused by the agent (Buckingham 1996). tion, combined with past qualitative observations of Although alligatorweed is found from the Gulf Coast north alligatorweed flea beetle presence (unpubl. data) at these into Arkansas (USDA NRCS), it is thought that the sites, met our selection criteria. Northern sites were Lake St. alligatorweed flea beetle only controls infestations in areas Joseph, Louisiana (328403900 N, 9181305800 W) and Openwood where average winter temperatures remain above 10 to 11.1 Lake, Mississippi (3282304600 N, 9084703700 W) and southern C (Coulson 1977, Julien et al. 1995) and herbivore sites were Maurepas Wildlife Management Area, Louisiana populations are able to overwinter and peak early in the (3088 05900 N, 9084802400 W) and Blind River, Louisiana subsequent growing season (Spencer and Coulson 1976, (308504200 N, 9084604300 W) (Figure 1). Sites consisted of lakes Coulson 1977). In areas outside the overwintering climate of (Openwood, St. Joseph), a wetland (Maurepas), and a river the alligatorweed flea beetle, insects must immigrate (Blind River). Alligatorweed abundance and coverage at annually to recolonize alligatorweed (Vogt et. al 1992). It sites was visually assessed prior to the study and determined has mostly been considered that in these outer areas two to be similar among sites. Within sites, we limited our study other introduced agents (Arcola malloi Pastrana and Amyno- areas to approximately 5-m2 patches. Samples were collect- thrips andersoni O’Neill) are tolerant to low temperatures and ed within 1.5 m of the shore and in water , 1m deep. provide more control than the alligatorweed flea beetle Sites were visited every 3 wk from March 2015 until the (Spencer and Coulson 1976, Vogt et. al 1992). This system presence of the alligatorweed flea beetle was documented to offers the opportunity to study an agent (alligatorweed flea assess abundance of flea beetles and associated herbivore beetle) in which 1) timing of attack has been suggested to be impact on alligatorweed. Thereafter, sites were visited at 2- important for control and 2) variable attack timing is wk intervals (6 2 d) with the exception of July and August displayed because of overwintering limits and the need to recolonize sites in its northern range (Spencer and Coulson sampling which was done at 3wk intervals for both southern 1976, Coulson 1977, Julien et al. 1995, Stewart et al. 1999). and northern sites. Our study period lasted from March Whether variability of control by the flea beetle is primarily until September at southern sites and March until August at a function of overwintering, phenology of alligatorweed in northern sites. We chose these study dates because our different locations, or a combination of the two is not fully intention was to capture early growing-season dynamics. understood. Sampling intervals corresponded with first insect presence, The objective of the current study was to quantify the so northern and southern sample schedules were different early-season timing and intensity of feeding by the alligator- for most of the study. weed flea beetle at sites in southern Louisiana and northern Louisiana/central Mississippi. Our prediction was that Alligatorweed flea beetle population dynamics alligatorweed–alligatorweed flea beetle dynamics would follow a pattern of early-season, high-intensity damage At each census date, 10 plants per site were examined in (insect populations would peak once with corresponding situ and the number of larvae and adult alligatorweed flea intensity of plant damage) at southern sites
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