Comparing the Effects of Spring and Fall Tillage on Larval Populations of a Beneficial Insect

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Commodities

• Vegetables: tomatoes

Practices

• Crop Production: cover crops, no-till
• Education and Training: demonstration, extension, farmer to farmer, on-farm/ranch research, participatory research, workshop
• Natural Resources/Environment: biodiversity
• Pest Management: biological control, integrated pest management, mulches - living, mulching - vegetative, prevention, traps, weed ecology
• Production Systems: organic agriculture
• Soil Management: green manures, organic matter

Granivorous ground beetles (Coleoptera:Carabidae) are abundant and ubiquitous throughout agricultural systems, and reduce weed seedbank densities. While much research has focused on adult activity patterns and the conservation biocontrol services they provide, little is known about their biology and habitat requirements during larval stages, despite the fact that adult recruitment is determined by factors that promote larval survival. We present results of larval pitfall trap surveys of Harpalus pennsylvanicus, a common weed seed predator across North America, from two experiments examining its phenology and distribution across tillage and cover cropping gradients in organic tomato systems. Larvae emerged 4-6 weeks after the adult activity peak, and larval activity density was up to 10 times higher in no-till crop environments than in cultivated areas. Compared with adults, larvae are relatively immobile and vulnerable to disturbance; thus, weed management strategies that rely on intense cultivation may undermine the ecosystem services they provide. Growers must balance competing priorities of immediate weed suppression and future biological conrol.

Introduction:

Increasing demand for produce raised without chemical inputs creates an imperative for agroecologists to develop stronger, ecologically based, cultural tools for farmers to manage weeds effectively and improve crop yields. An ecological approach to weed management combines several tactics including tillage, cover cropping and conservation biological control as an alternative to a simplified herbicide program (Westerman et al. 2005). Of these tactics, tillage is most heavily relied upon in herbicide-free systems, although soil disturbance eliminates food and habitat resources for natural enemies, creating a potential trade-off between weed suppression and biocontrol. Invertebrate seed predators can substantially reduce seedbanks and affect weed population dynamics (Davis et al. 2003, Westerman et al. 2006), but suffer high mortality due to heavy tillage (Purvis and Fadl 2002, Holland and Reynolds 2003). Nonetheless, they are abundant and ubiquitous in cropping systems across varied pest management regimes and geographical regions (Bohan et al. 2011), justifying widespread application of cultural techniques to enhance seed predation services in a variety of cropping systems (Landis 2011).

Ground beetles (Coleoptera:Carabidae) are the dominant seed predator taxa in many agricultural systems, and numerous studies have tested the effects of cover and tillage systems on seed predation services by adults (Gallandt et al. 2005, Pullaro et al. 2006, Shearin et al. 2008, Meiss et al. 2010, Ward et al. 2011). Despite the wealth of data on carabid adults in cover crop systems, little is known about distributions of larvae in heterogeneous environments, largely because of their cryptic, belowground lifestyle and difficulty identifying immature stages (Lovei and Sunderland 1996). The sparse information collected on larval phenology and life history of seed-feeding carabids in crop environments is largely observational (Kirk 1972a,b,c) or lab-based (Jorgenson et al. 1997, Hartke et al. 1999, Saska 2005), whereas few experimental studies have documented larvae in field investigations (Luff 1980, Traugott 2001, Noordhuis et al. 2001 Purvis and Fadl 2002, Frank et al. 2010).

Harpalus pennsylvanicus Dej. is an important focal species, being the most common carabid seed predator in many agricultural systems across North America (Barney and Pass 1986 Davis and Liebman 2003, Lundgren et al. 2006, Ward et al. 2011, Fox et al. 2013), with a peak activity period that directly coincides with the senescence of many summer annual weeds (Kirk 1972a). Larvae (Fig 1) emerge in late autumn, and are identifiable by their enlarged heads and mandibles (Tomlin 1975), acuminate laciniae, unequal claws (Bosquet 2010), and a signature shape of the frontal margin (Kirk 1972a). They actively forage for about 4 weeks before overwintering in small burrows where they cache weed seeds (Kirk 1972c). Larvae are relatively immobile and vulnerable compared to adults, and most mortality occurs before pupation (Kromp 1999). Due to high larval mortality, adult recruitment and weed seed biological control may be largely driven by local habitat factors that promote larval success, and the effects of varying cultural treatments on larval densities warrant investigation.

Here, we report on larval activity patterns of H. pennsylvanicus within two separate field experiments. First, we compare varying cultural weed control strategies in a market tomato system including tillage, plastic mulch, living mulch and roller crimped rye mulch. Second, we compare tillage practices and variety of fall cover crop types. We hypothesized that no-till treatments, particularly those containing a killed fall cover crop would provide optimal in-crop overwintering habitat, due to enhanced larval food resources and insulating thatch. We also document predictable emergence times for H. pennsylvanicus larvae in the Midwest, and discuss cultural treatments that may enhance overwintering survival.

1) Describe the active period of H. pennsylvanicus in market vegetable systems
2) Evaluate larval activity density in varied tillage and cover crop treatment combinations
3) Compare adult and larval activity densities between several fall cover crop species