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Cover crop seed preference of four common weed seed predators

Invertebrate seed predators (ISPs) are an important component of agroecosystems that help regulate weed populations. Previous research has shown that ISPs’ seed preference depends on the plant and ISP species. Although numerous studies have quantified weed seed losses from ISPs, limited research has been conducted on the potential for ISPs to consume cover crop seeds. Cover crops are sometimes broadcast seeded, and because seeds are left on the soil surface, they are susceptible to ISPs. We hypothesized that (1) ISPs will consume cover crop seeds to the same extent as weed seeds, (2) seed preference will vary by plant and ISP species, and (3) seed consumption will be influenced by seed morphology and nutritional characteristics. We conducted seed preference trials with four common ISPs [Pennsylvania dingy ground beetle ( Harpalus pensylvanicus ), common black ground beetle ( Pterostichus melanarius ), Allard’s ground cricket ( Allonemobius allardi ) and fall field cricket ( Gryllus pennsylvanicus )] in laboratory no choice and choice feeding assays. We compared seed predation of ten commonly used cover crop species [barley ( Hordeum vulgare ), annual ryegrass ( Lolium multiflorum ), pearl millet ( Pennisetum glaucum ), forage radish ( Raphanus sativus ), cereal rye ( Secale cereale ), white mustard ( Sinapis alba ), crimson clover ( Trifolium incarnatum ), red clover ( Trifolium pratense ), triticale (× Triticosecale ) and hairy vetch ( Vicia villosa )] and three weed species [velvetleaf ( Abutilon theophrasti ), common ragweed ( Ambrosia artemisiifolia ) and giant foxtail ( Setaria faberi )]. All four ISPs readily consumed cover crop seeds ( P < 0.05), but cover crops with hard seed coats and seed hulls such as hairy vetch and barley were less preferred. Our results suggest that farmers should select cover crop species that are avoided by ISPs if they plan on broadcasting the seed, such as with aerial interseeding.

Keywords

Introduction

Invertebrate seed predators (ISPs) such as carabid beetles (Coleoptera: Carabidae) and crickets (Orthoptera: Gryllidae) are key drivers of weed seed destruction (Westerman et al., Reference Westerman, Wes, Kropff and Van der Werf 2003; Kulkarni et al., Reference Kulkarni, Dosdall, Spence and Willenborg 2015a) and the subsequent reduction of weed emergence (White et al., Reference White, Renner, Menalled and Landis 2007; Kulkarni et al., Reference Kulkarni, Dosdall and Willenborg 2015b). ISPs can consume seeds before seed dispersal (pre-dispersal predation) or once the seeds have been shed and are on the soil surface (post-dispersal). Multiple generalist seed predators contribute to post-dispersal predation, which is a form of biological control that can reduce weed populations (Crawley, Reference Crawley and Fenner 1992). Previous research has shown that landscape context (e.g., proximity to field edge) and farm-management practices (e.g., reduced tillage practices) both influence ISP activity density and weed seed predation rates (Trichard et al., Reference Trichard, Alignier, Biju-Duval and Petit 2013; Petit et al., Reference Petit, Trichard, Biju-Duval, McLaughlin and Bohan 2017).

In addition to field research, laboratory feeding assays have been used to determine weed seed preference by ISPs, and results determined in the laboratory often translate directly to field preference (Honek et al., Reference Honek, Saska and Martinkova 2006; Petit et al., Reference Petit, Boursault and Bohan 2014; Ward et al., Reference Ward, Ryan, Curran and Law 2014). In general, constraints such as ISP body size and mouthpart strength determine which seeds can be consumed (Honek et al., Reference Honek, Martinkova, Saska and Pekar 2007; Lundgren, Reference Lundgren 2009). Several species of carabid beetles such as Amara aenea DeGeer (Ward et al., Reference Ward, Ryan, Curran, Barbercheck and Mortensen 2011), Anisodactylus sanctaecrucis Fabricius (White et al., Reference White, Renner, Menalled and Landis 2007), Harpalus affinis Schrankl (Honek et al., Reference Honek, Saska and Martinkova 2006) and crickets such as Gryllus pennsylvanicus Burmeister (Carmona et al., Reference Carmona, Menalled and Landis 1999) and Teleogryllus emma Ohmachi and Matsuura (Ichihara et al., Reference Ichihara, Inagaki, Matsuno, Saiki, Yamashita and Sawada 2012) have been evaluated for seed preference. However, the Pennsylvania dingy ground beetle (Harpalus pensylvanicus DeGeer) has been the model ISP species for seed preference work. Lundgren and Rosentrater ( Reference Lundgren and Rosentrater 2007) found H. pensylvanicus preferred species with small, dense seeds with hard seed coats [e.g., redroot pigweed (Amaranthus retroflexus L.); mass

0.33 mg; seed coat strength

47.81 MPa] compared with large seeds [e.g., ivyleaf morning glory (Ipomoea hederacea L.); mass

24.65 mg; seed coat strength

3.95 MPa]. Ward et al. ( Reference Ward, Ryan, Curran and Law 2014) found H. pensylvanicus consumed 71% of presented giant foxtail (Setaria faberi Herrm.) seeds compared with <1% of velvetleaf (Abutilon theophrasti Medik) seeds.

Though the material properties of seeds play a large role in determining seed preference, nutrient regulation has been proposed as another major factor of insect food selectivity (Behmer, Reference Behmer 2009). When multiple food sources are available, insects select those that optimize ratios of macronutrients such as lipids, carbohydrates and proteins. These nutrient ratios determine the health, development and evolutionary fitness of individual insects (Simpson et al., Reference Simpson, Clissold, Lihoreau, Ponton, Wilder and Raubenheimer 2015). Jensen et al. ( Reference Jensen, Mayntz, Toft, Clissold, Hunt, Raubenheimer and Simpson 2012) determined that the predatory carabid Anchomenus dorsalis (Pontoppidan) selected food to optimize a lipid-to-protein ratio of 0.36, which maximized the number of eggs a female could lay. Likewise, Harrison et al. ( Reference Harrison, Raubenheimer, Simpson, Godin and Bertram 2014) found that spring field crickets (Gryllus veletis Alexander and Bigelow) consume food sources that give a protein-to-carbohydrate ratio of 1 to 4.1 for males and 1 to 2.3 for females. Previous research has also shown how insects can maintain their required nutrient ratios by switching between nutritionally suboptimal but complementary foods (Behmer, Reference Behmer 2009).

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Laboratory seed preference of ISPs is generally studied in no choice and choice trials, but quantified in many different ways. In no choice trials, seed from a single plant species is offered to a captive insect, whereas seed from multiple plant species is offered in choice trials. In no choice trials, Lundgren and Rosentrater ( Reference Lundgren and Rosentrater 2007) presented 0.25 g of seed for each plant species they tested, whereas Ward et al. ( Reference Ward, Ryan, Curran and Law 2014) presented nine seeds regardless of seed size. In their choice trials, Honek et al. ( Reference Honek, Martinkova, Saska and Pekar 2007) presented 15 seeds of large-seeded species [e.g., great burdock (Arctium lappa L.)] and 30 seeds of small-seeded species [e.g., common lambsquarters (Chenopodium album L.)], while Ward et al. ( Reference Ward, Ryan, Curran and Law 2014) standardized by seed number in their choice trials and offered three seeds each of velvetleaf, giant foxtail and common lambsquarters. Quantifying consumed seeds is also variable among researchers. Some authors consider a seed consumed when >50% has been destroyed (e.g., Honek et al., Reference Honek, Martinkova, Saska and Pekar 2007), and others consider a seed consumed if the seed coat is cracked and part of the endosperm is damaged (e.g., Carmona et al., Reference Carmona, Menalled and Landis 1999). Although methods vary in the literature, most ISP research has focused on assessing the ecosystem service of weed seed destruction and characterizing ISP ecology (Kulkarni et al., Reference Kulkarni, Dosdall, Spence and Willenborg 2015a).

Relatively little research has been conducted on the negative effects of weed seed predators such as carabid beetles and crickets on crop seeds. One group of crops for which seed predation might be particularly relevant is cover crops. Cover crops are increasingly used in the USA to improve soil health, suppress weeds and provide other ecosystem services (Singer, Reference Singer 2008; Schipanski et al., Reference Schipanski, Barbercheck, Douglas, Finney, Haider, Kaye and White 2014; SARE, 2016; Wayman et al., Reference Wayman, Kissing Kucek, Mirsky, Ackroyd, Cordeau and Ryan 2016). In the Northeast USA, a large portion, if not most, of the land that is cover cropped is in a crop rotation with corn and soybean and the cover crops are seeded after corn and soybean are harvested in the fall. However, cover crops are also used extensively in small grain, forage and specialty crop production. For example, some farmers ‘frost seed’ red clover into wheat in early spring, while other farmers seed sudangrass and buckwheat in mid-summer between early and late season vegetables (Mohler and Johnson, Reference Mohler and Johnson 2009). Interseeding cover crops into corn and soybean in mid- to late summer is also becoming more popular (Belfry and Van Eerd, Reference Belfry and Van Eerd 2016; Blanco-Canqui et al., Reference Blanco-Canqui, Sindelar, Wortmann and Kreikemeier 2017; Curran et al., Reference Curran, Hoover, Mirsky, Roth, Ryan, Ackroyd, Wallace, Dempsey and Pelzer 2018; Youngerman et al., Reference Youngerman, DiTommaso, Curran, Mirsky and Ryan 2018). In addition to drill seeding, cover crops are seeded by broadcasting with a fertilizer spreader or by aerial seeding with airplanes (Fisher et al., Reference Fisher, Momen and Kratochvil 2011; SARE 2016). The method used by farmers often varies with farming system (e.g., drill seeding is more common in grain production whereas broadcast seeding is more common in vegetable production) and farm type (e.g., aerial seeding is more common on larger compared with smaller farms). Seeds on the soil surface are much more likely to be consumed by ISPs (White et al., Reference White, Renner, Menalled and Landis 2007; Kulkarni et al., Reference Kulkarni, Dosdall and Willenborg 2015b), so cover crop seeds may be susceptible to ISP predation when they are broadcast. Wilson et al. ( Reference Wilson, Allan and Baker 2014) reported losses of 48–98% of aerially seeded cover crop seeds 1 week after seeding and hypothesized that seed predators (e.g., by insects, rodents and birds) were responsible for these losses.

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Beyond seed placement (e.g., in furrow covered with soil compared with on the soil surface), the level of seed predation of cover crops will likely vary depending on a number of factors including the presence of seed predators, their activity levels and their seed consumption preferences. Given that ISP seed preference is likely based on seed size and seed coat strength, cover crops that are similar to preferred weed seeds could also be consumed. The goal of this study was to assess ISP seed preference for common cover crop species, and to compare their relative preference to common weeds that are known targets of ISPs. To quantify ISP preferences, a series of no choice and choice laboratory seed preference trials were conducted. We hypothesized that (1) ISPs will consume cover crop seeds to the same extent as weed seeds, (2) seed preference will vary by plant and ISP species, and (3) seed consumption will be influenced by seed morphology and nutritional characteristics.

Materials and methods

Laboratory experiments

Ten cover crop species and three weed species were used to test seed preference of four weed seed predators (Table 1). These plant species were selected because: (1) the cover crop species are used by farmers and the weed species commonly occur in cropping systems in the Northeast, and (2) they vary in several key seed traits including weight, size, compressive yield strength and oil and protein content. Weed seeds were collected from the Musgrave Research Farm in Aurora, NY (42°73′N, 76°63′W) in the fall of 2015 and kept in cold storage for the winter. Cover crop seeds were purchased from several seed companies including King’s ArgiSeeds (Lancaster, PA, USA) and Lakeview Organic Grain (Penn Yan, NY, USA). Germination of cover crop and weed seeds was tested prior to the preference trial to verify their viability, and all plant species had at least 60% germination. Dry seeds were used in all trials. Because size can influence seed viability in some species (Stanton, Reference Stanton 1984), which may affect ISP preference (Ward et al., Reference Ward, Ryan, Curran and Law 2014), within-species seed sizes were kept as similar as possible through visual inspection and careful seed selection for each experiment.

A Rainbow of Cannabis: What different colors tell us about a strain

Long before cannabinoid testing began, customers made their choices mostly based on smell and color. After that, taste and of course potency. Even today, with all the knowledge we have available when making our weed purchases, there is nothing more appealing than a jar of colorful buds.

So where do those amazing bud colors come from? In a word, anthocyanins.

Anthocyanins belong to a parent class of molecules called flavonoids and are synthesized via the phenylpropanoid pathway. They occur in all tissues of higher plants, including leaves, stems, roots, flowers and fruits. They are odorless and moderately astringent, and can appear red, blue or purple according to their pH.

Think of tree leaves in fall. As the temperature drops, they change from green to red, orange, yellow or gold. The same is true for cannabis: once the green fades, the colors appear.

Temperature plays a vital role too. Chlorophyll is the plant component vital to photosynthesis and cooler temperatures inhibit chlorophyll production. For cannabis, depending on the lineage of the strain, certain colors can appear when you drop the temperature and the light cycle shortens, simulating a change in season.

The ideal range to grow cannabis is a pH of 5.5-6.5, however, during flowering, you can lean one way or another to enhance or minimize certain anthocyanins to bring out certain colors. Additionally, different strains of cannabis come with different cannabinoid ratios, flavor profiles and anthocyanins.

Anthocyanins can be present in plant tissues, leaves and flowers. Sometimes, they even present in the trichomes themselves, which are the hairs or fine outgrowths or appendages on plants. They also can attract pollinating creatures like butterflies and bees, while deterring pests.

A common misconception is that strains with bold color are more potent. The truth is that color has nothing to do with potency.

In fact, buds that have been grown and harvested to their maximum potential can be so covered with trichomes that they almost appear white. Trichomes are packed with cannabinoids and terpenes so these flowers can be quite potent. White Widow or White Rhino are two strains with a propensity to become encrusted with trichomes.

However, anthocyanins are known to act as powerful antioxidants and are also thought to have analgesic, anti-inflammatory and neuroprotective properties. So while the presence of anthocyanins doesn’t change the potency of cannabinoids like THC levels, it might give the strain an added entourage effect on health.

Other plants high in these molecules include blackberries, blueberries, raspberries, goji berries, blood oranges, and cranberries. Cranberries especially are touted for their powerful antioxidant properties, due to anthocyanins.

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Strong and vibrant color also indicates that your cannabis is at its peak freshness, taste, and potency. If you are consuming your colorful bud orally in tinctures, oils, edibles, or capsules, you are also getting the nutritional benefits of carotenoids, anthocyanins and other flavonoids. But what about the actual color? Do different colors provide different effects? Your cannabis can come in a rainbow of colors, and yes, different shades can determine varied effects, taste, and even smell.

ROYGBIV – Taste the Rainbow

Red: Red hairs show up more frequently, but red buds and leaves are not nearly as common. Red marijuana is a genetically selected plant and is created by combining three kinds of cannabis – ruderalis, indica and sativa. It’s an extremely rare plant. The “Red” is best known for its hybrid effects, which simultaneously offer great relief to the body and the mind and is known for its intoxicating aroma.

There are also some lovely shades of pink, such as Predator Pink or Pink Kush, with actual pink and fuchsia hues. These are Indica-dominant hybrids, with powerful body-focused effects known to eliminate pain, insomnia, and appetite loss.

Orange: Carotenoids give cannabis those citrusy hues of yellow, gold, and orange. To get these colors, more alkaline conditions are required. If these colors are predominant in the plant, they will come out naturally as the flowering phase comes to an end.

Orange will mostly affect the hairs and buds, such as Agent Orange, Orange Crush, or Tangerine Dream. These strains are known for their aromas of fresh-cut citrus and are excellent mood enhancers.

Yellow: Carotenoids produce the warm hues found in many plants including carrots, squash, sweet potatoes, pumpkins, and tomatoes. Beta-carotene, lycopene, lutein, and zeaxanthin are among the more than 750 carotenoids have been identified and can be converted by the body into Vitamin A. Many flavonoids are also yellow and can influence the colors of cannabis.

Yellow strains include Lemon Haze, Golden Lemon, and Strawberry Banana. Known for their sweet and citrusy scents and high-THC content, they are said to produce happy, invigorating effects that sharpen creativity and sensory awareness.

Green: Green is the predominant color of most flowering plants, including cannabis, due to the presence of chlorophyll. Often plant tissues will have so much chlorophyll that its green color masks the presence of other pigments.

Cannabis strains that retain green as the dominant bud color include Green Crack and Green Goblin. With a tangy, fruity flavor redolent of mango, Green Crack is a great daytime strain known to fight fatigue, stress and depression.

Blue: Cannabis flowers with shades of blue are high in anthocyanins and are by far some of the most beautiful. Likewise, fruits and vegetables high in anthocyanins include blueberries, açai, raspberries, blackberries, and purple cabbage. Blue-hued cannabis can be achieved by picking a blue strain of cannabis seeds and letting it grow outdoors, naturally occurring as the temperature drops.

All blue cannabis descends from Dutch Passion’s Blueberry, developed in Amsterdam in the 1970s. Popular strains include Blue Haze, Blue Mystic and Blue Cheese. These predominant Indica strains are known for being heavy, often used for relaxation and for providing relief from muscle spasms, pain, or stress.

Indigo: There are some rare strains that are so dark they almost appear black. The origin of these genetics goes back to Vietnamese landraces, like Vietnamese Black. All other strains derived from hybrids, such as Black Willy and Black Tuna, share both the signature ebony buds and leaves.

In addition, black strains are noted for their intense psychedelic, cerebral highs. If you want visuals, this lineage is for you. The inky appearance comes from an overabundance of all colors in the leaves.

Violet: Purple strains of cannabis are probably the most popular, such as Granddaddy Purple, Purple Haze and Purple Urkle. Marijuana strains that appear purplish or blue as opposed to the traditional green cannabis, tend to be more fruity, due to the high number of anthocyanins.

Purple Orangutan (or Gorilla) has some of the strongest purple hues in the world. This mostly indica hybrid produces lush, chunky buds covered in trichomes and purple shades. Purple Gorilla flowers smell of fresh earth and an array of berries, with a taste reminiscent of grapes picked right from the vine.

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