Sticky Plant Traps Insects to Enhance Indirect Defence

Ecology Letters, (2012) doi: 10.1111/ele.12032 LETTER Sticky plant traps insects to enhance indirect defence

Abstract B. A. Krimmel1* and I. S. Pearse2 Plant-provided foods for predatory arthropods such as extrafloral nectar and protein bodies provide indirect plant defence by attracting natural enemies of herbivores, enhancing top-down control. Recently, ecol- ogists have also recognised the importance of carrion as a food source for predators. Sticky plants are widespread and often entrap and kill small insects, which we hypothesised would increase predator densities and potentially affect indirect defence. We manipulated the abundance of this entrapped insect carrion on tarweed (Asteraceae: Madia elegans) plants under natural field conditions, and found that carrion augmentation increased the abundance of a suite of predators, decreased herbivory and increased plant fitness. We suggest that entrapped insect carrion may function broadly as a plant-provided food for predators on sticky plants.

Keywords Enemy free space, glandular trichomes, indirect defense, omnivory, scavenging.

Ecology Letters (2012)

INTRODUCTION MATERIALS AND METHODS Extensive research has characterised the types of plant-provided We experimentally augmented carrion on tarweed (Asteraceae: Madia foods used by predators (W€ackers et al. 2005) such as pollen (Salas- elegans), an annual plant native to CA and OR. We then measured Aguilar & Ehler 1977; Wheeler 2001), extrafloral nectar (Janzen the responses of (1) predator abundance and diversity, (2) herbivore 1966; Bentley 1977; Heil et al. 2001) and protein bodies (Janzen damage and (3) plant lifetime fitness. At our study site, tarweed’s 1966). In addition to these non-prey food sources, carrion has major herbivore is the specialist caterpillar Heliothodes diminutiva recently been proposed as a ubiquitous and important resource for (Grote), which feeds largely on plant reproductive organs (Fig. 1) predators (Wilson & Wolkovich 2011). Plant trichomes commonly and can completely sterilise its host plants. The suite of predators entrap loosely associated invertebrates – ‘tourists’ (Moran & South- commonly found on tarweed includes the assassin bug Pselliopus spi- wood 1982; Southwood 1986), which die and become high-quality nicollis (Champion) (Fig. 1), two stilt bugs Hoplinus echinatus (Uhler) food sources for scavenging arthropods able to move on the sticky (Fig. 1) and Jalysus wickhami Van Duzee, the green lynx spider Peuce- plant surface (Wheeler & Schaefer 1982; Southwood 1986; Ander- tia sp. and the crab spider Mecaphesa schlingeri (Schick). All can navi- son & Midgley 2002; Sugiura & Yamazaki 2006). The goal of our gate tarweed’s sticky surface. study was to test whether entrapped carrion functions as a plant- provided food for defence by enhancing indirect defence by Study site scavenging predators. Sticky plants are common – 20–30% of vascular plants are esti- This study was conducted in an oak savannah and chaparral com- mated to produce glandular trichomes (Duke 1994) and many more munity within Stebbins Cold Canyon Nature Reserve, Solano Co, produce hooked trichomes. These trichomes are typically considered CA., a part of the University of California Natural Reserve System direct defences against herbivory (Levin 1973), but are thought to (North 38.50, West 122.10.28). The dominant members of the plant incur indirect costs by suppressing top-down control by predators, community include blue oak (Quercus douglasii), grey pine (Pinus probably by diminishing their foraging efficacy (Eisner et al. 1998; sabiniana), toyon (Heteromeles arbutifolia), poison oak (Toxicodendron Gassmann & Hare 2005). However, some of the few well-studied spp), manzanita (Arctostaphylos spp.), buck brush (Ceanothus cuneatus), arthropods commonly found on sticky plants in nature (e.g. green tarweed (M. elegans, Madia gracilis), California yerba santa (Eriodictyon lynx spiders, dicyphine plant bugs, stilt bugs, assassin bugs) can californicum) and gum plant (Grindelia camporum). move across the sticky plant surfaces without becoming entrapped (Wheeler 2001; Voigt et al. 2007; Romero et al. 2008). Further, The plant many are known to consume both living and dead prey items (Southwood 1973; Romero et al. 2008). Thus, we hypothesised that Common tarweed (M. elegans) is an annual flowering plant in the the benefits of attracting sticky plant-adapted predators for defence family Asteraceae. In Cold Canyon, it is very sticky, producing by producing glandular and hooked trichomes that entrap carrion abundant glandular trichomes. It generally emerges in the spring (Figs. 1, 2) could outweigh the costs of excluding certain generalist and flowers in mid-late summer, from approximately June through predators. September.

1University of California, Department of Entomology, Briggs Hall, One Shields 2Cornell Lab of Ornithology, 159 Sapsucker Woods Rd, Ithaca, NY, 14583, Avenue, Davis, CA, 95616, USA USA *Correspondence: E-mail: [email protected]

(a) (b)

Figure 1 Colour plate some of the major interactions of predators, an herbivore, and carrion on Madia elegans in nature. Hoplinus echinatus feeding on Drosophila corpse (a); Pselliopus spinicollis feeding on Drosophila corpse (b); Heliothodes diminutiva feeding on M. elegans ﬂower (c); P. spinicollis feeding on H. diminutiva (d). Photos taken by Sam Beck.

The herbivore Tarweed’s major herbivore in Cold Canyon is the noctuid moth H. diminutiva. Eggs are deposited primarily on developing buds and the caterpillars feed mostly on reproductive parts of the plant. Early and mid-instar larvae tend to develop inside ﬂower buds. Single caterpillars are capable of sterilising entire tarweed plants by consum- ing all of their ﬂowers and buds.

The predators We encountered five abundant species of predatory arthropod on tarweed: the assassin bug P. spinicollis, two stilt bugs H. echinatus and J. wickhami, the crab spider M. schlingeri and the lynx spider Peucetia sp. P. spinicollis nymphs and adults are highly mobile, and can be Figure 2 Hypothesised interactions connecting glandular trichomes to plant observed feeding both on H. diminutiva caterpillars and on insect fitness via carrion entrapment, increased predator abundance and reduced carrion under natural field conditions. We observed H. echinatus herbivory. The directions of the expected interactions are shown as +/À. The scavenging on insect carrion in the field, but have not observed it expected indirect effect of carrion addition on plant fitness is shown with a preying upon live insects; little is known about the ecology of this dashed line. Other indirect effects, such as trophic cascades, are expected to bug. J. wickhami is a predator of Lepidoptera (Jackson & Kester occur but not depicted in the flowchart for simplicity. 1996), although we did not observe it feeding. M. schlingeri was observed feeding on early instar H. diminutiva caterpillars in the Experimental design field. Although we did not observe Peucetia feeding, Peucetia spp. consume carrion and live prey items varying in size and taxonomic Carrion-addition experiment composition and are known to associate with glandular plants To manipulate carrion availability we selected 82 plants in a natural across broad geographical ranges (Vasconcellos-Neto et al. 2007). field and randomly assigned them to experimental (5 Drosophila spp.

© 2012 Blackwell Publishing Ltd/CNRS Letter Sticky plant traps insects to enhance indirect defence 3 fruit flies added per week) and control treatments (no fruit flies colonisation by adult assassin bugs and moths, and continued until added) (Online Supplementary Information). Our addition of 5 fruit plants senesced in September. As tarweed is a small plant, we were flies fell well within the natural variation of carrion on tarweed, able to count all arthropods and, for some species, their eggs, as which ranged from 0 to 40 entrapped insects per observation and well as plant reproductive organs on all plants each week. Addition- averaged 2.6. The experiment was initiated in early June, prior to ally, we recorded the number of chewed buds as a measure of caterpillar damage, and the number of successfully dehisced fruits as a measure of plant fitness; since tarweed is an annual that does not *** (a) reproduce clonally, changes in fruit production are likely to have strong fitness and population consequences. We also estimated the density of secreting glandular (viscid) trichomes and abundance of naturally entrapped carrion on all plants each week.

egg–days The carrion-addition experiment was established on June 7, 2011 P. spinicollis as a complete randomised design where half of 82 M. elegans plants 01530 were randomly assigned to either control or carrion addition treat- * ments. Each week through July, five freeze-killed fruit flies (Drosoph- ila spp.) were placed onto the glandular trichomes of each carrion augmented plant. Throughout the season, plants were monitored weekly to measure the abundance of carrion (experimentally and Spiders naturally entrapped), plant growth (# buds and flowers), herbivore

0 0.5 1 damage to buds and predator abundance (including P. spinicollis eggs, ** H. echinatus adults, nymphs and eggs, J. wickhami adults and nymphs, and the spiders Peucetia sp. and M. schlingeri). How sticky a plant is depends both on the number of glandular trichomes it bears and the extent to which they are actively secreting exudates. To capture

egg–days a functional combination of these two characteristics, we recorded H. echinatus rank-order estimates between 0 and 5 for each plant based on natu- 0 0.5 1 *** ral variation in viscid (actively secreting) trichome density each week (Online Supplementary Information). For all analyses, we condensed weekly ﬁeld observations into season-wide measurements on each M. elegans plant (Online Supplementary Information).

H. echinatus Caterpillar addition experiment 012 nymphs and adults On July 15, 2011, we set up an additional experiment aimed at quantifying the impact of herbivory on plant ﬁtness in this system. We selected 14 pairs of additional plants and randomly assigned . them to experimental (one caterpillar added, without any cage) and control (no caterpillar added) treatments. In the ﬁrst week of the experiment, two plants were lost for reasons that could not be

J. wickhami attributed to caterpillar feeding, resulting in 12 control and 14 nymphs and adults 0 0.5 1 Carrion Control addition Figure 3 (3a–b): Results from carrion-augmentation experiment. Carrion (b) increases predator abundance, reduces herbivory and increases plant ﬁtness. (a) Carrion attracts predators. The effects of experimentally added carrion to M. elegans plants on the abundance of four types of predatory arthropod: the assassin bug P. spinicollis (df = 1,80; P < .001), spiders P. viridans and M. schlingeri * (df = 1,80; P = .01), the stilt bug H. echinatus (eggs: df = 1,80; P = .004; nymphs and adults: df = 1,80; P < .001) and the stilt bug J. wickhami (df = 1,80; =

Damaged buds P .056). P. spinicollis was recorded as egg deposition on plants over the season. Spiders and J. wickhami were recorded as adult and nymph encounters over the 012 season. H. echinatus was recorded as eggs, adults and nymphs over the season. * Predatory arthropods were surveyed on 41 control and 41 carrion-augmented plants. Asterisks denote a significant effect of carrion addition (•P < 0.1, *P < 0.05, **P < 0.01, ***P < 0.001). (b) Carrion decreases herbivore damage and increases plant fitness. The effects of experimentally added carrion to M. elegans plants on herbivore damage to plant buds caused by H. diminutiva caterpillars and plant lifetime fitness, as measured by total dehisced fruits Dehisced fruits produced. Plant damage and fitness were surveyed on 41 control and 41 carrion- 0510 Carrion augmented plants. Asterisks denote a significant effect of carrion addition Control addition (•P < 0.1, *P < 0.05).

© 2012 Blackwell Publishing Ltd/CNRS 4 B. A. Krimmel and I. S. Pearse Letter caterpillar-augmented plants. These plants were scored each week in (a) the same way as in the carrion-addition experiment. carrion trmt control

RESULTS egg–days The addition of 5 dead fruit ﬂies (carrion) to plants each week over the growing season increased the abundance of all surveyed predatory arthropods associated with M. elegans plants by 76% to 450% (Fig. 3a). For P. spinoicollis, the most abundant predator, this effect was strong- P. spinicollis

est during the early growth season in June and July (Fig. 5). 048 The addition of carrion to M. elegans plants produced a 60% (b) decrease in bud damage caused by H. diminutiva, the dominant lepi- carrion abundance dopteran herbivore in this system (df = 2,69; P = 0.013, Fig. 3b), bud damage and increased lifetime fruit production by 10% (df = 2,69; P = 0.048, Fig. 3b). The number of damaged buds increased overall throughout the season as H. diminutiva abundance increased and more plants became reproductive (Fig. 6).

(a) Damaged buds 01234 024 Mean carrion counted per week Jun Jul Aug Sept

Figure 5 A time series by month showing: (a) the accumulation of P. spinicollis eggs on M. elegans plants for the carrion addition treatment (dashed line,

Carrion triangles) and control treatment (solid line, circles) over the season; and (b) natural entrapment of carrion (dotted line, squares) and bud damage from herbivory (solid line, circles). Error bars are +/À SE. accumulated over season over accumulated 50 100 150 200 250 300

0 (a) 20 40 60 80 100 * Viscid trichome density (b) Damaged buds 0246

(b) egg–days P. spinocollis P.

5 7.5 10 *** 2.5 Dehisced fruits 0

0 20 40 60 80 100 120 Caterpillar Control addition 0246 Natural carrion abundance Figure 6 The effect of a single experimentally added H. diminutiva caterpillar on Figure 4 The density of viscid trichomes, carrion abundance and predator eggs. M. elegans plants. Caterpillar addition (a) increased bud damage to plants and (b) (a) A regression of observed natural carrion accumulation on plant viscid decreased lifetime fitness, as measured by total dehisced fruits. Plant damage and trichome density (stickiness) on M. elegans plants (N = 82). Entrapped carrion fitness were surveyed on 12 control and 14 caterpillar addition plants in a paired * < was counted weekly over the growing season of M. elegans, viscid trichome design. Asterisks denote a significant effect of caterpillar addition ( P 0.05, *** < density was assessed at the same intervals, and both measures were integrated P 0.001). over the growing season. Both measures were square-root transformed prior to linear regression. (b) The relationship between naturally entrapped carrion on = Plants that had higher densities of viscid trichomes tended to M. elegans control plants (N 41) and the accumulation of P. spinicollis eggs over = < the growing season. The trend line shows the predicted values of a zero-inflated entrap more carrion than less viscid plants (df 2,68; P .001, Poisson regression model. P. spinicollis egg observations were square-root Fig. 4a). Within the control plants, the abundance of P. spinicollis transformed in the model but not in the Figure. eggs was greater on plants that had more naturally entrapped

© 2012 Blackwell Publishing Ltd/CNRS Letter Sticky plant traps insects to enhance indirect defence 5 carrion (df = 1,34; P < .0001, Fig. 4b). The abundances of other ACKNOWLEDGEMENTS predators were too low to assess temporal trends or correlations with naturally entrapped carrion on control plants. On average, we We thank Collin Edwards and Sam Beck for assistance with field recorded 2.6 pieces of naturally entrapped carrion per plant each work; Sam Beck for taking photos in the field; Jay Rosenheim, week. Natural carrion entrapment by plants remained fairly constant Richard Karban, Louie Yang, Alfred Wheeler Jr. and Tobin throughout the season, though it peaked in July (Fig. 5). Northfield for reading the manuscript and providing important In a separate experiment, we added a single H. diminutiva caterpil- suggestions and discussions; Alfred Wheeler Jr. for helping to lar to M. elegans plants to quantify the impact of herbivory on plant select the study system; Thomas Henry for identifying H. echinatus, fitness. Plants with a caterpillar experienced 50% more bud damage Christiane Weirauch for identifying P. spinicollis and Darrell Ubick than control plants (df = 14,11; P = 0.02, Fig. 6), and lifetime fruit for help identifying M. schlingeri; Virginia Boucher and the Univer- production fell by 50% (df = 14,11; P < 0.001, Fig. 6). sity of California Natural Reserve System (UCNRS) staff. Stebbins Cold Canyon is part of the UCNRS. Funding was provided by Jay Rosenheim through a USDA multistate research Hatch grant, a DISCUSSION NSF GRF awarded to B.A.K. and a Jastro-Shields Research Schol- Our results show that carrion entrapment by plants can enhance arship awarded to B.A.K. top-down control of herbivores by increasing the densities of their natural enemies. This indirect defence is not species-specific: experi- AUTHOR CONTRIBUTIONS mental plants attracted 5 species of predatory bugs and spiders. Given the widespread distribution of glandular and hooked tric- B.A.K. designed the study, assisted with fieldwork and wrote the man- homes across flowering plants and the ubiquity of scavenging by uscript. I.S.P. contributed to the experimental design, assisted with predators, we suggest that carrion is a widespread type of plant-pro- fieldwork, conducted the statistical analyses, prepared Figures and vided food for predators. This could help to explain why glandular wrote the original results and Online Supplementary Information. plants are so abundant, which is at odds with the conventional view Both authors discussed the results and commented on the manu- that stickiness is costly in the presence of predators (Eisner et al. script. 1998; Gassmann & Hare 2005). When protein is costly for plants to – produce, carbon-rich glandular exudates or hooked trichomes REFERENCES ‘tourist’ traps – could be a relatively cheap means of providing protein-rich foods for predators. Anderson, B. & Midgley, J. (2002). Digestive mutualism, an alternate pathway in – Although nectar and other well-known plant-provided foods for plant carnivory. Oikos, 102, 221 224. predators are produced directly by plants, insect carrion is captured Bentley, B. (1977). Extra-floral nectaries and protection by pugnacious bodyguards. Annu. Rev. Ecol. Syst., 8, 407–427. from the external environment. 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