Wiesenborn: Nitrogen Contents in Riparian 71

NITROGEN CONTENT IN RIPARIAN ARTHROPODS IS MOST DEPENDENT ON ALLOMETRY AND ORDER

WILLIAM D. WIESENBORN U.S. Bureau of Reclamation, Lower Colorado Regional Office, P.O. Box 61470, Boulder City, NV 89006

ABSTRACT

I investigated the contributions of body mass, order, family, and trophic level to nitrogen (N) content in riparian and collected near the Colorado River in western Arizona. Most variation (97.2%) in N mass among arthropods was associated with the allometric effects of body mass. Nitrogen mass increased exponentially as body dry-mass increased. Significant variation (20.7%) in N mass adjusted for body mass was explained by order. Ad- justed N mass was highest in Orthoptera, , Araneae, and Odonata and lowest in Coleoptera. Classifying arthropods by family compared with order did not explain signifi- cantly more variation (22.1%) in N content. Herbivore, predator, and detritivore trophic-levels across orders explained little variation (4.3%) in N mass adjusted for body mass. Within or- ders, N content differed only among trophic levels of Diptera. Adjusted N mass was highest in predaceous flies, intermediate in detritivorous flies, and lowest in phytophagous flies. Nitro- gen content in riparian spiders and insects is most dependent on allometry and order and least dependent on trophic level. I suggest the effects of allometry and order are due to exoskeleton thickness and composition. Foraging by vertebrate predators, such as insectivorous birds, may be affected by variation in N content among riparian arthropods.

Key Words: nutrients, spiders, insects, trophic level, exoskeleton, cuticle

RESUMEN

Se investiguo las contribuciones de la masa de cuerpo, orden, familia y el nivel trófico al con- tenido de nitógeno (N) en arañas e insectos riparianos (que viven en la orilla del rio u otro cuerpo de agua) recolectadaos cerca del Rio Colorado en el oeste del estado de Arizona. La ma- yoría de la variación (97.2%) en la masa (N) entre los artrópodos fue asociado con los efectos alométricos de la masa de cuerpo. La masa de nitrógeno aumentó exponencialmente con el au- mento de masa-seca del cuerpo. La variación significativa (20.7%) en la masa N ajustada por la masa del cuerpo se explica según el ordén del artrópodo. La masa ajustada N fue mas alta en Orthóptera, Hymenóptera, Araneae, Odonata y mas baja en Coleoptera. Al clasificar los ar- trópodos por familia comparado con el ordén no explica la variacion mayor significativa (22.1%) en el contenido de N. Los niveles tróficos de los herbívoros, depredadores y detritívoros en todos los ordenes explica la pequeña variación (4.3%) en la masa N ajustada por la masa del cuerpo. Entre los ordenes, el contenido N varía solamente entre los niveles tróficos de Diptera. El valor ajustado de la masa de N fue mayor para las moscas depredadores, intermedio para las moscas detritívoras y menor para las moscas fitófagas. El contenido de nitrógeno en arañas e insectos riparianos es mas dependiente sobre la alometría y ordén y menos dependiente so- bre el nivel trófico. Sugiero que los efectos de alometría y ordén son debidos al grosor y la com- posición del exo-esqueleto. El forraje por los depredadores vertebrados, como aves insectivoras, puede ser afectado por la variación del contenido N entre los artrópodos riparianos.

Nitrogen concentrations in organisms are de- predators by analyzing data compiled from vari- pendent on trophic level. This is most apparent ous sources. Concentrations of N in spiders and between plants and herbivores, because N com- insects were dependent on trophic level after con- prises 0.03-7% of dry mass in plants compared trolling for body length, representing allometry, with 8-14% in (Mattson 1980). Variation and taxonomic group, representing phylogeny in N concentration among and within plants, and (Fagan et al. 2002). Predators generally con- its effects on abundances of herbivores including tained higher %N than herbivores. Predaceous arthropods, especially agricultural pests, has arthropods may concentrate N from food similar been frequently examined (reviewed in Mattson to phytophagous arthropods. 1980; Scriber 1984). Fewer studies have consid- Variation in N concentration among spiders ered variation in N concentration among spiders and insects may affect foraging by arthropod- and insects. Bell (1990) and Studier & Sevick feeding vertebrates and the qualities of food they (1992) tabulated measurements of %N in various obtain. Diet protein has been implicated as affect- insects from different studies. Fagan et al. (2002) ing egg production (Ramsay & Houston 1997) and compared %N between arthropod herbivores and nestling growth (Johnston 1993) in insectivorous

72 Florida Entomologist 94(1) March 2011

birds. Identifying sources of variation in arthro- for Prosopis spp., which grew together. Each spe- pod N content may improve our understanding of cies was swept 10-15 min on 9 dates: 30 Apr, 14 the prey composition required to support May, 27 May, 08 Jun, 22 Jun, 30 Jun, 21 Jul, 4 of insectivorous wildlife. Aug, and 18 Aug 2009. All plant species were in I examined variation in N content among spi- flower or fruit except for P. fremontii. Arthropods ders and insects collected from trees and shrubs swept from plants were placed into plastic bags, established to restore riparian habitat for insec- kept in a refrigerator, and killed in a freezer. - tivorous vertebrates, especially birds. Variation ing insects were trapped with a Malaise trap in N mass was partitioned into various sources. I (Santee Traps, Lexington, KY) that was placed in first determined the allometric relationship be- the center of a plot supporting S. gooddingii and tween N mass and body dry-mass. After adjusting P. sericea and elevated 1 m aboveground with N mass for this relationship, N contents of arthro- fence posts. Trapped insects were collected into a pods were compared among orders and families dry plastic bottle containing a nitrogen-free, di- and among trophic levels across and within or- clorvos insecticide strip. Insects were trapped for ders. I interpreted N contents in relation to exosk- 6.1-7.3 h during 0855-1640 PDT on each of the eleton scaling and chemical composition and con- above dates except 30 Apr, 14 May, and 18 Aug cluded by applying the results to diets of insectiv- 2009. orous birds. Spiders and insects collected on each date were sorted under a microscope into morphotypes (sim- MATERIALS AND METHODS ilar-looking specimens). Representatives of each morphotype were placed into 70% ethanol for Arthropod Collections identification. I counted and split the remaining specimens of each morphotype into samples each Spiders and insects were collected next to the with an estimated maximum dry mass of 10 mg. Colorado River within Havasu National Wildlife Individual specimens with dry masses ≥10 mg Refuge in Mohave County, Arizona. Most arthro- were placed into separate samples. Arthropod pods were collected at an irrigated 43-ha riparian samples for N analyses were cleaned by vortexing restoration area (34°46’N, 114°31’W; elevation in water, transferred to filter paper with a Büch- 143 m) of planted or volunteer trees and shrubs ner funnel, dried 2 h at 80ºC, and stored in stop- 12 km southeast and across the river from Nee- pered vials. dles, California. Plots were planted during 2003- 2005 with cuttings that were taken from nearby Arthropod Identifications and Trophic Levels areas along the river and rooted in containers. The area is straddled by Topock Marsh (16 km2) Spiders and insects were identified to the low- and Beal Lake (0.9 km2), 2 impoundments con- est taxon possible, at least to family and typically taining mostly emergent cattails (Typhus sp., to . Vouchers of adult insects were deposited Typhaceae) and open water. Undeveloped areas of at the Bohart Museum of Entomology, University the surrounding floodplain support mostly natu- of California, Davis, and vouchers of spiders were ralized tamarisk (Tamarix ramosissima Ledeb., deposited at the California Academy of Sciences, Tamaricaceae) shrubs. The floodplain is flanked San Francisco. Arthropod taxa were classified by Sonoran desertscrub dominated by creosote into the trophic levels of herbivore, predator, and bush (Larrea tridentata (DC.) Cov., Zygophyl- detritivore based on published descriptions (Ta- laceaae). Maximum temperatures average 42.7ºC ble 1). Holometabolous insects were classified by during Jul, and minimum temperatures average larval diet. Herbivores included consumers of pol- 5.6ºC during Jan at Needles (DRI 2010). len, nectar, or honeydew (homopteran egesta). I collected arthropods from plants and trapped Predators included parasites and consumers of insects in flight. Arthropods were swept with a already-dead animals. 38-cm diameter muslin net from planted cotton- wood (Populus fremontii S. Watson, Salicaceae) Arthropod Nitrogen Estimates and Goodding’s black willow (Salix gooddingii C. Ball, Salicaceae) trees, planted narrow-leaved The mass of N in each arthropod sample was willow shrubs (Salix exigua Nutt.), volunteer estimated with the Kjeldahl method adapted honey mesquite (Prosopis glandulosa Torrey, Fa- from Isaac & Johnson (1976). Samples of dried ar- baceae) and screwbean mesquite (Prosopis pubes- thropods were weighed (±0.01 mg) with a mi- cens Benth.) trees, and volunteer arrowweed crobalance (model C-30, Cahn Instruments, Cer- shrubs (Pluchea sericea (Nutt.) Cov., Asteraceae). ritos, CA) and ground into water with a 5-mL I also swept arthropods from T. ramosissima bor- glass tissue homogenizer. Homogenized samples dering the plots. Additional arthropods on S. ex- were poured and rinsed with water, to a total vol- igua were swept from plants growing along a dirt ume of 20 mL, into 100-ml digestion tubes. I irrigation canal 2 km northwest of the restoration added 6 mL of concentrated sulfuric acid, contain- area. Plant species were swept separately except ing 4.2% selenous acid, and 3 mL of 30% hydrogen

Wiesenborn: Nitrogen Contents in Riparian Arthropods 73 % N 9.3 ± 1.2 9.2 9.0 8.6 9.1 ± 1.5 8.9 ± 1.2 10.5 ± 2.0 11.8 10.6 ± 0.9 13.0 12.1 ± 1.8 14.3 13.8 ± 0.1 12.3 ± 1.0 13.9 ± 2.7 14.6 11.0 ± 1.5 11.6 13.3 10.1 ± 2.2 11.2 ± 1.5 11.4 ± 0.0 14.6 10.1 10.6 12.5 Mean ± SD . . CONTENT

7.20 1.37 1.93 6.29 0.07 2.03 2.46 2.35 0.46 6.62 3.36 0.68 0.35 4.37 1.24 5.22 1.51 8.99 5.72 39.7 13.0 49.2 55.2 17.1 14.1 ramosissima 115.0

Mean body dry mass (mg) NITROGEN

Tamarix 3 FOR

; T, P P P P P P P P P P P P H H H H H H H H H H H H H H ANALYZED

Pluchea sericea AND

; S, RIZONA A 3-4 1 9 1-2 6 6-7 1 1-3 1 1 1 1 1 1-3 1-3 2 5 4 2 2 2-14 1 pubescens

67 28-41 19-22 11 IN sp.

P. per sample Trophic level No. specimens No. or RIVER

Misumenops 2 1 2 1 2 4 6 1 1 1 1 1 1 4 2 1 1 1 1 1 OLORADO C No. Samples No. Prosopis glandulosa THE 2

sp.; Thomisidae, NEAR E,SS S E 2 S S P S S S S F,G,PE P F,P,S 4 E,F,GG 6 T F 5 S S G,TG F,G,SG 2 F 9

Source 6 HABITAT

Habronattus 1 7 ; M, Malaise trap; P, — — — — — 6 sp. & sp. Genus RIPARIAN

Philodromus Habronattus Metaphidippus Misumenops Pachydiplax Acridinae Scudderia Largus Nysius Brochymena Thyanta Pselliopus Zelus Cicadellinae Gyponinae Opsius Typhlocybinae Oecleus Ormenis Chrysoperla Myrmelion FROM

Misumenops Salix gooddingii Metaphidippus ; G, 4,6 5,6 COLLECTED

sp.; Thomisidae, sp.; Salticidae, Myrmeliontidae Philodromidae Salticidae Thomisidae 2 families 3 families Tettigoniidae Lygaeidae Pentatomidae Cixiidae Flatidae Membracidae Populus fremontii ARTHROPODS ; F,

Hypsosinga Habronattus DULT 1. A Salix exigua Subfamily in Acrididae and subfamily or genus Cicadellidae. E, D, Detritivore; H, Herbivore; P, Predator. Reference for all (Borror et al. 1981) except Apioceridae (Cole 1969) and Andrenidae, Formicidae, and Tettigoniidae (Essig 1926). Salticidae, Araneidae, Adults and immatures. Immatures. 1 2 3 4 5 6 7 ABLE Homoptera Cicadellidae Araneae OdonataOrthopteraHeteroptera Libellulidae Acrididae Neuroptera Chrysopidae Order or suborder Family T

74 Florida Entomologist 94(1) March 2011 . % N 8.3 9.8 ± 1.2 6.6 ± 2.8 9.9 ± 2.0 7.8 ± 1.0 8.1 ± 4.6 9.2 ± 2.3 5.1 ± 1.5 9.8 8.8 8.5 21.2 11.4 11.7 11.5 11.6 10.9 ± 2.2 10.9 ± 1.8 11.7 14.1 16.7 13.4 14.0 Mean ± SD CONTENT

NITROGEN

FOR

3.01 4.75 6.26 52.87 42.3 0.39 1.31 2.37 1.68 15.0 13.8 7.66 1.01 1.74 0.76 7.42 5.57 2.71 33.5 7.23 4.54 28.8 10.6 Mean body dry mass (mg) ANALYZED Tamarix ramosissima .

3 ; T, AND

P P P P P P P P P P P P P D D D H H H H H H H RIZONA A IN

Pluchea sericea ; S, RIVER

6 2-4 2-8 1 1 4-5 2-6 2 1 2-3 1-2 7-9 2 6-16 1 3 9 1 1 1 6 1 17-113 P. pubescens per sample Trophic level OLORADO No. specimens No. C THE

Misumenops sp. 1 3 3 1 1 1 2 2 1 1 1 1 1 1 1 1 1 NEAR

13 13 No. Samples No. Prosopis glandulosa or 2 HABITAT

P F,P F,S M M M F,GF,GF,GM 2 M 2 M 1 F S E,SE S E 4 M M M E G Source RIPARIAN

Habronattus sp.; Thomisidae, 1 FROM

sp. Genus Algarobius Chilocorus Hippodamia Apiocera Proctacanthus Asyndetus Homoneura Minettia Eumacronychia Apatolestes Tabanus Zaira Acinia Perdita Formica Agapostemon Dieunomia Lasioglossum Bembix Cerceris Tachysphex Myzinum Polistes COLLECTED

Misumenops Salix gooddingii ; M, Malaise trap; P, Metaphidippus sp. & ; G, ARTHROPODS

DULT Coccinellidae Dolichopodidae Sarcophagidae Tabanidae Formicidae Sphecidae Tiphiidae Vespidae ) A Populus fremontii ONTINUED 1. (C Subfamily in Acrididae and subfamily or genus Cicadellidae. E, Salix exigua ; F, D, Detritivore; H, Herbivore; P, Predator. Reference for all (Borror et al. 1981) except Apioceridae (Cole 1969) and Andrenidae, Formicidae, and Tettigoniidae (Essig 1926). Salticidae, Habronattus sp.; Thomisidae, Araneidae, Hypsosinga sp.; Salticidae, Adults and immatures. Immatures. 1 2 3 4 5 6 7 ABLE Order or suborder Family Diptera Apioceridae Hymenoptera Andrenidae Coleoptera Bruchidae T Wiesenborn: Nitrogen Contents in Riparian Arthropods 75 peroxide and heated samples 1 h at 400ºC with a explained more variation in adjusted log(mg N) block digestor (model 2040, Tecator, Herndon, with the general linear test approach (Neter et al. VA). After cooling, water was added to 60 mL. 1996). This approach tests if mean square error in The ammonia concentration formed in the clear, an analysis of variance decreases significantly digested samples was measured by colorimetry, when the model becomes more complex. Samples against standards prepared from dried ammo- containing more than 1 family (3 samples of Ara- nium-sulfate, with a segmented flow analyzer neae, or spiders) were classified only to order. (model FS-4, OI Analytical, College Station, TX). Arthropod N-contents adjusted for body mass Salicylate, hypochlorite, and sodium nitroprus- were compared among trophic levels across and side were used as the indicator. I converted am- within orders or suborders. I compared N masses monia concentration to mg N. among trophic levels across orders or suborders I adjusted estimates of mg N in arthropod with analysis of variance. Separate analyses were samples with chitin samples containing known N performed within , Diptera, and Hy- masses. Chitin is a nitrogenous polysaccharide menoptera, the 3 orders or suborders with 2 or

(C8H13NO5)n abundant in arthropod exoskeleton, more trophic levels each containing more than 1 or cuticle (Neville 1975), that typically comprises sample. Analyses within orders or suborders 25-40% of exoskeleton dry-mass in insects (Rich- weighted adjusted values of log(mg N) by 1/s2 in ards 1978). Various masses (2, 4, 8, 16, 32, 64 mg) each trophic level to correct for uneven variances of powdered chitin (Tokyo Chemical Industry) among trophic levels (Neter et al. 1996). containing 6.89% N were weighed, placed in 20 mL water, digested, and measured for ammonia RESULTS within each batch (n = 4) of arthropod samples. I increased estimates of mg N in arthropod sam- Collected Arthropods ples in each batch to correct for the batch’s mean I collected 121 samples of spiders and insects underestimate of %N (5.76, 6.23, 6.44, 6.08%) in containing 1,490 specimens in 9 orders or subor- chitin samples. I calculated %N in arthropod ders, 33 families, and 43 subfamilies or genera samples as 100(mg N/mg dry mass). Two arthro- (Table 1). All of the arthropods collected were pod samples of Acinia and Chrysoperla with un- adults except for 8 samples in 3 taxa (families, usually low N concentrations (<0.9%) were ex- subfamilies, or genera) with adults and imma- cluded as outliers. Dry mass and mg N of each ar- tures and 6 samples in 1 taxon with only imma- thropod sample were divided by the number of tures. Body dry-masses of adult arthropods specimens in the sample to estimate dry mass ranged from 0.35 mg in Typhlocybinae leafhop- and N mass per specimen. pers (Cicadellidae) to 115 mg in the fork-tailed bush katydid Scudderia furcata Brunner (Tet- Statistical Analysis tigoniidae). Two orders or suborders (Orthoptera and Ho- Body masses of arthropods, transformed moptera) of collected spiders and insects were log(mg) to normalize residuals, were compared only herbivorous, 3 orders (Araneae, Odonata, among trophic levels with analysis of variance and Neuroptera) were only predaceous, and 4 or- (SYSTAT version 12, San Jose, CA). Nitrogen ders or suborders (Heteroptera, Coleoptera, masses in spiders and insects were analyzed se- Diptera, and Hymenoptera) included both trophic quentially. I first determined the relationship be- levels. All Coleoptera were predaceous except for tween N mass and body dry mass by regressing 1 sample. The only detritivores collected were log(mg N) against log(mg body mass) for each ar- flies (Diptera). Across orders or suborders, herbi- thropod sample. I verified that the relationship vores included 42 samples in 22 taxa, predators was allometric (exponential) by testing with an included 62 samples in 24 taxa, and 17 samples in approximate t test the null hypothesis that the re- 3 taxa were detritivores (Table 1). Trophic levels gression coefficient b = 1 (Neter et al. 1996). 1 contained arthropods with different body dry- Transformed N masses were adjusted for their al- masses (F = 25.5; df = 2, 118; P < 0.001). Preda- lometric relationship with transformed body tors were largest (back-transformed mean = 6.37 mass by adding the residuals from the regression mg) followed by herbivores (4.03 mg) and detriti- to the overall mean of transformed N mass (Sokal vores (0.55 mg). & Rohlf 1981). Adjusted, transformed N masses were com- Allometric Nitrogen Contents pared among arthropod orders with analysis of variance. were split into suborders Nitrogen mass in riparian spiders and insects Heteroptera and Homoptera, because the diges- was allometrically related to body dry mass tive systems of most homopterans have filter (Fig. 1). Transformed N mass per specimen in ar-

chambers that concentrate nitrogenous com- thropod samples was positively related (F = 4, 066; pounds (Borror et al. 1981). I tested if classifying df = 1, 119; P < 0.001) to transformed body dry- arthropods by family instead of order or suborder mass per specimen by: 76 Florida Entomologist 94(1) March 2011

Fig. 1. Mean N mass vs. mean body dry-mass in riparian arthropods from the lower Colorado River classified by family. Abbreviations are orders or suborders (in Hemiptera): A, Araneae; C, Coleoptera, D, Diptera; He, Het- eroptera; Ho, Homoptera; Hy, Hymenoptera; N, Neuroptera; Od, Odonata; Or, Orthoptera. Single point labeled Ara- neae represents mixed samples of Araneidae, Salticidae, and Thomisidae. Axes are log scales. Line fit to transformed data by linear regression weighted by sample size.

log mg N = -1.006 + 1.039(log mg dry mass) Nitrogen Content in Arthropod Orders Back-transforming this equation produced: Nitrogen mass adjusted for body mass in ripar- mg N = 0.0986(mg dry mass)1.039 ian arthropods (Fig. 2) differed (F = 3.64; df = 8, 112; P < 0.001) among orders or suborders. These The exponent (1.039 ± 0.016 SD) differs from taxonomic levels explained 20.7% of variation in unity (t* = 2.43; df = 119; P = 0.008), verifying adjusted N mass. Orthoptera (mean 14.0% N), that the relationship is exponential rather than Hymenoptera (12.4% N), Araneae (11.9% N), and linear. This allometric relationship explained Odonata (12.3% N) contained the highest ad- 97.2% of variation in N mass. Percentage of N in justed N contents, and Coleoptera (8.2% N) con- riparian arthropods (Table 1) increased as body tained the lowest adjusted N content. Orthoptera mass increased. were mostly immature slant-faced grasshoppers

Wiesenborn: Nitrogen Contents in Riparian Arthropods 77

pended on classification (Fig. 2). Across orders or suborders, N mass did not vary (F = 0.62; df = 2, 118; P = 0.54) among trophic levels. Trophic levels explained 1.0% of variation in N mass after ac- counting for body mass. Back-transformed means of adjusted N mass (and mean % N) were 0.413 mg (11.1% N) in herbivores, 0.397 mg (10.9% N) in predators, and 0.380 mg (9.44% N) in detriti- vores, the smallest arthropods collected. Within orders or suborders, N mass varied among trophic levels in Diptera (F = 4.60; df = 2, 35; P = 0.017) but not in Heteroptera (F = 0.62; df = 1, 12; P = 0.45) or Hymenoptera (F = 0.13; df = 1, 11; P = 0.91). Adjusted N contents in (Fig. 2) were lower in herbivores (mean 5.1% N) compared with predators (10.9% N) or detritivores (9.4% N). All phytophagous flies collected were 2 samples of the fruit fly (Tephritidae) (Snow), swept from P. fremontii. Adjusted N con- centrations in predaceous or parasitic flies (Apio- ceridae, Asilidae, Sarcophagidae, Tabanidae, and Tachinidae) and detritivorous flies (Dolichopo- didae and Lauxaniidae) were similar. Fig. 2. Nitrogen mass allometrically adjusted for body mass in riparian arthropods from the lower Colo- DISCUSSION rado River classified by order or suborder (in Hemi- ptera). Letters are means (± SE) and trophic levels: D, detritivores; H, herbivores; P, predators. Adjacent num- Allometric Nitrogen Contents bers are sample sizes. Y-axis is log scale. The allometric relationship between N mass and body mass in riparian arthropods resembles a similar relationship between exoskeleton mass (Acridinae) along with the sole katydid S. furcata. and body mass in terrestrial arthropods. Ander- Hymenoptera included ants (Formicidae), 2 fami- son et al. (1979) dissected the exoskeletons from 3 lies of bees (Andrenidae and Halictidae), and 3 species of immature and adult spiders, weighing families of wasps (Sphecidae, Tiphiidae, and between 25 mg and 1.2 g, and determined exosk- Vespidae). samples contained several fam- eleton dry-mass and body wet-mass were posi- ilies (Table 1). The only odonate collected was the tively related by: dragonfly Pachydiplax longipennis Burmeister. Coleoptera included 1 sample of the herbivorous g exoskeleton = 0.078(g body mass)1.135 seed beetle (Bruchidae) Algarobius prosopis Le- Conte, collected from Prosopis spp., and 6 sam- Body mass in spiders explained 94.1% (their r- ples containing 2 species of predaceous ladybird value squared) of variation in exoskeleton mass. beetles (Coccinellidae), Chilocorus cacti L. and Anderson et al. attributed this allometric rela- the widespread Hippodamia convergens Guerin- tionship to scaling. The exoskeleton of terrestrial Meneville. Insects in other orders, including the 2 arthropods must increase in thickness as body Hemiptera suborders, contained intermediate N weight increases to support the organism and concentrations (Fig. 2). withstand the stresses of bending and twisting Classifying arthropods by family instead of or- (Prange 1977; Anderson et al. 1979). der or suborder did not explain more variation in Allometric relationships between N mass and N mass adjusted for body mass. Error variance of body mass, and between exoskeleton mass and adjusted N mass did not decrease (F = 1.45; df = body mass, may be primarily due to exoskeleton 26, 86; P = 0.10) when arthropods were classified N. Trim (1941) estimated N concentrations of by family compared with order or suborder. Clas- 11.8% in abdominal cuticles of 2 Orthoptera spe- sifying arthropods by family instead of order or cies, approximating the mean concentration suborder explained 22.1%, a 1.4% improvement, (10.7%) in riparian arthropods. A large proportion of variation in adjusted N mass. of N in terrestrial arthropods likely resides within the exoskeleton due to its greater density Nitrogen Content in Trophic Levels compared with internal tissues and hemolymph. The allometric relationship between exoskeleton Differences in N content among the trophic mass and body mass may have produced the sim- levels of herbivore, predator, and detritivore de- ilar relationship between N mass and body mass. 78 Florida Entomologist 94(1) March 2011

A linear increase in N mass in internal tissues as Nitrogen Contents in Trophic Levels body mass increases would dampen the exponen- tial increase in cuticular N mass. The lower expo- I did not detect an overall difference in N con- nent relating N mass to body mass (1.039) com- centration among herbivorous, predaceous, and pared with the exponent relating cuticle mass to detritivorous arthropods after accounting for the body mass (1.135) may reflect this dampening. allometric effects of body mass. Trophic level did not appear to generally affect arthropod %N. This Nitrogen Contents in Orders or Suborders contradicts the overall difference in N concentra- tion between herbivorous and predaceous arthro- Exoskeleton composition may have contrib- pods detected by Fagan et al. (2002). Different re- uted to different N concentrations among orders sults may have been due to statistical methodol- of spiders and insects (Fagan et al. 2002). Arthro- ogy. Fagan et al. controlled for body length and pod cuticle is composed primarily of protein and taxonomic group, to account for phylogeny, chitin (Neville 1975), and concentrations of N are whereas I controlled only for body mass. Control- higher in the former. For example, I estimated ling for phylogeny is difficult, because different %N in arthropod cuticular protein from percent- frequencies of herbivores compared with preda- ages of amino acids in pronotal and abdominal cu- tors among taxonomic groups cause trophic level ticles of adult Tenebrio beetles (Andersen et al. and phylogeny to be confounded. Phylogeny and 1973; reported in Table 3.4 in Neville 1975) by as- trophic level cannot be statistically separated. suming the amino acids were bonded into Similar N contents between trophic levels polypeptides. The estimated N concentration of agree with the concept that most insects satisfy cuticular protein (17.4%) exceeded that of chitin nutrient requirements by adjusting food intake (6.89%). Based on the maximum range of chitin (Waldbauer 1968; reviewed in Simpson et al. concentration (10-60% of dry mass) in cuti- 1995). An example in riparian arthropods may be cle (Richards 1978; see also Table 1 in Hackman found in the 2 suborders of Hemiptera, insects 1974), and assuming cuticle is entirely chitin and with piercing-sucking mouthparts. Phytophagous protein, N concentrations in insect exoskeleton Heteroptera, such as Lygus leaf bugs (Backus et may vary from 11.1% to 16.4%. al. 2007), typically rupture, dissolve with saliva, Greater concentrations of protein in arthro- and ingest mesophyll from a variety of plant pod cuticle, producing higher N contents, have structures. All Homoptera are herbivorous, and been associated with concentrations of resilin many homopterans feed on phloem which is high (Andersen 1979). Resilin is a flexible, elastic in water and carbohydrates but low in other nu- protein that occurs in cuticle in near-pure con- trients including N. The Opsius stactogalus Fie- centrations or combined with other proteins ber leafhoppers collected here increase food in- and chitin (Richards 1978). I estimated as take, concentrate nutrients within their filter- above that resilin contains 19.0% N from per- chamber digestive tracts (Wiesenborn 2004), and centages of amino acids in resilin from Schisto- void excess water and sugars. Concentrations of cerca grasshoppers (Andersen 1966; reported in N in Homoptera, phytophagous Heteroptera, and Table 3.4 in Neville 1975). Various mechanical predaceous Heteroptera were similar despite dif- structures in arthropods are elastic due to resi- ferent diets and physiologies. lin (Table 2.1 in Neville 1975). Resilin is espe- An exception was Diptera. Herbivorous flies, cially prevalent in the wing tendons and hinges all Tephritidae, contained lower N concentra- of Odonata and Orthoptera (Andersen & Weis- tions than predaceous or detritivorous flies after Fogh 1964), primitive orders with synchronous considering body mass. Fagan et al. (2002) com- flight muscles. Andersen and Weis-Fogh also de- pared phylogenetic categories of herbivorous in- tected resilin in the abdominal sclerites of sects and found lower N concentrations in Schistocerca grasshoppers, presumably allow- Diptera and Lepidoptera, combined as the recent ing the abdomen to stretch. Abundances of resi- lineage Panorpida, after accounting for body lin in riparian Odonata and Orthoptera may length. The database analyzed by Fagan et al. in- have contributed to their high N contents. Al- cluded the herbivorous flies , Chlorop- though resilin has not been found in spiders idae, and Drosophilidae, each in a different su- (Andersen & Weis-Fogh 1964), the high degree perfamily separate from Tephritidae. The diver- of abdominal stretching by spiders (Browning sity of phytophagous Diptera found to contain 1942) suggests their cuticles contain a similar low N concentrations suggests N contents in flies elastic protein. Cuticles of Coleoptera are likely generally vary by trophic level. Fagan et al. less elastic. A dominant feature of beetles is the (2002) suggested several explanations for lower elytra, hardened front-wings that act only to N contents in herbivores than in predators. cover the folded hind-wings and abdomen. The These included the direct effects of diet N, indi- likely absence of resilin and resultant high con- rect effects of trophic niche unrelated to diet, and centrations of chitin, in elytra may have low- selection for low body N in response to low diet N. ered %N in Coleoptera. The A. picturata tephritids that I collected de- Wiesenborn: Nitrogen Contents in Riparian Arthropods 79 velop as larvae in the flower heads of Pluchea The importance of prey N-concentration to in- spp. (Foote et al. 1993), corresponding with the sectivorous birds that feed on more-diverse prey flowering P. sericea at the study site. Infestations is less clear. An example is the southwestern wil- by A. picturata reduce seed production (Alyokhin low flycatcher (Empidonax traillii (Audubon) ssp. et al. 2001), suggesting larvae eat ovaries or extimus Phillips), a migrant that winters in Cen- seeds. The species does not appear to concentrate tral America and breeds in southwestern U.S. ri- N from food, because its N concentration (5.1%) is parian habitats. Adult flycatchers ate mostly het- within the range (1-7% of dry mass) reported for eropterans, flies, and beetles but fed more odo- seeds (Mattson 1980). The structural or biochem- nates and beetles to nestlings (Drost et al. 2003). ical features correlated with low N concentration Diet N may be increased by including odonates, in A. picturata and other plant-feeding flies are especially dragonflies due to their large biomass. unknown. Low exoskeleton mass in tropical, her- Diets of nestling flycatchers in other localities bivorous beetles has been attributed to low diet contained more Diptera than those of adults N, short larval-development time, and high fe- (Durst et al. 2008) or prey compositions similar to cundity (Rees 1986). Equivalent N concentra- adults (Wiesenborn & Heydon 2007). The high-N tions in predaceous or parasitic flies and detritiv- orders of Araneae, Odonata, and Hymenoptera, orous flies suggest their diets contain similar taken together, were eaten with similar frequency amounts of N. by flycatchers at different localities and habitats. These orders comprised 21% of prey in California Arthropod Nitrogen as a Nutrient for Birds (Drost et al. 2003), 31% of prey in Arizona (Durst et al. 2008), and 21% of prey at 3 localities in Ar- Not all N in arthropods is digested by insectiv- izona and Nevada (Wiesenborn & Heydon 2007). orous birds. Bird diets are frequently determined In summary, N concentrations in riparian ar- by identifying undigested fragments of exoskele- thropods are primarily dependent on body mass ton in fecal samples (e.g., Wiesenborn & Heydon and order and less dependent on trophic level. 2007). Digestion of arthropod cuticle by verte- Variation in prey N concentration may affect for- brates likely depends on its sclerotization (Kara- aging by insectivorous birds and the qualities of sov 1990). Sclerotized proteins are bonded to- food they obtain. gether, frequently with chitin, forming an irre- versibly-hardened cuticle that cannot be hydro- ACKNOWLEDGMENTS lyzed into amino acids (Richards 1978). Unsclerotized proteins, like resilin, can be hydro- I am grateful to A. Stephenson, USBR Lower Colo- lyzed (Richards 1978). Relative proportions of rado Regional Laboratory, for measuring ammonia con- sclerotized and unsclerotized proteins vary centrations. I appreciate the help identifying insects provided by S. L. Heydon, L. S. Kimsey, and T. J. Zavort- greatly among species (Richards 1978) producing ink at the Bohart Museum of Entomology, and C. A. cuticles with different digestibilities. Arthropod Tauber and P. S. Ward at the Entomology Department, orders with high amounts of elastic protein, such UC Davis. I am grateful to J. E. O’Hara at Agriculture as Odonata and Orthoptera and probably Ara- and Agrifood Canada for identifying tachinids and to D. neae, may provide insectivorous birds with high Ubick for identifying spiders. I thank J. Allen of the U.S. concentrations of digestible protein. Fish and Wildlife Service for the collection permit. This Riparian arthropods presented insectivorous work was funded by the Lower Colorado River Multi- birds with prey containing a range (5.1-14.0%) of Species Conservation Program. N concentrations. Foraging by insectivorous birds in relation to prey N concentration can be difficult REFERENCES CITED to discern, because birds frequently forage in re- sponse to prey availability which is transitory ALYOKHIN, A. V., MESSING, R. H., AND DUAN, J. J. 2001. Utilization of the exotic weed Pluchea odorata and hard to estimate. Selective foraging may be (Asteraceae) and related plants by the introduced bi- inferred by comparing arthropods eaten by adults ological control agent Acinia picturata (Diptera: Te- with those concurrently captured by adults but phritidae) in Hawaii. Biocontrol Sci. Technol. 11: fed to nestlings. Insectivorous nestlings depend 703-710. on diet nutrients in addition to calories (Johnston ANDERSEN, S. O. 1966. Covalent cross-links in a struc- 1993). Adult great tits (Parus major L.) and blue tural protein, resilin. Acta Physiol. Scand. 66 (Suppl. tits (Parus caeruleus L.) in woodlands ate mostly 263): 1-81. Lepidoptera larvae but provided 3-9 day-old nest- ANDERSEN, S. O. 1979. Biochemistry of insect cuticle. lings with more spiders, earwigs (Dermaptera), Annu. Rev. Entomol. 24: 29-61. ANDERSEN, S. O., AND WEIS-FOGH, T. 1964. Resilin. A and flies (Cowie and Hinsley 1988). Including rubberlike protein in arthropod cuticle, pp. 1-65 In J. other arthropods, especially spiders, as prey may W. L. Beament, J. E. Treherne, and V. B. Wiggles- have augmented the low N content of Lepidoptera worth [eds.], Advances in Insect Physiology, vol. 2. (Fagan et al. 2002). Spiders also provide different Academic Press, London. amino-acid compositions (Ramsay & Houston ANDERSEN, S. O., CHASE, A. M., AND WILLIS, J. H. 1973. 2003). The amino-acid composition of cuticles from Tenebrio 80 Florida Entomologist 94(1) March 2011

molitor with special reference to the action of juve- the house martin Delichon urbica. Funct. Ecol. 7: nile hormone. Insect Biochem. 3: 171-180. 255-266. ANDERSON, J. F., RAHN, H., AND PRANGE, H. D. 1979. KARASOV, W. H. 1990. Digestion in birds: chemical and Scaling of supportive tissue mass. Q. Rev. Biol. 54: physiological determinants and ecological implica- 139-148. tions, pp. 391-415 In M. L. Morrison, C. J. Ralph, J. BACKUS, E. A., CLINE, A. R., ELLERSEICK, M. R., AND Verner, and J. R. Jehl [eds.], Avian Foraging: Theory, SERRANO, M. S. 2007. Lygus hesperus (Hemiptera: Methodology, and Applications. Studies in Avian Bi- Miridae) feeding on cotton: new methods and param- ology, no. 13. Cooper Ornithological Society, Los An- eters for analysis of nonsequential electrical pene- geles, CA. tration graph data. Ann. Entomol. Soc. America 100: MATTSON, W. J. 1980. Herbivory in relation to plant ni- 296-310. trogen content. Ann. Rev. Ecol. Syst. 11: 119-161. BELL, G. P. 1990. Birds and mammals on an insect diet: NETER, J., KUTNER, M. H., NACHTSHEIM, C. J., AND a primer on diet composition analysis in relation to WASSERMAN, W. 1996. Applied Linear Statistical ecological energetics, pp. 416-422 In M. L. Morrison, Models, 4th ed. McGraw-Hill, Boston, MA. 1408 pp. C. J. Ralph, J. Verner, and J. R. Jehl [eds.], Avian For- NEVILLE, A. C. 1975. Biology of the Arthropod Cuticle. aging: Theory, Methodology, and Applications. Stud- Vol. 4 of D.S. Farner [ed.], Zoophysiology and Ecolo- ies in Avian Biology, no. 13. Cooper Ornithological gy. Springer-Verlag, New York, NY. 448 pp. Society, Los Angeles, CA. PRANGE, H. D. 1977. The scaling and mechanics of ar- BORROR, D. J., DE LONG, D. M., AND TRIPLEHORN, C. A. thropod exoskeletons, pp. 169-181 In T. J. Pedley 1981. An Introduction to the Study of Insects, 5th ed. [ed.], Scale Effects in Locomotion. Academic Saunders, Philadelphia, PA. 827 pp. Press, New York, NY. BROWNING, H. C. 1942. The integument and moult cycle RAMSAY, S. L., AND HOUSTON, D. C. 1997. Nutritional of Tegenaria atrica (Araneae). Proc. R. Soc. London, constraints on egg production in the blue tit: a sup- B, Biol. Sci. 131: 65-86. plementary feeding study. J. Anim. Ecol. 66: 649- COLE, F. R. 1969. The Flies of Western North America. 657. University of California Press, Berkeley, CA. 693 pp. RAMSAY, S. L., AND HOUSTON, D. C. 2003. Amino acid COWIE, R. J., AND HINSLEY, S. A. 1988. Feeding ecology composition of some woodland arthropods and its of great tits (Parus major) and blue tits (Parus caer- implications for breeding tits and other passerines. uleus), breeding in suburban gardens. J. Anim. Ecol. Ibis 145: 227-232. 57: 611-626. REES, C. J. C. 1986. Skeletal economy in certain herbiv- DESERT RESEARCH INSTITUTE (DRI). 2010. Western U.S. orous beetles as an adaptation to a poor dietary sup- Climate Historical Summaries. Western Regional ply of nitrogen. Ecol. Entomol. 11: 221-228. Climate Center, Reno, NV [http://www.wrcc.dri.edu/ RICHARDS, A. G. 1978. The chemistry of insect cuticle, Climsum.html]. pp. 205-232 In M. Rockstein [ed.], Biochemistry of DROST, C. A., PAXTON, E. H., SOGGE, M. K., AND WHIT- Insects. Academic Press, New York, NY. FIELD, M. J. 2003. Food habits of the southwestern SCRIBER, J. M. 1984. Host-plant suitability, pp. 159-202 willow flycatcher during the nesting season, pp. 96- In W. J. Bell and R. T. Cardé [eds.], Chemical Ecology 103 In M. K. Sogge, B. E. Kus, S. J. Sferra, and M. J. of Insects. Sinauer, Sunderland, MA. Whitfield [eds.], Ecology and Conservation of the SIMPSON, S. J., RAUBENHEIMER, D., AND CHAMBERS, P. Willow Flycatcher. Studies in Avian Biology, no. 26. G. 1995. The mechanisms of nutritional homeosta- Cooper Ornithological Society, Los Angeles, CA. sis, pp. 251-278 In R. F. Chapman and G. de Boer DURST, S. L., THEIMER, T. C., PAXTON, E. H., AND [eds.], Regulatory Mechanisms in Insect Feeding. SOGGE, M. K. 2008. Age, habitat, and yearly varia- Chapman & Hall, New York, NY. tion in the diet of a generalist insectivore, the south- SOKAL, R. R., AND ROHLF, F. J. 1981. Biometry, 2nd ed. western willow flycatcher. Condor 110: 514-525. W. H. Freeman, New York, NY. 859 pp. ESSIG, E. O. 1926. Insects of Western North America. STUDIER, E. H., AND SEVICK, S. H. 1992. Live mass, wa- MacMillan, New York, NY. 1035 pp. ter content, nitrogen and mineral levels in some in- FAGAN, W. F., SIEMANN, E., MITTER, C., DENNO, R. F., sects from south-central lower Michigan. Comp. Bio- HUBERTY, A. F., WOODS, H. A., AND ELSER, J. J. chem. Physiol., A, Comp. Physiol. 103: 579-595. 2002. Nitrogen in insects: implications for trophic TRIM, A. R. 1941. Studies in the chemistry of the insect complexity and species diversification. American cuticle: some general observations on certain arthro- Nat. 160: 784-802. pod cuticles with special reference to the character- FOOTE, R. H., BLANC, F. L., AND NORRBOM, A. L. 1993. ization of the proteins. Biochem. J. 35: 1088-1098. Handbook of the Fruit Flies (Diptera: Tephritidae) of WALDBAUER, G. P. 1968. The consumption and utiliza- America North of Mexico. Comstock, Ithaca, NY. 571 tion of food by insects, pp. 229-288 In J. W. L. Bea- pp. ment, J. E. Treherne, and V. B. Wigglesworth [eds.], HACKMAN, R. H. 1974. Chemistry of the arthropod cuti- Advances in Insect Physiology, vol. 5. Academic cle, pp. 215-270 In M. Rockstein [ed.], The Physiolo- Press, London. gy of Insecta, 2nd ed. Academic Press, New York, NY. WIESENBORN, W. D. 2004. Mouth parts and alimentary ISAAC, R. A., AND JOHNSON, W. C. 1976. Determination canal of Opsius stactogalus Fieber (Homoptera: Ci- of total nitrogen in plant tissue, using a block di- cadellidae). J. Kans. Entomol. Soc. 77: 152-155. gestor. J. Assoc. Off. Anal. Chem. 59: 98-100. WIESENBORN, W. D., AND HEYDON, S. L. 2007. Diets of JOHNSTON, R. D. 1993. Effects of diet quality on the breeding southwestern willow flycatchers in differ- nestling growth of a wild insectivorous passerine, ent habitats. Wilson J. Ornithol. 119: 547-557.