Ingestion behavior The majority of research concerning feeding behaviors of terrestrial insects has investigated the effects of metals on feeding preference when individuals were exposed to various concentrations and combinations of metals. For herbivorous insects, this has focused mostly on antifeedant properties of metals on agricultural pests. Zinc, Cu, Ni, Se, and As have been evaluated individually, whereas several other studies examined combinations of metals.
Zinc sulfate has been extensively studied, particularly in experiments involving first-instar lepidopteran pests. Zinc salts are known to cause toxicity in insects and their abilities as feeding deterrents for agricultural pests were quantified. Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) responded to Zn sulfate treatments in choice experiments by avoiding meridic diet containing concentrations ≥0.1% ZnSO4 (Gahukar, 1975). Although variability in individual responses was high and not always significant, aversion increased with increasing ZnSO4 in the diet. These results are consistent with the results obtained by Sell & Bodznick (1971) for Heliothis virescens Fabricius (Lepidoptera: Noctuidae), which was deterred by concentrations ≥0.2% ZnSO4, and Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae), which was deterred by concentrations ≥0.1 m ZnSO4 (Salama & El-Sharaby, 1972). Similarly, Pollard & Baker (1997) demonstrated significant preference of Schistocerca gregaria (Forskål) (Orthoptera: Acrididae) and Pieris brassicae (L.) (Lepidoptera: Pieridae) for low Zn treatments over high Zn treatments in choice experiments. Behmer et al. (2005) came to the same conclusion in choice experiments, and further showed that S. gregaria learned associatively to avoid Zn-treated foods. Behavioral responses of insect predators and parasitoids to zinc sulfate-contaminated prey have not been reported.
Gustatory perception assays with adult Drosophila melanogaster Meigen (Diptera: Drosophilidae) showed that they also prefer low Zn treatments to high Zn treatments (Bahadorani & Hilliker, 2009). These authors observed the same pattern for Fe (II) and Fe (III), while adults preferred control diets to Cd- and Cu-contaminated diets significantly more often. Like adults, larvae also avoided feeding on high concentrations of heavy metals (Bahadorani & Hilliker, 2009).
Copper, as copper sulfate (CuSO4), has also been investigated as a feeding deterrent against agricultural pests. Mixing CuSO4 with lime creates a Bordeaux mixture effective as a fungicide (http://www.copper.org). El-Bassiouny (1991) investigated possible feeding deterrent properties for several lepidopteran species. These responded to CuSO4 feeding deterrents with mixed responses, depending on species. Oligophagous species [P. brassicae and Pieris napi (L.)] were deterred at lower concentrations (0.05–0.1 m CuSO4), whereas polyphagous species (Mamestra brassicae L. and Mamestra oleracea L.) (Lepidoptera: Noctuidae) were only inhibited by higher concentrations (0.2 m CuSO4) (El-Bassiouny, 1991). Pieris brassicae took shorter meals before feeding ceased, and experienced an increase in palpation frequency.
Some research has focused on hyperaccumulating plants and documented that high levels of metals in plant tissues may serve to deter herbivory. For example, herbivorous insects preferred Streptanthus polygaloides Gray (Brassicaceae) grown in low nickel (Ni) soils (15.6–76.5 μg g−1) vs. high Ni soils (1 820–7 960 μg g−1) (Jhee et al., 2005). These included the folivores Melanoplus femurrubrum (De Geer) (Orthoptera: Acrididae), Evergestis rimosalis Guenée (Lepidoptera: Pyralidae), and the rhizovore Delia radicum L. (Diptera: Anthomyiidae). Pieris rapae also preferred unamended to treated plants (180 and 7 400 mg Ni kg−1 soil, respectively) (Martens & Boyd, 1994). The feeding behaviors of aphids and other vascular feeding insects were not altered by Ni accumulation in plants (Boyd & Martens, 1999; Jhee et al., 2005).
The lepidopteran, Spodoptera exigua Hübner, exposed to different forms of Se were deterred from feeding by inorganic Se compounds (sodium selenate and sodium selenite) at LC30 values and greater for first and third instars (Vickerman & Trumble, 1999). By contrast, this same study revealed that organic Se compounds did not serve as feeding deterrents for third-instar S. exigua, though first instars preferred controls to these compounds 50–75% of the time. Sodium selenate accumulated by Brassica juncea (L.) Czern. (Brassicaceae) also effectively prevented Acheta domestica (L.) (Orthoptera: Gryllidae) feeding in choice experiments (Freeman et al., 2007) with 5× as many crickets preferring controls to treated leaves (546 ± 38 μg Se g−1 dry weight). At 10 mg kg−1 dry leaf weight, B. juncea with incorporated sodium selenate also successfully deterred Myzus persicae (Sulzer) (Hemiptera: Aphididae) feeding and prevented colonization (Hanson et al., 2004). When fed alfalfa with incorporated Se, first-instar S. exigua were unable to distinguish between low and high (2.88 ± 0.52 vs. 305.81 ± 52.14 μg g−1 plant dry weight) concentrations of Se compared with controls (Vickerman et al., 2002b). Fourth instars did not differentiate between low Se and controls, but avoided high Se plants. Alternatively, various polyphagous acridid grasshoppers chose low-Se Stanleya pinnata (Pursh) Britton (Brassicaceae) significantly more often than high-concentration alternatives (1 vs. 230 μg g−1 dry weight) in choice experiments (Freeman et al., 2007). A recently discovered biotype of a lepidopteran, Plutella xylostella Stanleyi (Lepidoptera: Plutellidae), was shown to withstand accumulated concentrations of 2 000 μg Se g−1 dry weight on S. pinnata and larvae showed no preference for low or high (47 vs. 792 μg g−1 dry weight) Se-treated plants in choice experiments, as opposed to P. xylostella G88 and P. rapae which avoided higher concentrations (Freeman et al., 2006).
Although there are many studies reporting the effects of As on insects, relatively few report behavioral impacts. In terms of ingestion behaviors, only a single paper was found. Rathinasabapathi et al. (2007) reported avoidance by Schistocerca americana (Drury) of lettuce contaminated with As when given a choice with low-As treated plants (46.14 ± 22 vs. 2.3 ± 0.2 mg kg−1). They showed adult S. americana took taste bites before rejecting highly contaminated lettuce, indicating As is detected through gustation.
In polluted areas, metals often exist as simple or complex mixtures. Migula & Binkowska (1993) investigated the ability of populations of Chorthippus spp. (Orthoptera: Acrididae) from heavily and weakly polluted sites to distinguish between Cd, lead (Pb), and Cd + Pb exposed diets. They found that grasshoppers locally adapted in weakly polluted sites did not have the ability to distinguish between leaves with different metal concentrations, whereas those from heavily polluted sites reduced their consumption rate with increasing Cd and Pb concentrations. This may indicate learned avoidance behavior in Chorthippus populations living in taxing environments. In a different experiment examining the effects of Cd alone, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) also experienced a significant decrease in feeding, as measured by the ‘leaf feeding damage index’ for treatment concentrations ranging from 0 to 300 mg kg−1 in Thlaspi caerulescens J. & C. Presl (Brassicaceae) varieties (Jiang et al., 2005).
A subset of research investigating metal impacts on ingestion behaviors examined plant biocontrol agents, with varying results. First-instar Bactra verutana Zeller (Lepidoptera: Tortricidae) exposed to purple nutsedge for up to 4 weeks were unaffected by Cd concentrations up to 18 μg g−1 (Quimby et al., 1979). Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae) exposed to 8.7 μg Cd g−1 alligatorweed showed an inability to distinguish between Cd contaminated and uncontaminated plants, though they did experience feeding depression when fed on Cd-contaminated leaves in choice experiments (Quimby et al., 1979).
Neochetina bruchi Hustache (Coleoptera: Curculionidae) is used in the control of water hyacinth, an emergent, metal-accumulating aquatic plant, and spends its life on the leaf surface. When separately exposed to Cd and Zn, Jamil et al. (1989a,b) found a significant decrease in the number of water hyacinth feeding lesions, reflecting a decrease in feeding activity with increasing exposure concentration for both metals. There was no significant difference between numbers of feeding lesions found in plants accumulating up to 89.5 and 165 μg Zn/100 g dry weight; however, lesions were significantly fewer when N. bruchi were fed on plants accumulating 232 μg Zn/100 g dry weight (Jamil et al., 1989a,b). Cd exposure accumulating to levels of 3.78, 6.20, and 66.70 μg/100 g dry weight showed the same pattern of feeding depression (Jamil et al., 1989a), with no effect of the lower concentrations on number of feeding lesions. This finding supports results by Quimby et al. (1979).
A different species of water hyacinth beetle, Neochetina eichhorniae Warner, had conflicting behavioral outcomes in the presence of Cd when compared with N. bruchi (Kay & Haller, 1986). Water hyacinth with 8.00 and 17.20 μg Cd g−1 leaves did not experience decreased feeding activity of N. eichhorniae when compared with controls. Neochetina eichhorniae feeding activity when exposed to 21.62 and 44.77 μg g−1 Cu and 5.89 and 9.84 μg g−1 Pb was also not significantly different from controls. Kay & Haller (1986) exposed beetles to contaminated water hyacinth for 10 days, vs. Jamil et al. (1989a,b) who exposed beetles for 7 days, and did not report feeding depression at any point during their assays. This implies that N. eichhorniae is more tolerant of metals uptake by water hyacinth than N. bruchi.
Other studies on the antifeedant effects of metals on beetles showed a consistent decrease in feeding activity as a result of dietary exposure. Third instars of Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) exposed to CuSO4 (El-Bassiouny, 1991) and Pb(NO3)2 (Kwartirnikov et al., 1999) significantly decreased feeding activity compared with controls, which were more pronounced for increasing concentrations of each antifeedant. Adult L. decemlineata exposed to Pb(NO3)2 also showed decreased feeding activity, though this result was less pronounced than for larvae (Kwartirnikov et al., 1999).
Complex preference experiments by Rokytová et al. (2004) revealed that adult Lochmaea capreae L. (Coleoptera: Chrysomelidae) did not alter feeding activity on birch leaves dipped in Cd (2–250 μg ml−1) and Pb (4–500 μg ml−1). For manganese (Mn) and Zn, feeding activity significantly decreased between low and high concentrations (100–500 and 10 000 μg Mn ml−1, and 80–400 and 8 000 μg Zn ml−1, respectively). They also avoided high concentrations of Mn and Zn more often than all concentrations of Cd and Pb. Another chrysomelid, Melasoma lapponica L. (Coleoptera: Chrysomelidae) showed a preference for very high (273.3 mg Ni kg−1 and 95.4 mg Cu kg−1) and very low (27.7 mg Ni kg−1 and 16.9 mg Cu kg−1) concentrations of Cu and Ni in willow foliage along a distance gradient from a smelter (Zvereva & Kozlov, 1996). For this species, some feeding was necessary on undamaged leaves before rejection, though damaged leaves with metal exposures were identified and rejected before feeding, possibly due to an increased release of deterrent substances (Zvereva & Kozlov, 1996).
Finally, a handful of studies have examined the effects of metals on soil-dwelling invertebrate feeding behavior. Orchesella cincta (L.) (Collembola: Entomobryidae) showed no significant preference for green algae diet contaminated with Pb(NO3)2 up to 1 600 μg g−1, and the authors concluded that avoidance was not necessary due to efficient excretion mechanisms already in place (van Capelleveen et al., 1986). Fountain & Hopkin (2001) reported significant avoidance of Pb-contaminated diet at 2 170 μg g−1 for Folsomia candida Willem (Collembola: Isotomidae), but no significant avoidance of diet contaminated with 406 μg g−1 Pb, consistent with van Capelleveen et al. (1986). Orchesella cincta was similarly unaffected by Mn in the diet up to 9.2 ± 0.3 μmol g−1 dry mass, though iron (Fe) caused a significant decrease in feeding activity, especially at higher concentrations (Nottrot et al., 1987). In a field study, Sphaeridia pumilis Krausbauer (Collembola: Sminthuridae), Parisotoma notabilis (Schäffer) (Collembola: Isotomidae), and Mesaphorura macrochaeta (Rusek) (Collembola: Onychiuridae) gut contents reflected preferential avoidance of the organic horizon where the majority of Cd, Pb, and Zn were concentrated (Gillet & Ponge, 2003).
Copper-contaminated diet significantly deterred F. candida at 1 500 μg g−1 dry weight in the laboratory (Filser & Hölscher, 1997), and Onychiurus armatus (Tullberg) (Collembola: Onychiuridae) with a 13.5% Cu solution-soaked diet (Filser et al., 2000). By contrast, Isotomurus palustris (Müller) (Collembola: Isotomidae) fed on diet with Cu contamination as often as on uncontaminated diet, and F. quadrioculata and F. manolachei preferred Cu-contaminated diet (Filser et al., 2000). Folsomia candida significantly avoided Cu-contaminated yeast at concentrations exceeding 10 μg g−1 (Fountain & Hopkin, 2001). Folsomia candida also avoided yeast contaminated with Cd at concentrations exceeding 28 μg g−1, and always preferred controls to Zn-contaminated diet (Fountain & Hopkin, 2001).
Taxis behavior Taxis is an oriented movement in response to a directional stimulus or a stimulus gradient. All of the available research investigating locomotory effects of pollution has focused on carabids and collembolans, with the exception of one study investigating pupation-site preference in D. melanogaster. Unfortunately, the studies appear to be contradictory, making deduction of patterns impossible. Bayley et al. (1995) found that Pterostichus cupreus L. (Coleoptera: Carabidae) larvae exposed to 500 μg Cu g−1 in soil and diet experienced severely impaired locomotion as adults. This resulted in decreased prey capture success as adults, despite the absence of antifeedant properties in the larval diet (Bayley et al., 1995). Though collembolans were shown to avoid Cu-contaminated diets, they tended to not avoid Cu-contaminated soils, the exceptions being M. macrochaeta and Folsomia manolachei Bagnall (Filser & Hölscher, 1997) and Pseudosinella alba (Packard) (Collembola: Entomobryidae) (Filser et al., 2000).
By contrast, Lock et al. (2001) sampled seven sites within the vicinity of an abandoned Pb-Zn mine and found no significant relationship between relative activity of carabids and measureable metal concentrations in soils. Activity was measured using pitfall traps in combination with diversity sampling to determine whether certain species were more active given particular soil metal concentrations. The lack of significance despite a gradient in total Pb, Cd, and Cu concentrations was attributed to these metals not being bioavailable to the predatory carabids (Lock et al., 2001).
Parisotoma notabilis exposed to a gradient of Cd, Pb, and Zn pollution in the field followed the distribution of its weakly polluted food source by shifting position in the soil from surface to deeper horizons, and thus avoided changing its feeding habits (Gillet & Ponge, 2003). Using methodology identical to that of Lock et al. (2001, 2003) determined that a location with similar contaminants as that of Gillet & Ponge (2003) showed no significant relationship between activity of collembolans and metal concentrations in soils. This difference in collembolan activity may have been caused by the high concentrations of Cd, Pb, and Zn reported in Gillet & Ponge (2003), compared with those in Lock et al. (2001). Other studies examining mixtures of metals in contaminated soils revealed that F. candida consistently avoided heavily contaminated soils, though there was high variability in individual response (Natal da Luz et al., 2004). Reporting an overall response of a conglomerate soil insect fauna, Gongalsky et al. (2009) found consistent avoidance of heavily contaminated soils. Collectively, the broadly contradictory results from the available research suggest there are many environmental factors that may influence movement. Thus, substantial opportunities exist for additional research on this topic.
In choice trials, Bahadorani & Hilliker (2009) found no significant difference in pupation-site preference in late-instar D. melanogaster when presented with normal food and food with a concentration of 70 mmol Zn l−1. Larvae did, however, significantly prefer normal food to Cu-contaminated food (20 mmol l−1). They also significantly preferred pupating in Fe (II)-contaminated food (70 mmol l−1) to non-contaminated food.
Oviposition To date, only five publications reported the effects of metals on ovipositional response in terrestrial insects. When given a choice between concentrations of ovipositional substrate exposed to hexavalent chromium (Cr VI) (50, 500, 1 000 μg g−1), Trumble & Jensen (2004) found that females of Megaselia scalaris Loew (Diptera: Phoridae) did not discriminate between control, low, and high concentrations. This occurred despite the observation that the highest level was toxic to the larvae. In a different study on a dipteran, Bahadorani & Hilliker (2009) reported that mated female D. melanogaster significantly decreased egg laying at relatively high concentrations of heavy metals (2 mmol l−1 Cd, 10 mmol l−1 Cu, 40 mmol l−1 Fe (II), 20 mmol l−1 Fe (III), and 30 mmol l−1 Zn). Interestingly, for both Fe and Zn, oviposition increased significantly relative to controls (0 mmol l−1 for each) at lower concentrations. This indicates that the female not only senses metals in the environment, but also knows which concentrations will maximize the fitness of her offspring.
Two studies were available that examined oviposition by S. exigua in response to Se. Females preferred to oviposit on low concentrations of Se-treated alfalfa (2.88 ± 0.52 μg Se g−1 dry weight) over controls (Vickerman et al., 2002b). However, adult females were unable to distinguish between low and high (305.81 ± 52.14 μg Se g−1 dry weight) concentrations of Se for oviposition, despite the fact that the high level was toxic. In a second study examining oviposition on Atriplex spp. plants which accumulate Se, S. exigua did not distinguish between plants that contained concentrations of Se that were toxic to their offspring (Vickerman et al., 2002a). For both M. scalaris and S. exigua, the inability to distinguish between lethal concentrations of Cr VI and Se, respectively, puts eggs and larvae at risk at exposed oviposition sites.
Finally, the newly discovered P. xylostella Stanleyi biotype does not differentiate between high and low concentrations of Se in S. pinnata when ovipositing. This is in direct contrast to a different ecotype, P. xylostella G88, and P. rapae, which avoided ovipositing on highly Se-contaminated plants (Freeman et al., 2006).