Metal concentrations of insects associated with the South African Ni hyperaccumulator Berkheya coddii （Asteraceae）

The high levels of some metals in metal hyperaccumulator plants may be transferred to insect associates. We surveyed insects collected from the South African Ni hyperaccumulator Berkheya coddii to document whole-body metal concentrations （Co, Cr, Cu, Mg, Mn, Ni, Pb, Zn）. We also documented the concentrations of these metals in leaves, stems and inflorescences, finding extremely elevated levels of Ni （4 700-16 000μg/g） and high values （5-34μg/g） for Co, Cr, and Pb. Of 26 insect morphotypes collected from B. coddii, seven heteropterans, one coleopteran, and one orthopteran contained relatively high concentrations of Ni （〉 500μg/g）. The large number of high-Ni heteropterans adds to discoveries of others （from California USA and New Caledonia） and suggests that members of this insect order may be particularly Ni tolerant. Nymphs of the orthopteran （Stenoscepa） contained 3 500 μg Ni/g, the greatest Ni concentration yet reported for an insect. We also found two beetles with elevated levels of Mg （〉 2 800 μg/g）, one beetle with elevated Cu （〉 70 μg/g） and one heteropteran with an elevated level of Mn （〉 200 μg/g）. Our results show that insects feeding on a Ni hyperaccumulator can mobilize Ni into food webs, although we found no evidence of Ni biomagnification in either herbivore or carnivore insect taxa. We also conclude that some insects associated with hyperaccumulators can contain Ni levels that are high enough to be toxic to vertebrates.

Host-herbivore studies of Stenoscepa sp. （Orthoptera：Pyrgomorphidae）, a high-Ni herbivore of the South African Ni hyperaccumulator Berkheya coddii （Asteraceae）

Nymphs of Stenoscepa sp. feed on leaves of the Ni hyperaccumulator Berkheya coddii at serpentine sites in Mpumalanga Province, South Africa. These sites contain Ni hyperaccumulators, Ni accumulators, and plants with Ni concentrations in the normal range. We conducted studies to： （i） determine the whole-body metal concentration of nymphs （including those starved to empty their guts）; （ii） compare Stenoscepa sp. nymphs against other grasshoppers in the same habitat for whole-body metal concentrations; and （iii）compare the suitability of Ni hyperaccumulator and Ni accumulator plants as food sources for Stenoscepa sp. and other grasshoppers. Stenoscepa nymphs had extremely high whole-body Ni concentrations （3 500μg Ni/g）. This was partly due to food in the gut, as starved insects contained less Ni （950 pg Ni/g）. Stenoscepa nymphs survived significantly better than other grasshoppers collected from either a serpentine or a non-serpentine site when offered high-Ni plants as food. In a host preference test among four Berkheya species （two Ni hyperaccumulators and two Ni accumulators）, Stenoscepa sp, preferred leaves of the Ni hyperaccumulator species. A preference experiment using leaves of three Senecio species （of which one species, Senecio coronatus, was represented by both a Ni hyperaccumulator and a Ni accumulator population） showed that Stenoscepa sp. preferred Ni accumulator Senecio coronatus leaves to all other choices. We conclude that Stenoscepa sp. is extremely Ni-tolerant. Stenoscepa sp. nymphs prefer leaves of hyperaccumulator Berkheya species, but elevated Ni concentration alone does not determine their food preference. We suggest that the extremely high whole-body Ni concentration of Stenoscepa nymphs may affect food web relationships in these serpentine communities.

High-nickel insects and nickel hyperaccumulator plants： A review

Insects can vary greatly in whole-body elemental concentrations. Recent investigations of insects associated with Ni hyperaccumulator plants have identified insects with relatively elevated whole-body Ni levels. Evaluation of the limited data available indicates that a whole-body Ni concentration of 500μg Ni/g is exceptional： I propose that an insect species with a mean value of 500μg Ni/g or greater, in either larval/nymphal or adult stages, be considered a ＂high-Ni insect＂. Using the 500μg Ni/g criterion, 15 species ofhigh-Ni insects have been identified to date from studies in Mpumalanga （South Africa）, New Caledonia and California （USA）. The highest mean Ni concentration reported is 3 500 μg Ni/g for nymphs of a South African Stenoscepa species （Orthoptera： Pyrgomorphidae）. The majority of high-Ni insects （66%） are heteropteran herbivores. Studies of high-Ni insect host preference indicate they are monophagous （or nearly so） on a particular Ni hyperaccumulator plant species. Much of the Ni in bodies of these insects is in their guts （up to 66%-75%）, but elevated levels have also been found in Malpighian tubules, suggesting efficient elimination as one strategy for dealing with a high-Ni diet. Tissue levels of Ni are generally much lower than gut concentrations, but up to 1200μg Ni/g has been reported from exuviae, suggesting that molting may be another pathway of Ni elimination. One ecological function of the high Ni concentration of these insects may be to defend them against natural enemies, but to date only one experimental test has supported this ＂elemental defense＂ hypothesis. Community-level studies indicate that high-Ni insects mobilize Ni into food webs but that bioaccumulation of Ni does not occur at either plant-herbivore or herbivorepredator steps. Unsurprisingly, Ni bioaccumulation indices are greater for high-Ni insects compared to other insect species that feed on Ni hyperaccumulator plants. There is some evidence of Ni mobilization into food webs by insect visitors to flowers ofNi hyperaccumulator plants, but no high-Ni insect floral visitors have been reported.

Nickel levels in arthropods associated with Ni hyperaccumulator plants from an ultramafic site in New Caledonia

Arthropods （mainly insects） were collected from a forest site that contained at least six species of Ni hyperaccumulators. Whole body Ni analysis was performed for 12 arthropod taxa, two of which were studied at different life cycle stages. We found two Nitolerant insects. The pentatomid heteropteran Utana viridipuncta, feeding on fruits of the Ni hyperaccumulator Hybanthus austrocaledonicus, contained a mean of 2 600 μg Ni/g in nymphs and 750μg Ni/g in adults. The tephritid fly Bactrocera psidii, feeding on pulp of Sebertia acuminata fruits that contained 6 900μg Ni/g, contained 420μg Ni/g as larvae that had evacuated their guts and significantly less （65μg Ni/g） as adults. European honeybees （Apis mellifera） visiting flowers of the Ni hyperaccumulator H. austrocaledonicus contained significantly more Ni （8-fold more） than those collected from flowers of Myodocarpus fraxinifolius, a non-hyperaccumulator. Our results show that some insects feed on Ni hyperaccumulator plants and that their feeding mobilizes Ni into local food webs.

Lygus hesperus （Heteroptera： Miridae） tolerates high concentrations of dietary nickel

Nickel hyperaccumulator plants contain unusually elevated levels of Ni （〉 1 000 μg Ni/g）. Some insect herbivores, including Lygus hesperus （Western tarnished plant bug）, have been observed feeding on the California Ni hyperaccumulator Streptanthus polygaloides. This bug may be able to utilize S. polygaloides as a host either through its feeding behavior or by physiological tolerance of Ni. This experiment determined the Ni tolerance of L hesperus by offering insects artificial diet amended with 0, 0.4, 1, 2, 4.5, 10, 20 and 40 mmol Ni/L and recording survival. Survival varied due to Ni concentration, with diets containing 10 mmol Ni/L and greater resulting in significantly lower survival compared to the control （0 mmol Ni/L） treatment. Insects tolerated diet containing as much as 4.5 mmol Ni/L, a relatively elevated Ni concentration. I conclude that L hesperus can feed on S. polygaloides because it is Ni-tolerant, probably due to physiological mechanisms that provide it with resistance to plant chemical defenses including elemental defenses such as hyperaccumulated Ni.

Host plant selection of Chrysolina clathrata （Coleoptera： Chrysomelidae） from Mpumalanga, South Africa

Hyperaccumulated elements such as Ni may defend plants against some natural enemies whereas other enemies may circumvent this defense. The Ni hyperaccumulator Berkheya coddii Roessler （Asteraceae） is a host plant species for Chrysolina clathrata （Clark）, which suffers no apparent harm by consuming its leaf tissue. Beetle specimens collected from B. coddii had a whole body Ni concentration of 260μg/g dry weight, despite consuming leaf material containing 15 100μg Ni/g. Two experiments were conducted with adults of this beetle species： a no-choice experiment and a choice experiment. In the nochoice experiment we offered beetles foliage of one of four speeies ofBerkheya： B. coddii, B. rehmannii Thell. var. rogersiana Thell., B. echinacea （Harv.） O. Hoffm. ex Burtt Davey, and B. insignis （Harv.） Thell. The two former species are Ni hyperaccumulators （defined as having leafNi concentration 〉 1 000 μg/g） whereas the latter have low Ni levels （〈 200 μg/g） in their leaves. Masses of beetles were monitored for 6 days. Choice experiments used growing stem tips from the same Berkheya species, placed into Petri dishes with five Chrysolina beetles in each, and the amount of feeding damage caused on each of the four species was recorded. Beetles in the no-choice experiment gained mass when offered B. coddii, maintained mass on leaves of the other Ni hyperaccumulator （B. rehmannii var. rogersiana）, and lost mass when offered non-hyperaccumulator leaves. In the choice test, beetles strongly preferred B. coddii to other Berkheya species. We conclude that C. clathrata may be host-specific on B. coddii.