Herbivore-constructed leaf shelters on Salix eriocarpa shoots affect arthropod communities

We observed the abundance of leaf shelters, aphids, other herbivores, and predators on willow trees, Salix eriocarpa, from May to October 2003. There was a positive correlation between the growth rate of aphids and the number of ants per shoot, suggesting ant attendance to aphids. Although the mean abundance of leaf shelters per shoot was rather low (1.7–2.2) throughout the observation period, aphids preferred to use shoots with leaf shelters compared with those without leaf shelters. The abundance of ants was positively influenced by the presence of leaf shelters and aphids from May to August. The abundance of other herbivores was positively influenced by leaf shelters, but negatively influenced by aphid presence from May to August. Furthermore, leaf shelters had a positive effect on the abundance of predators from July to October. These data suggest that a relatively low abundance of naturally occurring leaf shelters per shoot influenced the arthropod communities on S. eriocarpa, and the effect of those leaf shelters on each type of arthropod varied according to the season.     

Community‐wide impact of an exotic aphid on introduced tall goldenrod

1. The aphid Uroleucon nigrotuberculatum Olive, which is specialised to the tall goldenrod, Solidago altissima L., in its native range, has become a dominant species on the introduced tall goldenrod in Japan. How this exotic aphid influenced arthropod communities on the introduced tall goldenrod in aphid‐present (spring) and aphid‐absent (autumn) seasons was examined, using an aphid removal experiment. 2. In spring, aphid presence increased ant abundance because aphid honeydew attracted foraging ant workers. A significant negative correlation was found between the numbers of ants and herbivorous insects other than aphids on the aphid‐exposed plants, but no significant correlation was detected on the aphid‐free plants. Thus, the aphid presence was likely to decrease the abundance of co‐occurring herbivorous insects through removal behaviour of the aphid‐tending ants. There were no significant differences in plant traits between the aphid‐exposed and aphid‐free plants. 3. In autumn, the numbers of lateral shoots and leaves, and the leaf nitrogen content were increased in response to the aphid infestation in spring. Because of the improvement of plant traits by aphid feeding, the abundance of leaf chewers increased on aphid‐exposed plants. In contrast, the abundance of sap feeders decreased on the aphid‐exposed plants. In particular, the dominant scale insect among sap feeders, Parasaissetia nigra Nietner, decreased, followed by a decrease in the abundance of ants attending P. nigra. Thus, aphid feeding may have attenuated the negative impacts of the tending ants on leaf chewers. 4. Aphid presence did not change herbivore species richness but changed the relative density of dominant herbivores, resulting in community‐wide effects on co‐occurring herbivores through ant‐mediated indirect effects, and on temporally separated herbivores through plant‐ and ant‐mediated indirect effects. The aphid also altered predator community composition by increasing and decreasing the relative abundance of aphid‐tending ants in the spring and autumn, respectively.     

Tri-trophic level interactions affect host plant development and abundance of insect herbivores

Understanding the interactions among plants, hemipterans, and ants has provided numerous insights into a range of ecological and evolutionary processes. In these systems, however, studies concerning the isolated direct and indirect effects of aphid colonies on host plant and other herbivores remain rare at best. The aphid Uroleucon erigeronensis forms dense colonies on the apical shoots of the host plant Baccharis dracunculilfolia (Asteraceae). The honeydew produced by these aphids attracts several species of ants that might interfere with other herbivores. Four hypotheses were tested in this system: (1) ants tending aphids reduce the abundance of other herbivores; (2) the effects of ants and aphids upon herbivores differ between chewing and fluid-sucking herbivores; (3) aphids alone reduce the abundance of other herbivores; and (4), the aphid presence negatively affects B. dracunculifolia shoot growth. The hypotheses were evaluated with ant and aphid exclusion experiments, on isolated plant shoots, along six consecutive months. We adjusted linear mixed-effects models for longitudinal data (repeated measures), with nested spatial random effect. The results showed that: (1) herbivore abundance was lower on shoots with aphids than on shoots without aphids, and even lower on shoots with aphids and ants; (2) both chewing and fluid-sucking insects responded similarly to the treatment, and (3) aphid presence affected negatively B. dracunculifolia shoot growth. Thus, since aphids alone changed plant growth and the abundance of insect herbivores, we suggest that the ant–aphid association is important to the organization of the system B. dracunculifolia-herbivorous insects.     

Larvae of the green lacewing <Emphasis Type="Italic">Mallada desjardinsi</Emphasis> (Neuroptera: Chrysopidae) protect themselves against aphid-tending ants by carrying dead aphids on their backs

Larvae of the green lacewing Mallada desjardinsi Navas are known to place dead aphids on their backs. To clarify the protective role of the carried dead aphids against ants and the advantages of carrying them for lacewing larvae on ant-tended aphid colonies, we carried out some laboratory experiments. In experiments that exposed lacewing larvae to ants, approximately 40% of the larvae without dead aphids were killed by ants, whereas no larvae carrying dead aphids were killed. The presence of the dead aphids did not affect the attack frequency of the ants. When we introduced the lacewing larvae onto plants colonized by ant-tended aphids, larvae with dead aphids stayed for longer on the plants and preyed on more aphids than larvae without dead aphids. Furthermore, the lacewing larvae with dead aphids were attacked less by ants than larvae without dead aphids. It is suggested that the presence of the dead aphids provides physical protection and attenuates ant aggression toward lacewing larvae on ant-tended aphid colonies.     

Effects of aphids on foliar foraging by Argentine ants and the resulting effects on other arthropods

Abstract.  1. Although interactions between ants and honeydew-producing insects have received considerable study, relatively little is known about how these interactions alter the behaviour of ants in ways that affect other arthropods. In this study, field and greenhouse experiments were performed that examined how the presence of aphids ( Aphis fabae solanella ) on Solanum nigrum influenced the foraging behaviour of Argentine ants ( Linepithema humile ) and, in turn, modified the extent to which ants deter larval lacewings ( Chrysoperla rufilabris ), which are known aphid predators.
2. A field experiment demonstrated that the level of foliar foraging by ants increased linearly with aphid abundance, whereas no relationship existed between the level of ground foraging by ants and aphid abundance.
3. In the greenhouse, as in the field, foliar foraging by ants greatly increased when aphids were present. Higher levels of foliar foraging led to a twofold increase in the likelihood that ants contacted aphid predators. As a result of these increased encounters with ants, lacewing larvae were twice as likely to be removed from plants with aphids compared with plants without aphids. Once contact was made, however, the behaviour of ants towards lacewing larvae appeared similar between the two experimental groups.
4. Argentine ants drive away or prey upon a diversity of arthropod predators and parasitoids, but they also exhibit aggression towards certain herbivores. Future work should attempt to quantify how the ecological effects that result from interactions between honeydew-producing insects and invasive ants, such as L. humile , differ from those that result from interactions between honeydew-producing insects and native ants.     

Ant- and plant-mediated indirect effects induced by aphid colonization on herbivorous insects on tall goldenrod

We studied the indirect effects of an aphid Uroleucon nigrotuberculatum on density and performance of herbivorous insects through tending ants and modification of plant traits on a tall goldenrod Solidago altissima in Japan. To examine ant-mediated indirect effects of the aphid on the leafhopper and geometrid moth caterpillars, we conducted an experiment in which we manipulated aphid densities. The aphid decreased the density of these herbivorous insects through ant-mediated indirect effects, because honeydew scattered by the aphid-attracted ants that then removed them. To examine plant-mediated indirect effects of the aphid on two temporally separated insects, a scale insect and a grasshopper, we compared the density and performance of these herbivorous insects on aphid-inoculated plants and aphid-free plants. Aphid-induced plant modifications had different effects on the scale insect and grasshopper. The aphid indirectly decreased the density and survivorship of the scale insect. On the other hand, the number of grasshoppers increased as a result of the increased number of leaves and the increased nitrogen content induced by prior aphid feeding. However, aphid infestation did not affect the survival of the grasshopper. Thus, the aphid has large indirect effects on co-occurring herbivorous insects through the removal behavior of tending ants and on temporally separated herbivorous insects through changes in quality and quantity of the tall goldenrod.     

Preference for protected feeding sites by larvae of the willow-feeding leaf beetle Galerucella lineola

1. Free-living insects are often thought of as more vulnerable to environmental hazards than concealed insects, such as galling or mining insects. The possibility that larvae of the free-living leaf beetle Galerucella lineola seek out existing plant structures and thereby become partly concealed was explored.
2. Neonate larvae of G. lineola frequently feed in rolled-in margins of young leaves of their host plant, Salix viminalis . In addition to nutritional benefits from feeding on young leaves, larvae may also gain protection against adverse weather conditions and general predators by feeding in the leaf rolls. Field and laboratory experiments were conducted to test these hypotheses.
3. Artificial shelters were constructed and cohorts of neonate larvae were placed on experimental plants. In all experiments, larvae preferred to feed in shelters, even when shelters were constructed on mature leaves.
4. In one of the experiments, fewer larvae disappeared when shelters were provided. In a predator exclusion experiment, however, no differences in predator-inflicted mortality on G. lineola were found between shelter-containing shoots and control shoots.
5. A laboratory experiment showed increased protection from desiccation when shelters were present; growth rate was higher for larvae feeding on plants with shelters.
6. Thus, free-living insects may not always be as exposed to environmental hazards as is often assumed. In particular, young larvae may take advantage of preformed structures on their host plant and feed in a concealed microhabitat. Because mortality, in general, is high during early instars, shelter-seeking behaviour may increase survival significantly. The existence of preformed shelters may therefore be a plant characteristic that should be considered when exploring the environmental risks associated with the free-living habit.     

Species-specific bird functions in a forest-canopy food web

Bird functions in a forest-canopy food web were evaluated by a large-scale field experiment using 'canopy' enclosures. By controlling the presence of two bird species, great tits (Parus major; foliage gleaner) and nuthatches (Sitta europaea; trunk gleaner), in the enclosures, their effect on predatory insects (ants), herbivorous insects (Lepidoptera larvae) and producers (oak trees) was quantified. Great tits reduced the density of Lepidoptera larvae and, indirectly, leaf damage, but had no impact on ants. Nuthatches decreased the density of ants but did not influence either Lepidoptera larvae or leaf damage. These results highlight species-specific functions of birds in the maintenance of forest ecosystems.     

Predator and parasitoid attacking ant-attended aphids: effects of predator presence and attending ant species on emerging parasitoid numbers

Interaction between a predator and a parasitoid attacking ant-attended aphids was examined in a system on photinia plants, consisting of the aphid Aphis spiraecola, the two ants Lasius japonicus and Pristomyrmex pungens, the predatory ladybird beetle Scymnus posticalis, and the parasitoid wasp Lysiphlebus japonicus. The ladybird larvae are densely covered with waxy secretion and are never attacked by attending ants. The parasitoid females are often attacked by ants, but successfully oviposit by avoiding ants. The two ants differ in aggressiveness towards aphid enemies. Impacts of the predator larvae and attending ant species on the number of parasitoid adults emerging from mummies per aphid colony were assessed by manipulating the presence of the predator in introduced aphid colonies attended by either ant. The experiment showed a significant negative impact of the predator on emerging parasitoid numbers. This is due to consumption of healthy aphids by the predator and its predation on parasitized aphids containing the parasitoid larvae (intraguild predation). Additionally, attending ant species significantly affected emerging parasitoid numbers, with more parasitoids in P. pungens-attended colonies. This results from the lower extent of interference with parasitoid oviposition by the less aggressive P. pungens. Furthermore, the predator reduced emerging parasitoid numbers more when P. pungens attended aphids. This may be ascribed to larger numbers of the predator and the resulting higher levels of predation on unparasitized and parasitized aphids in P. pungens-attended colonies. In conclusion, a negative effect of the predator on the parasitoid occurs in ant-attended aphid colonies, and the intensity of the interaction is affected by ant species.     

Effects of plant protease inhibitors, oryzacystatin I and soybean Bowman-Birk inhibitor, on the aphid Macrosiphum euphorbiae (Homoptera, Aphididae) and its parasitoid Aphelinus abdominalis (Hymenoptera, Aphelinidae)

Transgenic plants expressing protease inhibitors (PIs) have emerged in recent years as an alternative strategy for pest control. Beneficial insects such as parasitoids may therefore be exposed to these entomotoxins either via the host or by direct exposure to the plant itself. With the objective of assessing the effects of PIs towards aphid parasitoids, bioassays using soybean Bowman-Birk inhibitor (SbBBI) or oryzacystatin I (OCI) on artificial diet were performed on Macrosiphum euphorbiae-Aphelinus abdominalis system. OCI significantly reduced nymphal survival of the potato aphid M. euphorbiae and prevented aphids from reproducing. This negative effect was much more pronounced than with other aphid species. On the contrary, SbBBI did not affect nymphal viability but significantly altered adult demographic parameters. Enzymatic inhibition assays showed that digestive proteolytic activity of larvae and adults of Aphelinus abdominalis predominantly relies on serine proteases and especially on chymotrypsin-like activity. Immunoassays suggested that OCI bound to aphid proteins and accumulated in aphid tissues, whereas SbBBI remained unbound in the gut. Bioassays using M. euphorbiae reared on artificial diets supplemented with both OCI and SbBBI showed a fitness impairment of Aphelinus abdominalis that developed on intoxicated aphids. However, only SbBBI was detected in parasitoid larvae, while no PI could be detected in adult parasitoids that emerged from PI-intoxicated aphids. The potential impact of PI-expressing plants on aphid parasitoids and their combined efficiency for aphid control are discussed.     

The response of an aphid tending ant to artificial extra-floral nectaries on different host plants

Sap-feeding homopterans, which reduce the fitness of their host plants, are often tended by ants that feed on their honeydew. The composition of the honeydew varies with both the aphid and the host plant. Extra-floral nectaries (EFNs) are believed to have evolved to attract attending ants, protecting the hosts, but it is unknown if EFNs on different plants have the same impact on the relations between an aphid species feeding on those plants and its tending ant. Experimental research was conducted to examine the attraction of Tapinoma erraticum scout ants to honeydew from the aphid Aphis gossypii feeding on two different plants, Prunus amygdalus and Mentha piperita, negligence of tending the aphids, and survival of the aphids in the presence of artificial EFNs. The scout ants were significantly more attracted to artificial nectar dispensed on P. amygdalus leaves than on M. piperita, or aphids on both plants and water. They neglected aphids in the presence of artificial EFNs on M. piperita but not on P. amygdalus. The aphid population on M. piperita did not statistically change in the presence of artificial EFNs during the 8 days of the third experiment. On P. amygdalus, the aphids succeeded in developing fully to winged form. In conclusion, the responses of the ants tending aphids to the presence of artificial EFNs were influenced by the host plant.     

Aphid-ant interaction reduces chrysomelid herbivory in a cottonwood hybrid zone

In a cottonwood (Populus) hybrid zone, Chaitophorus aphids attract aphid-tending ants which subsequently reduce herbivory by the leaf-feeding beetle, Chrysomela confluens. Observations and experimental manipulations of aphids and beetle larvae on immature cottonwood trees demonstrated that: 1) via their recruitment of ants, aphids reduced numbers of beetle eggs and larvae on the host; 2) these interactions occurred within a few days of the host being colonized by aphids; and 3) although aphid colonies were ephemeral, their presence resulted in a 2-fold reduction in beetle herbivory. The aphid-ant interaction is most important in the hybrid zone where 93% of the beetle population is concentrated (for reasons unrelated to aphids and ants). Because beetle defoliation of immature trees is high (ca. 25%), the indirect effect of aphids in reducing herbivory is likely more beneficial to trees in the hybrid zone than in adjacent pure zones where beetle herbivory is virtually absent. Tree genotype likely affects the impact of the aphid-ant interaction on trees within the hybrid zone, since levels of herbivory differ between sympatric Fremont and hybrid cottonwoods.     

The effects of the invasive Argentine ant (Linepithema humile) and the native ant Prenolepis imparis on the structure of insect herbivore communities on willow trees (Salix lasiolepis)

Abstract 1. We examined the relative effects of the invasive Argentine ant, Linepithema humile, and a common native ant, Prenolepis imparis, on the community of herbivorous insects occurring on willow trees, Salix lasiolepis in Northern California, U.S.A. 2. Using paired control and treatment branches from which we excluded ants and other non‐volant predators, we found that effects of Argentine ants on the herbivore community were generally similar to those of P. imparis. Argentine ants and P. imparis suppressed the damage by skeletonising insects by 50%, but had little effect on most other external‐feeding or internal‐feeding guilds. 3. The abundance of aphids was 100% greater in the presence of Argentine ants, but there was no effect on aphid numbers in the presence of P. imparis. Late season aphid numbers were substantially higher in the presence of Argentine ants, but not P. imparis. 4. The effects of Argentine ants on skeletonising insects and aphids combined with the overwhelming abundance of Argentine ant workers, suggests that they may have substantial, but often overlooked, effects on the herbivore communities on other plant species in or near riparian habitats in which they invade.     

The effect size of aphid‐tending ants in an agricultural tri‐trophic system

Most studies regarding ant–aphid interactions focus only on the direct effects of ants on tended aphids and aphidophagous predators, or the indirect effects on the host plant. Studies evaluating the effects of aphid‐tending ants on more than one trophic level are rare and evaluate only the presence or absence of such effects. Here we assessed the effect sizes of ants in a tri‐trophic system (common bean plants, aphids and lacewing larvae). We tested if the presence of aphid‐tending ants has positive effects on aphid abundance and host‐plant production and negative effects on aphid predator abundance. We also hypothesized that aphid‐tending ants affect more intensely trophic levels that are more directly related to them (i.e., first aphids, then aphid predators and then host plants). We tested these hypotheses in field mesocosms experiments using the presence and absence of ants. We found that aphid‐tending ants have great positive effects on final aphid abundance. Ants also positively affected the number of seeds; however, it was not possible to measure the effect size for this trophic level. Furthermore, ants had negative effects on lacewing larvae only at first release. The effect size of ants was greater for aphids, followed by lacewing larvae, and with no effects on the number of seeds produced. Ants positively affect aphids and host‐plant production, probably by way of honeydew collection preventing the development of entomophagous/saprophytic fungi. On the other hand, ants negatively affect lacewing larvae by excluding them from the host plant. In natural systems, several ant species may attend aphids, differently affecting the organisms of the various trophic levels within the ant–aphid interaction, thereby obscuring the real effect size of ants. Assessing the effect size of aphid‐tending ants on the organisms involved in ant–aphid interactions provides more realistic information about the effects of this interaction on natural systems.     

Why do ants shift their foraging from extrafloral nectar to aphid honeydew?

When aphids parasitize plants with extrafloral nectaries (EFNs) and aphid colony size is small, ants frequently use EFNs but hardly tend aphids. However, as the aphid colony size increases, ants stop using EFNs and strengthen their associations with aphids. Although the shift in ant behavior is important for determining the dynamics of the ant–plant–aphid interaction, it is not known why this shift occurs. Here, we test two hypotheses to explain the mechanism responsible for this behavioral shift: (1) Extrafloral nectar secretion changes in response to aphid herbivory, or (2) plants do not change extrafloral nectar secretion, but the total reward to ants from aphids will exceed that from EFNs above a certain aphid colony size. To judge which mechanism is plausible, we investigated secretion patterns of extrafloral nectar produced by plants with and without aphids, compared the amount of sugar supplied by EFNs and aphids, and examined whether extrafloral nectar or honeydew was more attractive to ants. Our results show that there was no inducible extrafloral secretion in response to aphid herbivory, but the sugar concentration in extrafloral nectar was higher than in honeydew, and more ant workers were attracted to an artificial extrafloral nectar solution than to an artificial aphid honeydew solution. These results indicate that extrafloral nectar is a more attractive reward than aphid honeydew per unit volume. However, even an aphid colony containing only two individuals can supply a greater reward to ants than EFNs. This suggests that the ant behavioral shift may be explained by the second hypothesis.     

Dissecting components of population-level variation in seed production and the evolution of masting behavior

Theory predicts that at higher trophic levels species are especially vulnerable to habitat fragmentation due to small population size and dependence on particular prey species. Using aphids as model organism, we tested the hypothesis that herbivore abundance increases in fragmented habitats as a result of reduced predator and parasitoid densities. In a 3 year-study, we examined the population dynamics of aphids with respect to host plant abundance and ant nest density in experimentally fragmented calcareous grasslands at two sites in the northern Jura mountains. Fragments of different size (area: 20.25 m², 2.25 m² and 0.25 m²) were isolated by a 5 m wide strip of frequently mown vegetation and corresponding control plots were situated in the adjacent undisturbed grassland. Aphid density was higher in fragments than in control plots. This was a combined result of a higher frequency of aphid-infested plants and larger aphid colonies in fragments than in control plots. Furthermore, a larger proportion of aphid colonies was ant-attended in fragments than in control plots. Aphid colonies were also more frequently visited by ants in fragments than in control plots in one of the 3 years. Parasitoid pressure on aphids was not influenced by the experimental fragmentation. Neither were aphid species richness and diversity affected by the fragmentation. Our study shows that even small-scale habitat fragmentation can have profound effects on the abundance of herbivorous insects. The effect on aphid density was consistent over 3 years and two sites with slightly different aphid communities.     

Pymetrozine causes a nontarget pest, the Colorado potato beetle (Coleoptera: Chrysomelidae), to leave potato plants

Pymetrozine is a selective insecticide that targets aphids. Published assessments of the effects of pymetrozine on nontarget organisms focus mainly on predatory insects, and they rarely indicate toxicity. In a laboratory bioassay, survival of Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), larvae was not affected by pymetrozine exposure. We subsequently used pymetrozine to implement low-aphid-density treatments in a field experiment that involved separate manipulations of Colorado potato beetle density. Unexpectedly, the addition of Colorado potato beetle adults and eggs did not increase the densities of Colorado potato beetle larvae in plots that were sprayed with pymetrozine (applied with water and an adjuvant). In control plots sprayed with water and adjuvant (without pymetrozine), addition of Colorado potato beetles increased densities of their larvae. Data collected on a smaller scale suggest that a behavioral mechanism underlies the population-level pattern: Colorado potato beetle larvae become more active and are less likely to remain on a host plant after exposure to pymetrozine. Thus, potato, Solanum tuberosum L., growers who use pymetrozine against aphids also might benefit in terms of Colorado potato beetle control.     

Evergreen foliage allows early hatching in a pine processionary moth and escape from predation

1. Seasonal variation in leaf quality and climate conditions often imposes constraints on the temporal occurrence of tree‐feeding insect larvae, but the seasonal effects of predation have received limited attention. In temperate climate zones, both the abundance and activity of predators can be expected to vary over time. 2. The study reported herein examined the impact of temporal variation in predator activity levels on the life history of an herbivorous insect feeding on a constant food source: previous‐year needles of Scots pine (Pinus sylvestris L.). In field experiments, the survival and growth rates of colonies of Thaumetopoea pinivora Treitschke larvae that had been manipulated to hatch at three different dates were compared. Eggs of T. pinivora usually hatch by mid‐April in southern Sweden, which is earlier than most other herbivorous insects that overwinter as eggs in this region. 3. Predator exclusion experiments indicated that larvae which hatched later than April experienced a higher level of predator activity, mainly by ants. The final larval size and the timing of pupation were not affected by hatching date. First instar larvae were more extensively preyed on than second instars. 4. The life history of herbivore species can be affected by seasonal variation in predation pressures. This study suggests that early hatching in a lepidopteran species can allow a temporal escape from predation during the vulnerable early life stages.     

Leaf beetle larvae,Plagiodera versicolora (Coleoptera: Chrysomelidae), show decreased performance on uninfested host plants exposed to airborne factors from plants infested by conspecific larvae

Undamaged plants are known to suffer less damage from herbivores when previously exposed to airborne factors from neighboring plants that are either infested or artificially damaged. However, to date, the effects of such a defensive phenomenon on performance of herbivorous insects have not been clearly shown. Here, we studied such effects in an interaction between a willow plant, Salix eriocarpa Franchet et Savatier (Salicales: Salicaceae), and a specialist leaf beetle, Plagiodera versicolora (Laicharting) (Coleoptera: Chrysomelidae). In a wind tunnel, uninfested willow plants were placed downwind of willow plants infested by leaf beetle larvae for 4 days. As a control, we placed uninfested plants downwind of uninfested plants in the tunnel. After exposure, downwind plants were served to leaf beetle larvae. Pupal weight, larval survival rates, and the leaf area consumed by larvae all decreased significantly, and larval developmental duration increased significantly, when larvae fed on willow plants downwind of infested plants were compared with those downwind of uninfested plants. These results showed that airborne factors from infested willow plants negatively affected the performance of leaf beetle larvae. Further studies are needed to identify the active factor(s) from the infested willow plants affecting the performance of leaf beetle larvae.     

Cry1Ab protein levels in phytophagous insects feeding on transgenic corn: implications for secondary exposure risk assessment

Enzyme-linked immunosorbent assays (ELISA) and bioassays were used to estimate levels of Cry1Ab protein in four species of phytophagous insects after feeding on transgenic Bt-corn plants expressing Cry1Ab protein or artificial diets containing Cry1Ab protein. The level of Cry1Ab in insects feeding on sources containing the Cry1Ab protein was uniformly low but varied with insect species as well as food source. For the corn leaf aphid, Rhopalosiphum maidis (Fitch), feeding on diet solutions containing Cry1Ab protein, the level of the protein in the aphid was 250–500 times less than the original levels in the diet, whereas no Cry1Ab was detected by ELISA in aphids feeding on transgenic Bt-Corn plants. For the lepidopteran insects, Ostrinia nubilalis (Hübner), Helicoverpa zea (Boddie), and Agrotis ipsilon (Hufnagel), levels of Cry1Ab in larvae varied significantly with feeding treatment. When feeding for 24 h on artificial diets containing 20 and 100 ppm of Cry1Ab, the level of Cry1Ab in the larvae was about 57 and 142 times lower, respectively, than the original protein level in the diet for O. nubilalis, 20 and 34 times lower for H. zea, and 10 to 14 times lower for A. ipsilon. Diet incorporation bioassays with a susceptible insect (first instar O. nubilalis) showed significant Cry1Ab bioactivity present within whole body tissues of R. maidis and O. nubilalis that had fed on diet containing a minimum of 20 ppm or higher concentrations (100 or 200 ppm) of Cry1Ab, but no significant bioactivity within the tissues of these insects after feeding on transgenic Bt-corn plants. The relevance of these findings to secondary exposure risk assessment for transgenic Bt crops is discussed.