The Influence of Planting Periods on Herbivore and Natural Enemy Abundance on Yellow Sticky Traps in Bt Maize Fields.

Knowledge of the insect densities during crop development is necessary for adopting appropriate measures for the control of insect pests and minimizing yield losses. Within integrated pest management programs, crop rotation has been carried out in recent years, but this current trend delays the planting period for Bt maize. The small amount of available information regarding the influence of sowing Bt maize early or late on the seasonal abundance of herbivores prompted us to study these aspects in two current common planting periods in northeastern Spain in 52 maize fields over three consecutive years. We sampled the fields planted on different dates with sticky yellow traps. Our results show that only the abundances of herbivore thrips, other than Frankliniella occidentalis, and Syrphidae were significantly different between the two planting periods. Moreover, when we performed yearly analyses, we found significant effects of the planting period on Coccinellidae and Chrysopidae in 2015 and on Aeolothrips sp. in 2016 and 2017. In most of the taxa, the abundance peaks in earlier growth stages, which is related to pollination (before or during). Only the abundances of Stethorus punctillum and Syrphidae peak later in the season. In addition, F. occidentalis, aphids, Syrphidae, and Coccinellidae registered higher abundance in fields sown in the late planting period. These results highlight the effects of sowing in different planting periods on insect dynamics in Bt maize and can be used to identify the abundance of certain pests and natural enemies in specific phenological stages of maize, which may allow producers to adopt better-integrated management and thus avoid reaching the level of economic damage.

ß-carotene and Bacillus thuringiensis insecticidal protein differentially modulate feeding behaviour, mortality and physiology of European corn borer (Ostrinia nubilalis).

Maize with enhanced ß-carotene production was engineered to counteract pervasive vitamin A deficiency in developing countries. Second-generation biofortified crops are being developed with additional traits that confer pest resistance. These include crops that can produce Bacillus thuringiensis Berliner (Bt) insecticidal proteins. Currently, it is unknown whether ß-carotene can confer fitness benefits through to insect pests, specifically through altering Ostrinia nubilalis foraging behaviour or development in the presence of Bt insecticidal toxin. Therefore the effects of dietary ß-carotene plus Bt insecticidal protein on feeding behaviour, mortality, and physiology in early and late instars of O. nubilalis larvae were investigated. The results of two-choice experiments showed that irrespective of ß-carotene presence, at day five 68%-90% of neonates and 69%-77% of fifth-instar larvae avoided diets with Cry1A protein. Over 65% of neonate larvae preferred to feed on diets with ß-carotene alone compared to 39% of fifth-instar larvae. Higher mortality (65%-97%) in neonates fed diets supplemented with ß-carotene alone and in combination with Bt protein was found, whereas <36% mortality was observed when fed diets without supplemented ß-carotene or Bt protein. Diets with both ß-carotene and Bt protein extended 25 days the larval developmental duration from neonate to fifth instar (compared to Bt diets) but did not impair larval or pupal weight. Juvenile hormone and 20-hydroxyecdysone regulate insect development and their levels were at least 3-fold higher in larvae fed diets with ß-carotene for 3 days. Overall, these results suggest that the effects of ß-carotene and Bt protein on O. nubilalis is dependent on larval developmental stage. This study is one of the first that provides insight on how the interaction of novel traits may modulate crop susceptibility to insect pests. This understanding will in turn inform the development of crop protection strategies with greater efficacy.

Planting period is the main factor for controlling maize rough dwarf disease.

Maize rough dwarf virus (MRDV) is one of the main yield-limiting factors of maize in the Mediterranean. However, knowledge about the interactions between the agroecosystem and the virus-vector-host relationship continues to be limited. We used multi-model inference to test a landscape-scale approach together with variables measured in the field, and we estimated the effects of early and late planting on MRDV incidence. The results revealed that the virus incidence increased by 3% when the planting was delayed, and this increase was coincident with the first peak of the vector population. The variables at the field and landscape scales with a strong effect on virus incidence were the proportions of grasses in adjacent crops, in uncultivated areas, and in edges close to maize plants. Grass plant cover in the edges also affected virus incidence, but these effects varied with the planting period. These findings provide new insights into the causes of MRDV incidence and may provide some guidance to growers to reduce losses caused by the virus. Among the recommendations to be prioritized are early planting, management of grasses at field edges, and non-overlapping cultivation of maize and winter cereals in the same area.

The Impact of Environmental Stress on Bt Crop Performance.

Bt crops have been grown commercially for more than two decades. They have proven remarkably effective in the control of target insect pests. However, Bt crops can become less effective under various forms of environmental stress. Most studies in this area have considered the effect of environmental stress on Bt insecticidal protein levels or target pest mortality, but not both, resulting in a lack of mechanistic analysis. In this review, we critically examine previous research addressing the impact of environmental stress on the effectiveness of Bt crops. We find that the body of research data is not sufficiently robust to allow the reliable prediction of the performance of Bt crops under extreme climatic conditions.

Carotenoids moderate the effectiveness of a Bt gene against the European corn borer, Ostrinia nubilalis.

We assessed the effectiveness of a biofortified maize line (4BtxHC) which accumulates high levels of antioxidant carotenoids that also expressed the insecticidal Cry1Ac Bacillus thuringiensis (Bt) gene against the European corn borer Ostrinia nubilalis. This line had been previously engineered to accumulate carotenoids specifically in the seed endosperm, whereas the Bt gene was expressed constitutively. The concentrations of Bt toxin (Cry 1Ac) in the leaves of the 4Bt and 4BtxHC lines were not significantly different at 47±6 µg/g of fresh weight (FW); neither were they in the kernels of both lines (35±3 µg/g FW). The kernels and leaves were toxic to the larvae of O. nubilalis. However, the insecticidal activity was substantially lower (ca. 20%) than that of lines that expressed only Bt in spite that the two lines showed a quantity of toxin not significantly different in kernels or in leaves. Although the reduced effectiveness of Cry1Ac in kernels may not be entirely surprising, the observation of the same phenomenon in vegetative tissues was unexpected. When semi-artificial diets containing kernels from 4Bt supplemented with different levels of ß-carotene were used in insect bioassays, the ß-carotene moderated the effectiveness of the Bt similarly to the plant material with carotenoid enrichment. To elucidate the biochemical basis of the reduced effectiveness of Bt toxin in the carotenoid-enriched plants, we measured the activity of three enzymes known to be implicated in the detoxification defence, namely, catalase, superoxide dismutase and glutathione S-transferase. Whereas Cry1Ac expression significantly increased SOD and CAT enzymatic activity in the absence of carotenoids, carotenoids, either in 4BtxHC or in artificial diets enriched with ß-carotene, significantly lowered CAT activity. Carotenoids can therefore moderate the susceptibility of the maize borer O. nubilalis to Cry1Ac, and we hypothesize that their role as antioxidants could explain this phenomenon via their scavenging of reactive oxygen species produced during Cry1Ac detoxification in the larvae. The involvement of this mechanism in the decreased mortality caused by Cry1Ac when carotenoids are present in the diet is discussed.

Innate and Learned Prey-Searching Behavior in a Generalist Predator.

Early colonization by Zyginidia scutellaris leafhoppers might be a key factor in the attraction and settling of generalist predators, such as Orius spp., in maize fields. In this paper, we aimed to determine whether our observations of early season increases in field populations of Orius spp. reflect a specific attraction to Z. scutellaris-induced maize volatiles, and how the responses of Orius predators to herbivore-induced volatiles (HIPVs) might be affected by previous experiences on plants infested by herbivorous prey. Therefore, we examined the innate and learned preferences of Orius majusculus toward volatiles from maize plants attacked by three potential herbivores with different feeding strategies: the leafhopper Z. scutellaris (mesophyll feeder), the lepidopteran Spodoptera littoralis (chewer), and another leafhopper Dalbulus maidis (phloem feeder). In addition, we examined the volatile profiles emitted by maize plants infested by the three herbivores. Our results show that predators exhibit a strong innate attraction to volatiles from maize plants infested with Z. scutellaris or S. littoralis. Previous predation experience in the presence of HIPVs influences the predator's odor preferences. The innate preference for plants with cell or tissue damage may be explained by these plants releasing far more volatiles than plants infested by the phloem-sucking D. maidis. However, a predation experience on D. maidis-infested plants increased the preference for D. maidis-induced maize volatiles. After O. majusculus experienced L3-L4 larvae (too large to serve as prey) on S. littoralis-infested plants, they showed reduced attraction toward these plants and an increased attraction toward D. maidis-infested plants. When offered young larvae of S. littoralis, which are more suitable prey, preference toward HIPVs was similar to that of naive individuals. The HIPVs from plants infested by herbivores with distinctly different feeding strategies showed distinguishable quantitative differences in (Z)-3-hexenal, (E)-2-hexenal, and methyl salicylate. These compounds might serve as reliable indicators of prey presence and identity for the predator. Our results support the idea that feeding by Z. scutellaris results in the emission of maize's HIPVs that initially recruit Orius spp. into maize fields.

Host selection by the autoparasitoid Encarsia pergandiella on primary (Bemisia tabaci) and secondary (Eretmocerus mundus) hosts.

In autoparasitoids, females are generally primary endoparasitoids of Hemiptera, while males are hyperparasitoids developing in or on conspecific females or other primary parasitoids. Female-host acceptance can be influenced by extrinsic and/or intrinsic factors. In this paper, we are concerned with intrinsic factors such as nutritional status, mating status, etc. We observed the behavior of Encarsia pergandiella Howard (Hymenoptera: Aphelinidae) females when parasitizing primary (3rd instar larvae of Bemisia tabaci Gennadius [Homoptera: Aleyrodidae]) and secondary hosts (3rd instar larvae and pupae of Eretmocerus mundus Mercet [Hymenoptera: Aphelinidae]) for a period of 1 h. Females had different reproductive (virgin or mated younger) and physiological (fed elder or mated elder) status. Virgin females killed a large number of secondary hosts while investing a long time per host. However, they did not feed upon them. Mated females killed a lower number of secondary hosts and host feeding was observed in both consuming primary and secondary hosts. It was common to observe host examining females of all physiological statues tested repeatedly stinging the same hosts when parasitizing, killing or rejecting them. Fed elder females parasitized more B. tabaci larvae than E. mundus larvae or pupae, while investing less time on the primary host than on the secondary host. They also parasitized more B. tabaci larvae than mated elder females, while investing less time per host. The access of females to honey allowed them to lay more eggs.

Paradoxical EU agricultural policies on genetically engineered crops.

European Union (EU) agricultural policy has been developed in the pursuit of laudable goals such as a competitive economy and regulatory harmony across the union. However, what has emerged is a fragmented, contradictory, and unworkable legislative framework that threatens economic disaster. In this review, we present case studies highlighting differences in the regulations applied to foods grown in EU countries and identical imported products, which show that the EU is undermining its own competitiveness in the agricultural sector, damaging both the EU and its humanitarian activities in the developing world. We recommend the adoption of rational, science-based principles for the harmonization of agricultural policies to prevent economic decline and lower standards of living across the continent.

Modification of hormonal balance in larvae of the corn borer Sesamia nonagrioides (Lepidoptera: Noctuidae) due to sublethal Bacillus thuringiensis protein ingestion.

Bacillus thuringiensis (Bt) corn, Zea mays L., is highly efficient against the corn borer Sesamia nonagrioides (Lefèbvre) (Lepidoptera: Noctuidae) when the larvae feed only on the transgenic plants. However, when they feed on Bt leaves during only part of their development, thus ingesting sublethal amounts of Bt toxins, some larvae survive. A previous study reported a prolonged development and precocious diapause induction in larvae fed on a diet with sublethal amounts of Cry1Ab protein. To determine whether these effects were accompanied by a modification of the hormonal balance, S. nonagrioides larvae were fed on sublethal amounts of Bt protein provided in Bt leaves or in the diet. The larvae that survived had higher levels of juvenile hormone (JH), whereas their level of ecdysteroids did not increase sufficiently to allow pupation, leading to a longer larval development and more larval molts. This response may be considered a defense mechanism that allows some larvae to survive toxin ingestion; it is similar the response to insecticidal toxins or viruses observed in other larvae. Changes in the hormone levels in diapausing larvae were undetectable, probably because these changes were masked by the higher level of JH in the hemolymph of diapausing larvae and because of lack of ecdysteroid titer increase, a phenomenon that is usually observed a few days before pupation in nondiapausing larvae. These results should be taken into account in the establishment of non-Bt refuges to prevent development of Bt-resistance in S. non-agrioides populations.

Response of Mythimna unipuncta males to components of the Sesamia nonagrioides pheromone.

Several sympatric lepidopteran species feed on maize plants, and the different components of their species-specific female sex pheromones may play a role in attracting conspecifics and/or deter heterospecific males. In this study, we analyzed the content of Mythimna unipuncta pheromone glands and tested the response of males to components of their own pheromone blend and that of Sesamia nonagrioides in the wind tunnel. Whole pheromone glands, and lures where (Z)-9-hexadecenyl acetate, Z)-11-hexadecen-1-ol or (Z)-9-hexadecenyl acetate + (Z)-11-hexadecen-1-ol were added to the major component, (Z)-11-hexadecenyl acetate, elicited significantly higher responses by M. unipuncta males than lures with main component alone, although the levels varied with concentration. In the field a rather different outcome was observed, as the addition of other compounds found in the female pheromone gland did not improve trap catch over lures with only (Z)-11-hexadecenyl acetate. The addition of (Z)-11-hexadecenal, a compound of the S. nonagrioides pheromone, to (Z)-11-hexadecenyl acetate significantly reduced attraction of M. unipuncta males both in the wind tunnel and in the field, as well as the number of sympatric clover cutworm, Discestra trifolii, under field conditions. The addition of (Z)-9-hexadecenyl acetate, a minor component of the M. unipuncta pheromone blend, reduced the number of S. nonagrioides captured in field traps that were baited with the S. nonagrioides lure. The significance of such inhibition in the reproductive isolation of sympatric species that attack maize is discussed.

Responsiveness of arthropod herbivores and their natural enemies to modified weed management in corn.

Alteration of weed flora as consequence of the deployment of genetically modified herbicide-tolerant crops may affect higher trophic levels in agrosystems. A 4-yr study is being conducted in Spain to investigate interrelations between weeds and associated arthropods in corn fields. In a first step, the work aimed to detect the most responsive arthropods to weed management changes. To identify the most responsive arthropods, arthropod composition and abundance in herbicide-tolerant corn plots treated twice with glyphosate and untreated plots were compared for 2 yr. Plots were sampled seven times during the season by visual inspection and pitfall and yellow sticky traps to estimate abundance and activity of the main arthropod herbivores, predators, and parasitoids. As intended, the abundance and composition of weed flora was strongly altered by the differential herbicide treatments. Several groups of arthropods responded to the weed changes but in variable directions. Whereas leafhoppers and aphids were more abundant on herbicide-treated plots, the contrary was found for phytophagous thrips. Among predators, Orius sp., spiders, and trombidids were more abundant on treated plots, whereas nabids and carabids were more abundant in untreated plots; the same case was found for carabids and spiders caught in pitfall traps. Among parasitoids, ichneumonids were more abundant in untreated plots and mymarids in treated plots. These results cannot be interpreted in terms of nontarget effects of postemergence treatments with broad-spectrum herbicides; for this, a comparison with conventional weed management practices should be done and this is the current step in the study.

Factors affecting the natural duration of diapause and post-diapause development in the Mediterranean corn borer Sesamia nonagrioides (Lepidoptera: Noctuidae).

Sesamia nonagrioides responds to photoperiod for diapause termination at high temperature with a Type II response curve, and only photoperiods longer than 12:12 terminate diapause. However, these photoperiods never occur in the field when diapausing larvae are competent to terminate diapause. Under a temperature similar to the natural field temperatures diapause terminates spontaneously in approximately 4 months, which ensures that the larvae reach the middle of winter without pupation. S. nonagrioides larvae pupate after going through a specific number of light-dark cycles or days: the required day number (RDN) for diapause completion. This RDN could be modulated by temperature but more research is necessary to clarify this point. In post-diapause development, when a suitable temperature threshold is considered, the absolute accumulation of heat is more important than whether the temperature received is fluctuating or constant. The temperature threshold for diapause and post-diapause development was lower than the temperature threshold of larvae in continuous development. This is important for adjusting phenological models in S. nonagrioides and in other species, and may explain why in many cases adults appear in the field when the supposed temperature threshold for development has not been attained.

Inhibition of pheromone response in Sesamia nonagrioides by the pheromone of the sympatric corn borer, Ostrinia nubilalis.

Sesamia nonagrioides Lef. and Ostrinia nubilalis (Hübn.) are two corn borers present in the Mediterranean area that share a similar feeding habitat. The female sex pheromones of the two species consist of (Z)-11-hexadecenyl acetate, (Z)-11-hexadecen-1-ol, (Z)-11-hexadecenal and dodecyl acetate (77:8:10:5 w/w) and (Z)-11-tetradecenyl acetate and (E)-11-tetradecenyl acetate (97:3 w/w) respectively. Although the pheromones share no common components, previous work had shown the inhibition of the response by O. nubilalis males to their own pheromone owing to the presence of S. nonagrioides pheromone. In this study, the pheromone of O. nubilalis and its two components separately were shown to inhibit the attraction of S. nonagrioides males to the synthetic female pheromone in both laboratory wind-tunnel bioassay and field trapping studies. In the wind tunnel, the number of contacts of S. nonagrioides males with the source were significantly (P < 0.05) reduced by the single pheromone components of O. nubilalis. In the field, the addition of 1% of O. nubilalis pheromone significantly (P < 0.05) reduced the catches of S. nonagrioides males. The components of both pheromones also elicited electroantennographic responses from antennae of male S. nonagrioides moths. The ecological consequences and the possibilities of applying this cross-inhibition for mating disruption techniques are discussed.

Pheromone antagonism in the European corn borer moth Ostrinia nubilalis.

Mixing the sex pheromones of the Mediterranean corn borer, Sesamia nonagrioides, and the European corn borer, Ostrinia nubilalis, results in significantly lower captures of O. nubilalis when compared to traps loaded with its pheromone alone. Rubber septa loaded with a constant concentration of the pheromone of O. nubilalis and different percentages of the S. nonagrioides pheromone (from 1 to 100%) causes dose-dependent antagonism in the field. Electroantennograms of O. nubilalis males showed high antennal responses to its own pheromone components, followed by smaller responses to the major, [(Z)-11-hexadecenyl acetate (Z11-16:Ac)], and two minor components [dodecyl acetate (12:Ac) and (Z)-11-hexadecenal (Z11-16:Ald)] of the S. nonagrioides pheromone. There was almost no response to the S. nonagrioides minor component (Z)-11-hexadecenol (Z11-16:OH). Field tests that used traps baited with the O. nubilalis pheromone plus individual components of S. nonagrioides showed that Z11-16:Ald causes the antagonism. Adding 1% Z11-16:Ald to the pheromone of O. nubilalis reduced oriented flight and pheromone source contact in the wind tunnel by 26% and 83%, respectively, and trap captures in the field by 90%. The other three pheromone components of S. nonagrioides inhibited pheromone source contact but not oriented flight of O. nubilalis males and did not inhibit capture in the field. Cross-adaptation electroantennogram suggests that Z11-16:Ald stimulates a different odor receptor neuron than the pheromone components of O. nubilalis. We conclude that Z11-16:Ald is a potent antagonist of the behavioral response of O. nubilalis.

Within-plant distribution and sampling of single and mixed infestations of Bemisia tabaci and Trialeurodes vaporariorum (Homoptera: Aleyrodidae) in winter tomato crops.

In several areas of Spain, the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), and the sweet potato whitefly, Bemisia tabaci (Gennadius), coexist in tomato, Lycopersicon esculentum Miller. For integrated pest management decision-making, it is important to know the abundance of each species, because they exhibit different abilities to transmit viruses, are susceptible to different biological control agents, and have different responses to insecticides. This study was conducted to provide information on the vertical distribution of T. vaporariorumn and B. tabaci in tomato plants grown in greenhouses in winter and to determine the optimal sampling unit and the sample size for estimating egg and nymphal densities of both whitefly species. Eggs of T. vaporariorum were mainly located on the top stratum of the plant, whereas B. tabaci eggs were mainly found on the middle stratum. Nymphs of both species mainly concentrated in the bottom stratum of the plant. When pest abundance and low relative variation were considered, the bottom stratum was selected as the most convenient for sampling nymphs of both whitefly species. Conversely, the same two criteria indicated that either the top or the middle strata could be used when sampling T. vaporariorum and B. tabaci eggs. Several different sampling units were compared to optimize the estimation of nymphal and egg densities in terms of cost efficiency. One disk (1.15 cm in diameter) per leaflet collected from the top stratum of the tomato plant was the most efficient sampling unit for simultaneously estimating the egg densities of the two whitefly species.

Six years after the commercial introduction of Bt maize in Spain: field evaluation, impact and future prospects.

We carried out a 6-year-field evaluation to assess potential hazards of growing Compa, a transgenic Bt maize variety based on the transformation event CG 00256-176. Two categories of hazards were investigated: the potential of the target corn borer Sesamia nonagrioides to evolve resistance to Bt maize and effects on non-target organisms. In order to address the first hazard, dispersal capacity of the corn borer was measured and our results indicated that larvae move to plants other than those onto which the female oviposited - even to plants in adjacent rows - in remarkable numbers and they do so mostly at a mature age, suggesting that mixing Bt and non-Bt seeds in the same field would not be a very useful deployment strategy to delay/prevent resistance. In addition, adults move among fields to mate and males may do so for up to 400 m. Three different aspects of potential non-target effects were investigated: sub-lethal effects on the target S. nonagrioides, effects on non-target maize pests, and effects on maize-dwelling predators. Larvae collected in Bt fields at later growth stages, in which event 176 Bt maize expresses Bt toxin at sub-lethal concentrations, had longer diapause and post-diapause development than larvae collected in non-Bt fields, a feature that might lead to a certain isolation between populations in both type of fields and accelerate Bt resistance evolution. Transgenic maize did not have a negative impact on non-target pests in the field; more aphids and leafhoppers but similar numbers of cutworms and wireworms were counted in Bt versus non-Bt fields; in any case differences in damage or yield were recorded. We observed no difference in the numbers of the most relevant predators in fields containing transgenic or no transgenic maize.

Predatory fauna in cornfields and response to imidacloprid seed treatment.

The composition and abundance of predatory fauna in corn, Zea mays L., were studied by field visual sampling and pitfall traps over a 5-yr period. In visual samplings, the most abundant groups were Araneae, Heteroptera, Carabidae, Coccinellidae, and Staphylinidae, whereas the prevalent predators caught in pitfall traps belonged to Carabidae, Araneae, Dermaptera, and Opilionidae. The most abundant species or genera in the prevalent groups, except in Arachnida, were identified. Application of the insecticide imidacloprid as a seed dressing is common in the study area to prevent wireworm and cutworm damage. By comparing predator composition and abundance in treated and untreated cornfields during the 5 yr, we assessed the impact of imidacloprid seed treatment on predatory fauna. Among the prevalent predator groups found in visual sampling, Araneae, Coccinellidae, and Staphylinidae were not affected by the imidacloprid treatment, whereas Carabidae was only moderately affected in one of the 5 yr studied. On the contrary, Heteroptera was more drastically reduced by the imidacloprid, but the effect varied with the year. Incidence of European corn borer, Ostrinia nubilalis (Hübner), may be increased as result of such heteropteran reduction. In pitfall traps, only Staphylinidae resulted in lower numbers as consequence of the treatment, whereas the rest of the most abundant predator groups, Carabidae, Araneae, Dermaptera, Opilionidae, Trombididae, and Heteroptera, were not caught in significantly different numbers in treated or untreated plots.