Concerns about possible undesired environmental effects of transgenic crops have prompted numerous evaluations of such crops. So-called Bt crops receive particular attention because they carry bacteria-derived genes coding for insecticidal proteins that might negatively affect non-target arthropods. Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects. Indeed, Cotesia marginiventris, a parasitoid of lepidopteran pests, lived longer and parasitized more pest caterpillars in the presence of aphid-infested Bt maize than in the presence of aphid-infested isogenic maize. Hence, depending on aphid pest thresholds, the observed increased susceptibility of Bt maize to aphids may be either a welcome or an undesirable side effect.

No abstract

Intake of sugar-rich foods by adult parasitoids is crucial for their reproductive success. Hence, the availability of suitable foods should enhance the efficacy of parasitoids as biological control agents. In situations where nectar is not readily available, homopteran honeydew can be a key alternative food source. We studied the impact of honeydew feeding on the longevity of the larval endoparasitoids Cotesia marginiventris, Campoletis sonorensis and Microplitis rufiventris, all natural enemies of important lepidopteran pests. Females of these wasps lived longer when feeding on honeydew produced by the aphid Rhopalosiphum maidis on barley compared to control females provided with water only. However, they lived shorter than females fed with a sucrose solution. Further investigations with C. marginiventris showed that access to honeydew also increases the number of offspring produced, but less so than access to a sucrose solution. Moreover, it was found that females of this species need to feed several times throughout their life in order to reach optimal longevity and reproductive output. Analyses of the sugars in the honeydew produced by R. maidis on barley revealed that it contains mainly plant-derived sugars, but also several aphid-synthesized sugars. The sugar composition of the honeydew changed as a function of aphid colony size and time a colony had been feeding on a plant. In general, the higher the aphid infestation, the smaller the percentage of aphid-synthesized sugars in the honeydew. Experiments with honeydew sugar mimics allowed us to reject the hypothesis that the relatively poor performance of the parasitoid on a honeydew diet was due to the sugar composition. Instead, the results from additional feeding experiments with diluted honeydew showed that the nutritional value of pure honeydew is primarily restricted by its high viscosity. The possible consequences of these findings for biological pest control are discussed.Z usammenfassung Die Aufnahme von zuckerreicher Nahrung durch adulte Parasitoide ist entscheidend fu¨r ihren reproduktiven Erfolg. Daher sollte die Verfu¨gbarkeit von geeigneter Nahrung die Effizienz von Parasitoiden als Mittel der biologischen Kontrolle steigern. Wenn Nektar nicht leicht verfu¨gbar ist, kann der Honigtau von Homopteren eine entscheidende alternative Nahrungsquelle sein. Wir untersuchten den Einfluss des Honigtaukonsums auf die Lebensdauer der larvalen Endoparasitoide Cotesia marginiventris, Campoletis sonorensis und Microplitis rufiventris, die natu¨rliche Feinde wichtiger Schadlepidopteren sind. Verglichen mit Kontroll-Weibchen, die nur Wasser erhielten, lebten die Weibchen dieser Wespen la¨nger, wenn sie von der Blattlaus Rhopalosiphum maidis (auf Gerste) produzierten Honigtau aufgenommen hatten. Allerdings lebten sie ku¨rzer als Weibchen, die mit einer Saccharose-Lo¨sung gefu¨ttert wurden.Ã Corresponding author.

Estudou-se a resposta funcional de Trichogramma pretiosum Riley parasitando ovos de Tuta absoluta (Meyrick) com idades de 12 a 36, 36 a 60 e > 60 h, em folhas de tomate industrial IPA-5, com exposição dos ovos a uma fêmea pelo período de um dia. A partir do número de ovos parasitados em diferentes densidades foi feita a linearização do parasitismo por meio da equação de Woolf, obtendo-se desta o tempo de manipulação (Th) de 0,77 h e a taxa de ataque (a') de 0,80 h-1. Baseando-se nesses resultados, testou-se o tipo de resposta apresentada pelo parasitóide que se ajustou melhor ao Tipo II: Na = Nt [1 - exp (-0,80/(1+0,0258Nt))], para ovos de 12 a 36 h de idade. O número máximo estimado de ovos parasitados foi de 31,1, próximo ao número observado de 30 ovos parasitados na densidade de 60 ovos por folíolo. O Th correspondeu a 3,22% do tempo total de exposição. Um aumento na taxa de parasitismo foi observado até a densidade de 55 ovos/folíolo, caracterizando uma resposta funcional do Tipo I para baixas densidades [ÿ = 4,53 + 0,288 (± 0,054)x], enquanto que em altas densidades, a taxa de parasitismo foi estável nas densidades de 56 a 80 e de 81 a 100 ovos/folíolo (média ± DP = 20,1 ± 4,01 ovos). Em ovos com idades de 36 a 60 ou > 60 h, o parasitóide não apresentou incremento de parasitismo significativo com a densidade.

One important factor determining the efficacy of parasitoids is the way they exploit different host patch. This study evaluated the response of females of

An. Soc. Entomol. Brasil 29(4): 831-834 (2000) Aphelinus gossypii Timberlake (Hymenoptera: Aphelinidae) AttackingAphis gossypii Glover (Hemiptera: Aphididae) on Cotton Plants in Brazil ABSTRACT -This study aimed to verify the parasitoids species related to Aphis gossypii Glover, in herbaceus cotton seeded in na experimental area of UFPE campus, from February to December 1998. A total of 1,880 mumified aphids were collected. Two species of parasitoids were identified following adult emergence. This is the first report of the occurence of Aphelinus gossypii attacking A. gossypii on cotton in Brazil.KEY WORDS: Insecta, parasitoid, aphid.RESUMO -Com a finalidade de se verificar as espécies de parasitóides associadas a Aphis gossypii Glover, mudas de algodão herbáceo foram mantidas em vasos de fevereiro a dezembro de 1998, em uma área experimental do campus da UFPE. Durante o período coletou-se um total de 1.880 pulgões mumificados que foram isolados em cápsulas de gelatina incolor. Após a emergência dos parasitóides adultos identificaram-se duas espécies, Aphelinus gossypii Timberlake e Lysiphlebus testaceipes Cresson. Destas, Aphelinus gossypii é assinalada pela primeira vez associada a Aphis gossypii sobre algodão no Brasil.PALAVRAS-CHAVE: Insecta, parasitóide, afídeo.