Density dependent parasitism of Delphacodes kuscheli (Homoptera: Dephacidae) eggs by Anagrus flaveolus (Hymenoptera: Mymaridae): Influence of egg patchiness and density

LILJESTHRÖM, G.; EDUARDO GABRIEL VIRLA

BIOCONTROL SCIENCE AND TECHNOLOGY

Lugar: New York; Año: 2004 vol. 14 p. 107 - 115

0958-3157

Many herbivorous insects display a marked tendency to aggregate and previous work has shown that the parasitoids likely to be more effective for pest control programs are those that concentrate their attacks on denser host patches. However, as herbivore density changes different spatial patterns usually emerge in their distribution. In Tucuma´n province, Argentina the mymarid Anagrus flaveolus is a native egg parasitoid of Delphacodes kuscheli, which oviposits endophitic eggs and is the only demonstrated vector of the MRCV virus that seriously affects maize (Zea mays). Winter is the critical period for maize colonization from oat (Avena sativa) crops where D. kuscheli overwinters.We analyzed, under field conditions and during the winter critical period, percentage parasitism by A. flaveolus when host patch density increases following two different patterns: an increase in the batch size, and an increase in the number of infested plants with similar mean batch size. In order to generate a heterogeneous egg patchiness, 144 oat plants were arranged in 36 groups of four plants each, and four types of groups were created by placing one, two, three or four infested plants per group. Each group was characterized by the number of infested plants as well as by the number of eggs. If one, two, three or all four plants were infested, the group was denominated G1, G2, G3 or G4, respectively. Considering only G1, the number of eggs per group increased only if the batch size increased. On the other hand, considering all G1, G2, G3 and G4 groups, the increase in patch density from G1 to G4, was essentially due to an increase in the number of infested plants. The groups were symmetrically distributed in the field in a homogeneous 200-m2 area (the position of each group was randomly assigned), and allowed for oviposition for 48 h since A. flaveolus only attacks eggs that have three or less days of development. We found that almost all infected plants were parasitized by A. flaveolus but parasitism never surpassed 64% of the available eggs, suggesting that the parasitoid selectively attacks eggs inserted in particular places of the oat plant, and/or never saturates a batch. We also found that parasitism was spatially direct density dependent when only G1 was considered, as well as when G1, G2, G3 and G4 were considered together. In the first case, an increase in the number of eggs in a group of four plants represents an increase in the batch size, and higher batch size received higher parasitism. In the second case, it represents an increase in the number of infested plants, and higher patch density received higher parasitism. Although the proportion of infected plants parasitized was similar, infected plants with similar number of eggs received higher parasitism when placed in denser groups (G3 and G4) than when placed in the less dense groups (G1 and G2). Behavioral mechanisms leading to a restriction on re-dispersal, like an area-restricted search behavior, as well as supernormal stimulus of contiguous host egg batches in certain places that may require a higher threshold before mutual interference has an effect, could be explained these results.vorous insects display a marked tendency to aggregate and previous work has shown that the parasitoids likely to be more effective for pest control programs are those that concentrate their attacks on denser host patches. However, as herbivore density changes different spatial patterns usually emerge in their distribution. In Tucuma´n province, Argentina the mymarid Anagrus flaveolus is a native egg parasitoid of Delphacodes kuscheli, which oviposits endophitic eggs and is the only demonstrated vector of the MRCV virus that seriously affects maize (Zea mays). Winter is the critical period for maize colonization from oat (Avena sativa) crops where D. kuscheli overwinters.We analyzed, under field conditions and during the winter critical period, percentage parasitism by A. flaveolus when host patch density increases following two different patterns: an increase in the batch size, and an increase in the number of infested plants with similar mean batch size. In order to generate a heterogeneous egg patchiness, 144 oat plants were arranged in 36 groups of four plants each, and four types of groups were created by placing one, two, three or four infested plants per group. Each group was characterized by the number of infested plants as well as by the number of eggs. If one, two, three or all four plants were infested, the group was denominated G1, G2, G3 or G4, respectively. Considering only G1, the number of eggs per group increased only if the batch size increased. On the other hand, considering all G1, G2, G3 and G4 groups, the increase in patch density from G1 to G4, was essentially due to an increase in the number of infested plants. The groups were symmetrically distributed in the field in a homogeneous 200-m2 area (the position of each group was randomly assigned), and allowed for oviposition for 48 h since A. flaveolus only attacks eggs that have three or less days of development. We found that almost all infected plants were parasitized by A. flaveolus but parasitism never surpassed 64% of the available eggs, suggesting that the parasitoid selectively attacks eggs inserted in particular places of the oat plant, and/or never saturates a batch. We also found that parasitism was spatially direct density dependent when only G1 was considered, as well as when G1, G2, G3 and G4 were considered together. In the first case, an increase in the number of eggs in a group of four plants represents an increase in the batch size, and higher batch size received higher parasitism. In the second case, it represents an increase in the number of infested plants, and higher patch density received higher parasitism. Although the proportion of infected plants parasitized was similar, infected plants with similar number of eggs received higher parasitism when placed in denser groups (G3 and G4) than when placed in the less dense groups (G1 and G2). Behavioral mechanisms leading to a restriction on re-dispersal, like an area-restricted search behavior, as well as supernormal stimulus of contiguous host egg batches in certain places that may require a higher threshold before mutual interference has an effect, could be explained these results.