Material and Methods
Plant MaterialIn 1995 cuttings of 16 L. alba populations from all over Guatemala (Table 1) were transplanted in an experimental field at ICTA (Instituto de Ciencia y Tecnologia Agricola), Cuyuta, Guatemala. The term 'collection source' in Table 1 specifies the origin of plant material, which was collected from house gardens ('cultivated') or from the wild. Cuttings were planted at distances of 75 × 50 cm in a randomized block design with three replications, 16 plants per replication.The decision to collect cuttings instead of seeds from wild populations derived from the extremely low germination rates found for L. alba of only 1-2%. Herbarium specimens of each population were deposited at the Universidad del Valle, Guatemala City, Guatemala. 90 days after transplantation, leaves and flowers were harvested and dried in a greenhouse at an average temperature of 32°C for 4 days. The temperature in the greenhouse was regulated by a ventilator.
Morphological CharactersThree out of several morphological characters were especially interesting in the context of essential oil composition: leaf shape, width of the leaf, and length of the flower stem. The length of the flower stem, measured at the 4th node from the top, is defined as the distance between the axilla and the node of the involucre.
The abilities of Myiopharus doryphorae (Riley) and M. aberrans (Townsend) (Diptera: Tachinidae) to discriminate between parasitized and non‐parasitized Colorado potato beetle (host) Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) larvae, were investigated under laboratory and field conditions. Laboratory experiments showed that both Myiopharus species have a significantly greater frequency for larvipositing in non‐parasitized hosts over parasitized ones. Direct field observations of larvipositional behavior of both Myiopharus species over three growing seasons showed effective restraint from larviposition into parasitized hosts, while larviposition into non‐parasitized ones occurred readily. Avoidance of previously‐parasitized hosts occurred after the larvipositing flies briefly landed on host larvae without attempting to insert the larvipositor.
The low levels of superparasitism which occurred in the caged experiments and in the field appeared to be due to a breakdown of the larvipositing parasitoids' restraint when they met only parasitized hosts or when many parasitoids competed for reduced numbers of hosts late in the season.
Arthropods were sampled from feverfew [Tanacetum parthenium (L.) Schultz-Bip], Echinacea purpurea (L.) Moench, Echinacea pallida (Nutt.) Nutt., Valeriana officinalis L., and St. John' s wort (Hypericum perforatum L.) during 1998-2001. In addition, arthropods were sampled on tansy (Tanacetum vulgare L.) from 2001-2004. In general, 50-60 arthropod species where collected and identified among all of the medicinal plant species. Among the predators, Orius insidiosus (Say) (Hemiptera: Anthocoridae), Geocoris punctipes (Say) (Hemiptera: Lygaeidae) and spiders were most abundant from 1998-2004.The three-cornered alfalfa hopper, Spissistilus festinus (Say), was the most abundant herbivore found from 1998 to 2001. Orius insidiosus and G. punctipes were 3-4 times more abundant on T. parthenium than on any other medicinal plant species. Based on the numbers of predatory arthropods found on T. parthenium, this crop could be suitable as a companion or "banker" plant to attract and maintain populations of predators, especially O. insidiosus and G. punctipes. Whitefly nymphs attacked by predators with piercing/sucking mouthparts are easily identified using a microscope because of the general appearance of the carcass left by the predators. Thus, populations of predators on T. parthenium suppressed Bemisia tabaci populations on E. purpurea when these crops were planted as companion crops.
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