Johanna Nifosi, 2011

Universidad Metropolitana

Main factors limiting population growth of Lema daturaphila

Agricultural methods often use pesticides to control herbivory, but this can cause major problems such as pollution and eutrophication of water bodies, among other things. Newer agricultural methods apply other ecofriendly methods to suppress herbivory, such us biological control of pests, that is based on a natural trophic chain. One type of biological control is known as Conservation Biological Control. This method utilizes non-crop vegetation located around the actual crop (Buffer strips), that serves as the habitat of natural enemies of the pest. However, in order to reach its maximum efficiency, it is important to evaluate the suitability for each crop. Crop plants from the economically important family Solanaceae have specialist herbivores that may entirely defoliate plants and cause extensive economic losses, but in the wild Solanaceous herbivores are often suppressed by natural enemies. Understanding how natural systems regulate herbivores may provide insights into regulating pests in agricultural systems. In this study, I examine what regulates populations of the leaf beetle Lema daturaphila (Coleoptera) on its main local host, Physalis longifolia (known as wild tomatillo, Solanaceae). Prior work identified some of the main predators and parasites of these beetles, but also found that most mortality occurs in the egg stage, with only 28 percent of eggs hatching. In order to determine the key factors we exposed herbivore eggs to potential predators varying the time of day and type of exposure (by covering plants with different sized mesh bags at different times). We found that multiple types of predators were involved, from large insects like grasshoppers and adult lacewings to tiny predators like thrips. The greatest sources of Lema egg mortality were tiny predators, small diurnal predators, and small nocturnal predators, but grasshopper consumption of Lema eggs became substantial by late summer and was the most frequently witnessed egg mortality factor captured on video camera. We did not observe the eggs being parasitized. While herbivore suppression by multiple predators was wholly effective in controlling Lema populations in the wild, the large impact of small predators may be difficult to replicate under Conservation Biological Control strategies, as their mobility is limited. Only the winged adult stage would readily disperse from non crop vegetation into the crop areas, and result in a time delay before predator populations could build up substantially in the crop area.

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