why butterflies show specificity in choosing host plants?

Question

why butterflies use a perticular feed plant to grow?

Answer 1

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Answer 2

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Answer 3

A highly publicized study performed by Losey et al. at Cornell University showed that Bt corn pollen had a negative impact on viability of monarch butterfly larvae (1). Though the authors of the scientific correspondence published in the journal Nature (5/20/99) reported only preliminary findings obtained from one assay, the report has touched a nerve and increased the volume on a widespread and somewhat rancorous debate among scientists, farmers, environmentalists, the agbiotech industry, anti-technology advocates, and nature lovers.

The din is not surprising considering the beautiful monarch butterfly, well known for its twice yearly migration across the North American continent, is an unofficial symbol of conservation in the US and has been dubbed the "Bambi of the insect world." Monarch numbers have been declining for decades; consequently, the recent Nature report served to further heighten fear that genetic engineering in general and Bt corn, specifically, pose a potential threat to monarchs as well as biodiversity in general.

Contrary to ominous headlines such as "Butterfly Research Proves GMO Threat to Biodiversity" that suggest the monarch species is in peril, the Nature report primarily confirms what is already known. The potential for toxic effects of Bt corn on moth and butterfly larvae has been reported in the literature since at least 1986. "Both the Environmental Protection Agency and the US Department of Agriculture have been provided data on the potential for impacts on nontarget species from Bt pollen for years," according to Val Giddings, BIO Vice President of Food and Agriculture (2).

However, despite the monarch larvae's susceptibility to Bt toxin in the laboratory, the literature is devoid of studies confirming significant mortality of larvae on milkweed near Bt cornfields in natural settings. And based on what is known about monarch migration, egg laying, and feeding patterns, experts predict little impact on monarch larvae beyond the edges of Bt corn fields for several reasons:

Monarchs prefer to fly in open meadows, prairies, and wetlands and choose to lay their eggs on small milkweed plants found in fence rows, ditches, and pastures. It is unlikely, therefore, that monarch eggs would be laid on milkweed in the middle of a field of tall corn, assuming any milkweed could be found there. Milkweed is considered a noxious weed that farmers routinely eradicate from their fields with broad-spectrum herbicides. Also, most corn pollen stays within the cornfield; any pollen drift to nearby milkweed is limited because corn pollen is relatively heavy, the majority falling within eight rows. According to industry reports, ongoing monitoring of Bt cornfields reveals little pollen falling on adjacent milkweed leaves (3).

Timing

Corn produces pollen over a period of five to ten days. Based on known monarch migratory behavior, the majority of larvae are not present when corn is shedding pollen.

Preference

Monarch larvae in the Nature study ate significantly less when pollen, both Bt and untransformed, was placed on the leaf surface. Thus it appears likely that monarch larvae in the wild would avoid pollen-tainted milkweed leaves, if given a chance. Likewise, the presence of pollen on milkweed leaves may also discourage adult butterflies from laying eggs on these plants.

Dose

The Cornell scientists manually dusted corn pollen onto lightly misted milkweed leaves to "visually match densities on milkweed leaves collected from corn fields." Though the actual "dose" of Bt pollen was not measured in this trial, the amount of pollen found on milkweed leaves located several meters away in ditchrows adjacent to corn fields would likely be much less, thereby diluting the dose-response effect on larvae dining there.

Ironically, the backlash against growing Bt corn might in turn prove detrimental not only to the monarch, but to other components of the ecosystem as well. If protesters succeed in their quest to reduce or ban the planting of genetically modified corn, the recent trend toward reduced use of chemical insecticide sprays for the European corn borer (ECB) could be reversed. Since the introduction of Bt-protected corn, farmers reported fewer insecticide treatments for target pests were required, which should be beneficial for many species of insect, including the monarch.

Interestingly, some farmers have unintentionally limited Bt pollen drift and increased insect bio-diversity by planting non-Bt refugia around boundaries of Bt-crop fields. Planted in this manner, refugia offer several benefits. In addition to providing a source of susceptible mates for insects resistant to Bt corn, refugia supply prey for beneficial insects and a buffer zone that limits Bt pollen dispersal outside the field. Also, farmers may one day have the option of planting varieties of Bt corn, currently being developed, that target Bt gene expression to the corn stalk, thereby eliminating risks associated with the presence of Bt protein in pollen.

Efforts by conservation groups to curtail dwindling monarch populations are largely directed at habitat preservation and other conservation measures. As one commentator suggested, we should "become a nation of Johnny Milkweeds, planting milkweed seeds wherever we can" and thereby accomplish for monarch butterflies what the campaign to erect bluebird houses across the US has successfully achieved for bluebirds.

ISB; July, 1999