Insect Notes
If you've been recently anywhere near trees out in Long Island (or surely other locations in the Northeast), you've probably had some caterpillars string down onto your head or clothing. We're experiencing quite an outbreak of cankwerworms that are defoliating many trees (mostly oaks). I remember a week or so ago there were so many in the forest patch I was walking in that their frass (insect feces) falling on leaves and ground sounded like rain.
Many of the caterpillars are the Fall Cankerworm, Alsophila pometaria (Harris). The larvae vary in color from apple green to dark brownish green in color with a dark middle stripe and three narrow white lines on each side. Fall cankerworms have three pairs of fleshy prolegs at the end of the abdomen (as opposed to the Spring Cankerworm, Paleacrita vernata). The Spring and Fall Cankerworms larvae emerge in the Spring, when foliage is young. When they are about to pupate, they descend to the ground on silk threads. The larvae then burrow into the ground to a depth of one to four inches, spin a silken cocoon and pupate. The pupae remain in the soil until the late fall or early spring. The names of two species refer to when the adults emerge: the Spring cankerworms emerge in the early spring the following season, the Fall cankerworms emerge as adults in late fall of the same season, often during warmer periods in October through early December. The females are wingless and a dull grey color. They crawl up on tree trunks to await a winged male.
My advisor, Doug Futuyma, found that populations of the Fall Cankerworm are formed by a minority of sexual individuals and a majority of parthenogenetic females - females that reproduce asexually. Because females are wingless and many asexual, a population can be composed of many very differentiated strains, specialized on different host plants (oak, maple, etc), with different adult emergence time and hatching times. [Mitter, C., D. J. Futuyma, J. C. Schneider and J. D. Hare. 1979. Genetic Variation and Host Plant Relations in a Parthenogenetic Moth. Evolution 33: 777-790.]
For those in the NorthEast of the US, there is another exciting insect event: the emergence of periodical cicadas (Magicicada sp.). The typical cicadas have yearly reproductive cycles. Periodical cicadas live most of their lives in a immature stage below ground feeding on root sap, and then emerge in large numbers. Then after a period of 13 or 17 years, depending on the species, a huge number of adults emerge. There are several species, each with distinct morphology. Some have a period of 13, others 17 years. Curiously, species with similar periods are not the most closely related, rather morphologically similar species with 13 and 17-periods form closely related species pairs. 17-year nymphs grow more slowly during the first four years of life than do 13-year cicadas. An individual species may have different populations, in different localities, all with similar periods, but with emergence dates falling on different years. This pattern gives rise to different broods, that rarely reproduce with each other (but there are exceptions). This year is the emergence of brood X, of the 17-year species. So far it hasn't been reported on Long Island, but other states are already experiencing the explosion. Check Cicada mania for updates.
Why should they emerge simultaneously in large numbers? The common explanation is that by emerging in large number, predators or parasites become rapidly satiated leaving many alive. The more interesting question for me is why 13/17 years? Why not some other numbers? It seems to have to do with the fact that they are prime numbers, which reduces the occurence of co-emergence of broods, or the co-emergence of the cicadas and predators. [Cox, RT and CE Carlton. 1998. A commentary on prime numbers and life cycles of periodical cicadas AMERICAN NATURALIST 152: 162-164] But then why not other prime numbers, such as 3,5 or 7? Lloyd and Dybas postulated a parasitoid with a life cycle of many years duration, nearly synchronized with the ancestral periodical cicada and exerting a counter-selection pressure favoring a longer life cycle. A race between the parasitoid and the cicadas would have culminated in the 13-year cycle, and the parasitoid would have gone exticnt. [Monte Lloyd and Henry S. Dybas. 1966. The Periodical Cicada Problem. II. Evolution 20: 466-505.]
Many of the caterpillars are the Fall Cankerworm, Alsophila pometaria (Harris). The larvae vary in color from apple green to dark brownish green in color with a dark middle stripe and three narrow white lines on each side. Fall cankerworms have three pairs of fleshy prolegs at the end of the abdomen (as opposed to the Spring Cankerworm, Paleacrita vernata). The Spring and Fall Cankerworms larvae emerge in the Spring, when foliage is young. When they are about to pupate, they descend to the ground on silk threads. The larvae then burrow into the ground to a depth of one to four inches, spin a silken cocoon and pupate. The pupae remain in the soil until the late fall or early spring. The names of two species refer to when the adults emerge: the Spring cankerworms emerge in the early spring the following season, the Fall cankerworms emerge as adults in late fall of the same season, often during warmer periods in October through early December. The females are wingless and a dull grey color. They crawl up on tree trunks to await a winged male.
My advisor, Doug Futuyma, found that populations of the Fall Cankerworm are formed by a minority of sexual individuals and a majority of parthenogenetic females - females that reproduce asexually. Because females are wingless and many asexual, a population can be composed of many very differentiated strains, specialized on different host plants (oak, maple, etc), with different adult emergence time and hatching times. [Mitter, C., D. J. Futuyma, J. C. Schneider and J. D. Hare. 1979. Genetic Variation and Host Plant Relations in a Parthenogenetic Moth. Evolution 33: 777-790.]
For those in the NorthEast of the US, there is another exciting insect event: the emergence of periodical cicadas (Magicicada sp.). The typical cicadas have yearly reproductive cycles. Periodical cicadas live most of their lives in a immature stage below ground feeding on root sap, and then emerge in large numbers. Then after a period of 13 or 17 years, depending on the species, a huge number of adults emerge. There are several species, each with distinct morphology. Some have a period of 13, others 17 years. Curiously, species with similar periods are not the most closely related, rather morphologically similar species with 13 and 17-periods form closely related species pairs. 17-year nymphs grow more slowly during the first four years of life than do 13-year cicadas. An individual species may have different populations, in different localities, all with similar periods, but with emergence dates falling on different years. This pattern gives rise to different broods, that rarely reproduce with each other (but there are exceptions). This year is the emergence of brood X, of the 17-year species. So far it hasn't been reported on Long Island, but other states are already experiencing the explosion. Check Cicada mania for updates.
Why should they emerge simultaneously in large numbers? The common explanation is that by emerging in large number, predators or parasites become rapidly satiated leaving many alive. The more interesting question for me is why 13/17 years? Why not some other numbers? It seems to have to do with the fact that they are prime numbers, which reduces the occurence of co-emergence of broods, or the co-emergence of the cicadas and predators. [Cox, RT and CE Carlton. 1998. A commentary on prime numbers and life cycles of periodical cicadas AMERICAN NATURALIST 152: 162-164] But then why not other prime numbers, such as 3,5 or 7? Lloyd and Dybas postulated a parasitoid with a life cycle of many years duration, nearly synchronized with the ancestral periodical cicada and exerting a counter-selection pressure favoring a longer life cycle. A race between the parasitoid and the cicadas would have culminated in the 13-year cycle, and the parasitoid would have gone exticnt. [Monte Lloyd and Henry S. Dybas. 1966. The Periodical Cicada Problem. II. Evolution 20: 466-505.]
posted by André Levy @ 9:20 PM
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