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Determining PMI with Forensic Entomology

Determining PMI with Forensic Entomology. This is will require a working brain, and a calculator. The plan of attack. Insect life cycles and development times are predictable based on environmental temperature

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Determining PMI with Forensic Entomology

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  1. Determining PMI withForensic Entomology This is will require a working brain, and a calculator.

  2. The plan of attack • Insect life cycles and development times are predictable based on environmental temperature • This only works because insects can’t maintain a constant body temperature, so their metabolism is very dependent on ambient temperatures • (Why wouldn’t this work for mammals?) • This concept is called physiological time

  3. Degree Days • We need a metric that combines time and temperature, thus measuring how much energy is available. • We consider only temperatures above a certain threshold, below which development does not occur. (base temp) • So we simply multiply the number of degrees above the base temp times the number of days. Thus, Degree-Days, or DD. • DDs accumulated over a number of days are called ADD

  4. DD – Simple Example • For five days, the average temperature is 10 degrees above the base temperature. • 50 ADD • For 25 days, the average temp is 2 degrees above the base temp. • 50 ADD • Both represent the same amount of physiological time • Conclusion: • Eggs laid at the beginning of the interval in either scenario would be at the same stage at the end of the interval (even though the times are different)

  5. DDs for Stages by Species • Each stage of development requires a certain amount of DDs. • This varies by stage and by species • Example: Species A (green bottle fly) • Requires 12.5 DD from egg-laying to emergence as first instar larva • Requires 13.9 MORE DDs to complete 1st instar larval stage. • SO, from egg-laying to beginning for 2nd instar stage (end of 1st) requires ???? • Keep in mind, this is NOT a number of days, but a measure of the energy requirement

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