Forensic Palynology – Use of Pollen to Solve Crimes
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
Determination of time of death (T.O.D.) is one of the most important pieces of information that homicide investigators need in solving murder investigations. This is normally determined by the Medical Examiner (M.E.), but if the victim has been dead for a long time the M.E. cannot use algor mortis, livor mortis, or rigor mortis to assist in the determination of T.O.D. If the body has started to decompose, then examination of the types of insects present on and in the body can be used to help pinpoint T.O.D. In such cases the M.E. might consult a forensic entomologist to identify when the victim was murdered. If the body is to the skeletal stage of decomposition, then it may be necessary to consult a forensic anthropologist to assist in determining T.O.D., as well as cause of death and identification of the remains. One type of evidence that can be used in some cases to narrow the window of T.O.D. is pollen. This is because pollen of any given species of plant growing in a particular area is produced only during a specific time of the year. Therefore, the presence of certain types of pollen and the absence of other types of pollen on the clothes of a corpse compared to the types of pollen found around the body might indicate the T.O.D. of the victim. Forensic palynology is the use of pollen and fungal spores to solve crimes and scientists who study this type of evidence are known as forensic palynologists.
Pollen is the male gametophyte (produces male gametes or sperm) of seed bearing plants. Transfer of pollen from the pollen producing structures (the anthers) to the pollen receiving structures (stigmas in flowering plants) is called pollination. Pollination and subsequent fertilization of the ovules of a plant is necessary for the production of seed and fruits in many economically important cultivated plants. Unfortunately, pollen also can act as a powerful allergen and causes allergic rhinitis in many people.
Pollen can be transferred from one plant to another in several different ways, but the principal means by which pollen is transferred is by wind (anemophilous plants) and by insects such as bees, ants, butterflies, and moths (entomophilous plants). Pollen produced by anemophilous (wind-pollinated) plants is normally dry and small, or has modifications that permit it to stay in air currents for long periods. Pollen of anemophilous plants is produced in large quantities and will be widely distributed in areas where these plant species grow. Pollen produced by entomophilous plants is often large and “sticky” which makes it easier to be carried on the body parts of pollinators. This type of pollen is usually not distributed widely but remains close to the areas around where the plants are growing.
Pollen is normally short lived, remaining viable for only a few hours to a few days. However, the outer layer (exine) is composed of an extremely tough, durable material called sporopollenin which makes pollen resistant to extremely harsh conditions and permits it to remain identifiable for centuries. The shape, structure, and external scalloping of pollen grains make it possible in many cases to identify the source of pollen to the genus or even to the species level. Because pollen remains in the environment for extended periods of time and can be quite unique in its appearance, it can provide the forensic scientist with useful forensic evidence in many criminal investigations.
Different plants produce pollen at different times of the year. And because plants are often restricted to particular regions, countries, or continents, it is possible to use this information in forensic analyses. For example, pollen can be used to know something about where someone has been and when they were there. Pollen sticks to many different fabrics and as mentioned above is persistent in the environment for extended periods of time. Therefore, the identity of pollen grains can tell the forensic palynologist much information about the type of plant species nearby and the time of year that a crime may have occurred.
The first report of the use of forensic palynology to solve a crime was in solving a murder case in Austria in 1959. The murder was solved by correlating the abundance of a particular type of pollen found on the victim with that found at the crime scene and on the murderer. Since pollen is so small and abundant, it is almost always present at a crime scene; thus, pollen can be a valuable tool in forensic investigations. Fungal spores can also be very useful in forensic palynology. This is because fungal spores are extremely abundant in nature and can often be identified to the taxonomic level of genus; identifiable presence and specificity when present at a crime scene just as is true with pollen.
New Zealand is currently the only country that uses forensic palynology routinely to solve crimes. Forensic palynology is not routinely used in the United States in solving crimes even though studies would suggest that this field of study could contribute significantly in the solving of crimes. One reason for this view is that previous studies have shown that soil, honey, and polsters (cushion plants that collect pollen, dust, etc.; e.g. a cushion moss would be an example of a polster) from different regions possess a unique spectrum of pollen called a pollen fingerprint, making the United States an ideal country to use forensic palynology. The lack of any significant use of forensic palynology in solving crimes has led to a great deal of skepticism in the population about the usefulness of pollen and fungal spores as forensic tools. There is certainly precedence for using pollen in assisting in criminal investigations; we just need to rethink some of our biases about the usefulness of microscopic structures (trace evidence) in solving crimes. DNA is considered the gold standard of evidence in criminal investigations. Perhaps we in the U.S. are failing to avail ourselves fully of a potent tool in the arsenal of the forensic scientist; the use of pollen and mold spores in solving crimes.
Consult the references below for additional information on the field of forensic palynology:
Mildenhall, D.C. et al. (editorial) (2006). Forensic Palynology: Why do it and how it works. Forensic Science International 163:163-172.
Mildenhall, D.C. (2006). Hypericum pollen determines the presence of burglars at the scene of a crime: An example of forensic palynology. Forensic Science International 163:231-235.
Passcrelli, Lilian. (2016) Forensic Palynology in Argentina. An alternative treatment for tape method is proposed. Peertechz J. Forsenci Sci. Technol. 2(1):001-003.
Weber M., Ulrich S. (2016) Forensic Palynology: How Pollen in Dry Grass Can Link to a Crime Scene. In: Kars H., van den Eijkel L. (eds) Soil in Criminal and Environmental Forensics. Soil Forensics. Springer, Cham.
Wiltshire, P. E. J. (2016) Protocols for forensic palynology. Palynology. Vol. 40(1):4-24.