Tom Turpin
Professor of
Purdue University


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Insect frass

Iris borer
Iris borer frass

Photo credit: John Obermeyer

Anyone recognize the long-running children’s TV program on PBS called “Sesame Street”? Probably most of us do. That’s because in the years following the 1969 introduction of the show it has been estimated that over 95 percent of U.S. preschoolers have seen it. That’s a lot of kids, parents and grandparents who know about “Sesame Street” and its Muppet characters.

One of the recurring features of the show has been the word of the day. That segment featured a special guest such as Adam Sandler, LL Cool J or Kelly Ripa. The well-known guest and a Muppet, often Elmo, would team up to explain and demonstrate the meaning of a word.

So let’s do an entomological version of the “Sesame Street” word of the day – without a guest star to help explain the meaning.  Today’s word is “frass.” Frass is not a word you hear every day. In fact, I would wager that it is a word that many of us have never heard, much less used. That’s because “frass” is a word used mainly in scientific circles. 

insect frass
Fall armyworm frass

Photo credit: John Obermeyer

Most dictionaries define “frass” as excrement or other refuse left by insects and insect larvae. In other words, frass is insect manure – the poop of insects. The word apparently showed up in the mid-1800s and was based on a German word that meant “devouring as a beast does.” 

A number of terms exist for waste material discharged from animal bodies after digestion, including “feces,” “stool,” “dung” and “droppings.” Informally, terms such as “cow pies” and “road apples” are used to describe excrement from cows and horses, respectively. 

Exactly why a specific term for insect manure was needed is not clear. But for sure, “frass” can be used to identify insects even if the insects are no longer present. Such information is useful to help solve problems with pest insects. Home gardeners generally will recognize the black pellets – the frass of hornworm caterpillars – that fall to the ground or collect on leaves of tomato plants.  Sometimes hornworm frass is noticed before the green caterpillars are seen, but where there is frass there are bound to be caterpillars!

Insect pest control specialists and gardeners are likely to encounter insect frass more often than most people; however, almost all of us have noticed fly specks from time to time. The poet Karl Shapiro references insect frass in his poem “The Fly” with the line “You dot all whiteness with diminutive stool.”
Ladybugs are also prone to leave little spots of frass on surfaces where they crawl.  Such behavior just adds insult to their presence inside our homes during the fall and winter months.

Manure accumulation is always a problem when animals such as pigs and chickens are raised in confined areas. Some of us remember cleaning the manure out of chicken houses and barns using shovels and pitchforks. The same problem exists when people grow crickets and mealworms for the fishing bait and pet food market but on a smaller scale, to be sure; however, when large numbers of insects are being produced the frass accumulates.  

So like large-animal producers before them, insect producers have begun to promote insect frass as a fertilizer product. The percentage of nitrogen, potassium and phosphorus is not as high as in bird or mammal manure, but it does have fertilizer value. 

You can buy cricket frass under the name of Cricket Poo or Kricket Krap. Mealworm frass is generally called mealworm castings. Casting is the term that has been used to describe earthworm manure, a product that has long been used as a dry fertilizer or to make a nutrient tea.

So while entomologists seem to be happy using the word “frass” in day-to-day conversation, it’s not a word I hear often from my friends. So you can imagine that some years ago I was a bit surprised when a young woman who had taken my introductory entomology class said the most important thing she learned in the class was the word “frass.”  According to the student, she was in the habit of exclaiming – no “expletive deleted” – ever so often. Now she says “no frass,” and people including her mother think that a college education has been good for her!  Frassinating isn’t it?    


Writer: Tom Turpin
Editor: Keith Robinson