As many of us already know, wild and native trout thrive in clean, cold water environments. One of the reasons they do so is because of the abundance of insect life that is also found in these same habitats. Diverse insect life is of significant importance to wild and native trout. As trout habitats ebb and flow from season to season, going through years of drought, extensive rains, and typical changes that occur year over year, it is important to maintain assorted biota in order to give the trout the best chance for survival. Some years, the species of abundance may be amphipods, some years may be terrestrial insects, others years could focus more on stoneflies or mayflies. It is absolutely critical to keep as wide a range of invertebrates as possible in any given aquatic ecosystem in order to avoid disaster during these yearly changes that are becoming more frequent. This is one reason why insects have always been so resilient. 


Trout feed mainly on aquatic insects, and this is a widely undisputed fact. While crustaceans, terrestrial insects, smaller fish species, and even mice can show up in stomach contents, aquatic insects still make up the bulk of it. A report released this year by Idaho Fish & Game on Idaho bull trout surprisingly discovered that this carnivorously-portrayed fish still retained an incredible amount of insects in its diet during the summer months in spawning tributaries. 87% of its stomach contents were comprised of aquatic insects and 11% were terrestrial insects. Trout are opportunists at best when it comes to feeding, but the thriving bug life underwater and hatches throughout the year are really what drives the majority of their diet, no matter what type of trout. 

While all plants and animals serve as indicators of changes in the environments, insects do so at a micro-level, which can show the change that is coming on a macro-scale. Just a few of the threats facing our aquatic insects are urbanization, habitat degradation which can be agricultural or industrial, recreational use, and environmental changes ( i.e. climate change). As the world becomes more populated and more areas see development, losses or mitigations of creeks and streams will occur. This can fragment habitat in addition to making it disappear altogether. As deforestation happens due to industrialization, or agricultural reasons like logging occur, lack of leaf litter availability and quality can reduce the number of aquatic insects present in a particular food web. Agricultural runoff can also negatively impact populations by tainting the water supply for the insects. 

In the past two years, especially, outdoor recreation has become more popular. While this can be both encouraging and discouraging, proper use of the resource and taking extra precautions to make sure you are not disrupting the ecosystem is merited. Dragging watercraft on river bottoms, removing rocks from the water, and motors used in shallow or grassy riverbeds can all cause disruptions. In even the smallest bodies of water, activities such as ORV, ATV & dirt bike use can also severely disturb the sensitive insect inhabitants. 

With more and more summers of dramatic heat, severe drought, and wildfires occurring, the trickle-down effect has potentially devastating impacts on our trout fisheries. Most aquatic insects are detritovores, meaning they shred and consume leaves in order to gain nutrients. In areas where wildfires have occurred, there is a significant decrease in the number of fallen leaves making their way into bodies of water and an increase in ash present in the water. Extreme heat also contributes to the faster breakdown of leaf matter in water by microbes which also takes away that important insect food source.  In areas where drought is occurring, sometimes insects are forced into a diapause state or are naturally driven to leave the area altogether due to genetic instinct as the environment around them is changing. We often talk about “insect drift” in angling, but it is important to understand that drift is also a biological tool used by insect populations to move when conditions change and become undesirable. In “active drift” immature insects may choose to let the currents carry them downriver to an area where there may be more water, more leaf matter, less disruption, ash, or pollution. In poor water years, agricultural impacts can also be felt by insect populations due to what is known as “passive drift” caused by increased flows from dams and diversions, causing an inadvertent sweeping of the insect population downriver to what may or may not be a suitable habitat due to substrate changes or the chance that the water source could dry up by the end of the agricultural growing season. 

One great way to keep an eye on what is happening is to use insects as biological indicators. Taking inventory of the types of insects in bodies of water on a consistent basis is an incredible tool that can indicate a disruption in the food web or sense change in a habitat. One great example of this is the South Fork Initiative, a program of the Henry’s Fork Foundation which has begun surveying the macroinvertebrates found in the South Fork of the Snake River, to see if there are regular increases or decreases in the taxa that they are monitoring, and investigate what is causing the disruption. Many state departments of environmental quality and other organizations or universities already do this and provide chances for volunteer opportunities if one is interested in getting involved. A more specific insect example that has had many eyes on it in recent years is the Western glacier stonefly. This particular stonefly has an extraordinarily narrow range of water temperatures in which it may survive, regaling its habitat to approximately 16 streams in the U.S. Stoneflies, by nature, tend to genetically disperse mainly through drift because they are poor fliers, which limits its opportunities to adapt as its unique habitat in glacial outflow water deteriorates. Snowpack increases in some winters may not be enough to counter the loss of glaciers in the stonefly’s endemic streams. The species is now listed as threatened under the Endangered Species Act. Monitoring the local fauna and gaining regulatory status such as this can help in some ways when it comes to preventing the development or avoiding detrimental agricultural or recreation impacts on certain bodies of water. 

Parallel to the types of insects that are most important to trout anglers in fishing, the same “big three” groups of insects are also the most in danger of decline due to the aforementioned factors. EPT, or Ephemeroptera (mayflies), Trichoptera (caddisflies), and Plecoptera (stoneflies) are the most vulnerable and sensitive groups of insects when it comes to change. Odontata (damselflies & dragonflies), are also highly sensitive to disturbance and have many species that are considered endangered or threatened. Diptera (true flies), Hemiptera (true bugs), and Coleoptera (beetles) are slightly more hardy, although, with insects, there is always an exception. Because insects are soft-bodied, they uptake any changes in their water sources cutaneously or through their gills as they respirate, making even the most subtle changes quickly realized.  As water temperatures warm and algae blooms become more common, we may start to see insects die off as they get trapped or lose food sources. In mayfly species, one of the biggest factors is the adult life span when it comes to their opportunities for dispersal in adverse conditions. Mayfly adults only live for about a day, but may have a staggered emergence throughout a river system based on water depth and temperatures. Due to their limited lifespan and mating opportunities, they are less likely to travel as adults to look for a more suitable habitat. This is true for most lentic species or species that come from moving water. Lotic insects are from more still or standing water environments and have to adapt more quickly because lakes and pools tend to dry up more often, although this is changing with our human manipulation of moving water.  Many of the more endangered species are considered specialists due to their ecological niche and limited temperature survival ranges or food sources. The factors that can alter aquatic insect growth, development, and reproduction are innumerable. 

Insect phenology, or their life cycle calendar, is that of being univoltine, bivoltine, or multivoltine. This translates to mean that insects emerge and complete their life cycles in one year, twice per year, or multiple times per year. One thing that may start to occur in warming water more frequently in stoneflies, mayflies, and caddisflies is a generational shift with more hatches happening in a shorter period of time. While the may sound exciting to the angler, it is not good news for the overall health of the stream. When this happens, more oxygen is required for the immature stage, which is not easy in a warming water source. They expend more energy to grow and the emerging insect may be smaller in size due to restrictions in its ability to grow. Another reason this is bad is the exact reason that was earlier mentioned in regards to trout needing a diverse food source- some years can be bad. A low water year, an oil spill or a fire can be offset in broods of insects that have a cushion of emergences behind the one that is predominantly impacted. 

Whether we realize it or not, we are currently living in the sixth mass extinction on our planet. A study by Francisco Sánchez-Bayo and Kris Wyckhuys estimates a whopping 33% of aquatic insects are threatened to become extinct if something drastic doesn’t change.  Insects are the main source of food for so many groups of vertebrates, and they are essentially the anchor in all food webs. If insects become extinct, there will be consequences on a much larger scale, not just for trout, but for all of us. To quote the former CEO of Orvis, Perk Perkins, “If we are going to benefit from our natural resources, we must be willing to take action to protect them”. It’s time to step up and start caring about the bugs as much as we care about the fish.

About the Author: Maggie Heumann is a member of the Jackson Hole Trout Unlimited and the Idaho Wildlife Federation Board. She helped found Artemis Sportswomen and is an active advocate for conservation. She currently manages the Orvis Jackson Hole store and gets out to fish and collect bugs as much as possible with her husband, Hunter, a fishing guide, and her dogs, Bug, Missy, & Cotton.

References

Baumann, R.W., A.R. Gaufin, and R.F. Surdick. (1977). The stoneflies (Plecoptera) of the Rocky Mountains. Memoirs of the American Entomological Society 31:1-208.

Bilton, D.T., J.R. Freeland, and B. Okamura. (2001). Dispersal in Freshwater Invertebrates. Annual Review of Ecology and Systematics 32:159-181.

Bowersox, B. (2021). The Diverse Diet of Idaho’s Bull Trout. Retrieved from:

https://idfg.idaho.gov/blog/2021/08/diverse-diet-adult-bull-trout.

Grafe, C.S., C.A. Mebane, M.J. McIntyre, D.A. Essig, D.H. Brandt, and D.T. Mosier. (2002). The Idaho Department of Environmental Quality Water Body Assessment Guidance, Second Edition-Final. Idaho Department of Environmental Quality; Boise, Idaho. Retrieved from: http://www.deq.idaho.gov/water/data_reports/surface_water/monitoring/wbag_02_entire.pdf 

Griffith, M.B., Barrows, E.M., and S.A. Perry. (1998). Lateral dispersal of adult insects (Plecoptera, Trichoptera) following emergence from headwater streams in forested Appalachian catchments. Annals of the Entomological Society of America 91:195-201.

Naman, S.M., J.S. Rosenfeld, J.S. Richardson. (2016). Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes. Can. Journal of Fish. & Aquatic Sciences 73: 1292-1305. 

NatureServe. (2009). Zapada glacier. Retrieved from http://www.natureserve.org/explorer/.

National Park Service (NPS). (2008). Glacier National Park: Nature and Science. Retrieved From: http://www.nps.gov/glac/naturescience/index.htm.

Owen, J. (2011). Short Life of British Mayfly Halved by Climate Change. Retrieved from: https://www.nationalgeographic.com/science/article/111026-mayflies-climate-change

Polhemus, D. (1993). Conservation of Aquatic Insects: Worldwide Crisis or Localized Threats? American Zoology 33: 588-598.

Sánchez-Bayo, F. and Wyckhuys K.A.G. (2019). Worldwide decline of the entomofauna: A review of its drivers. Biological Conservation 232: 8-27. 

U.S. Fish & Wildlife Service. (2019). Western glacier stonefly (Zapada glacier). Retrieved from: https://www.fws.gov/mountain-prairie/es/westernglacierstonefly.php.

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