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Adult Emergence

Rick Hafele on Adult Insect Emergence Timing is Everything !

Whoever coined the phrase, timing is everything, must have been a fly fisher. A lot of what we do and how successful we are is due to timing. There is the timing of the seasons, and the corresponding changes in streamflows and water temperature. There is the timing of fish runs and spawning, factors that determine where fish will be and what they’ll be doing. And for anglers looking for those special days of wild surface feeding activity, there is the perplexing timing of insect hatches. How do aquatic larvae, tucked under rocks in a riffle, know that today is the day to suddenly abandon the relative safety of the stream or lake bottom and rise up to the surface for their final orgy of reproduction? And why at three o’clock instead of ten o’clock? These questions are related to some of the most fundamental factors controlling insect life cycles and behavior. They are also part of a large complex subject that we can only scratch the surface of in the space of this column. Nonetheless, time waits for no one, so here goes.

Adult insect emergence has both a long-term seasonal cycle and short-term day-to-day patterns. Let’s first take a look at the factors that affect long-term seasonal patterns. If you’ve been fishing for a while you know there is a consistent pattern to insect hatches within each season. Blue-winged Olives hatch before the March Browns, which are followed by Pale Morning Duns, etc. This pattern is repeated year after year, but exactly when this cycle of hatches starts can vary from year to year and from stream to stream by several weeks. If you were planning a trip to fish the Pale Morning Dun hatch on the Big Horn River, it would be nice to know you’ll be there at the right time. Is it possible to know ahead of time if a particular hatch will be early or late?

The primary factor controlling the timing of seasonal emergence cycles is water temperature. While that may sound simple, the way temperature interacts with growth and development, and ultimately the timing of adult emergence, is anything but simple. For one thing, the way temperature affects development differs widely between different species.

Days to Egg Hatch for a Clinger Mayfly
Eggs reared at
Temp. °C Days to Egg Hatch
Everything starts with the egg stage. For many species the number of days eggs incubate before hatching increases as temperature decreases. Some species of clinger mayflies (Family: Heptageniidae) provide a good example of this response. When eggs are reared at 20 °C they hatch in about 15 days, but when they are reared at 6 °C they take at least 80 days to hatch (see chart). For other species eggs remain dormant until a minimum threshold temperature is reached at which time the eggs hatch regardless of how long they spent at a lower temperature. The eggs of the burrowing mayfly Ephoron album, for example, will not hatch until water temperatures reach 10 °C.

Monitoring the response of egg development at different constant temperatures in a lab is one thing, but in nature temperature generally varies over a 24-hour period. When this is taken into account, eggs for some species hatch sooner than expected. For example, the greater the 24-hour range in water temperature the less time it took for the eggs of some water boatmen species to hatch.

Besides triggering when eggs hatch, temperature also affects the duration of hatching, a potentially important factor in synchronizing adult emergence. This is illustrated by studies of some Baetis (Blue-winged Olive) eggs. When held at 3 °C eggs began hatching after 119 days and hatching extended over a 34-day period. When eggs were held at 22 °C hatching began after just seven days, and was completed for all eggs in just three days.

But wait, there’s more. Egg diapause, a period of arrested development or dormancy (it’s a bit like puberty!), is often induced or terminated by temperature. In a simple example, eggs of some species remain dormant until temperatures drop to or near freezing, which breaks the diapause and the eggs then begin developing. Thus, eggs that may have been laid over a period of weeks all start development at nearly the same time.

Interesting – right? It gets even more interesting after the eggs hatch, because temperature also has an important, yet varied, affect on feeding and growth of the newly hatched larvae. Since insects are cold-blooded, in general, growth rates are lower at cold temperatures and increase as temperature increases. There are many deviations, however, from this simple response. Some species, for example, remain very active in cold water and complete most of their growth with temperatures at or near freezing, a phenomenon common to many stoneflies, some of which have been shown to complete two thirds of their growth under ice at temperatures less than 1 °C. While this doesn’t help synchronize adult emergence, it does point out that many species can grow at temperatures typically thought to be too low for significant growth.

For species that over-winter as larvae, but do not grow significantly through the winter, a rapid increase in growth tends to occur as temperatures begin to warm in the spring. One way such temperature changes help synchronize adult emergence is through a process called differential temperature-growth response. That’s a fancy way to say that as temperatures rise in the spring younger or smaller individuals have a lower temperature threshold for growth than larger or older individuals. Thus, growth is stimulated first in the youngest individuals allowing them to catch up in size and development to individuals that were larger and more mature. The net result is that the majority of individuals in a population all reach maturity at nearly the same time. Declining temperatures may also help synchronize emergence. For example, the closer individuals of Chaoborus americanus (phantom midges) are to emergence, the more a small drop in temperature slows their development. This results in highly synchronized emergence, and reduces the chance of adults emerging during periods of unsuitably cold air temperatures.

There’s at least one more way temperature may affect long-term seasonal emergence patterns. Changes in growth hormones apparently tell larvae when to stop growing and to start developing adult tissue for emergence. These changes in growth hormones are often triggered by temperature. Some researchers have found that this shift from larval growth to adult tissue development occurs when temperature exceeds some critical threshold, regardless of how large or old the larvae are at the time. This synchronizes adult emergence and also explains why adults often decrease in size over their emergence period. Take Callibaetis, or Speckle-winged Quills, for example. They typically have two or three emergence periods spread out from April or May through September and October. Those that emerge in the spring are considerably larger than those that emerge in the summer or fall. If a minimum temperature is needed to trigger adult tissue development, then those nymphs growing through the winter and emerging in the spring spend more time growing as nymphs before that minimum temperature is reached. During the summer or fall the threshold temperature is reached much sooner, and thus the final nymph stage, and adults, are smaller by comparison.

There are other factors besides temperature that affect long-term emergence patterns. One of the most important is photoperiod (the number of hours of daylight and darkness over a 24-hour period). Photoperiod plays a role, though apparently a smaller one, in the timing and synchronization of emergence. For example, photoperiod significantly affected larval development of some dragonflies when they were reared at constant temperature, but had little or no effect on the same larvae when they were reared under natural fluctuating temperature regimes. This suggests that in more constant temperature environments (spring creeks for example) photoperiod becomes a significant factor in synchronizing emergence for some species. Other species that live in constant temperature environments, like spring creeks, show little or no response to photoperiod, and tend to have very poor emergence synchronization, instead showing nearly continuous development and emergence throughout the year.

Now you have a few examples of the way temperature and photoperiod may affect long-term seasonal emergence patterns. Hopefully, you can also see why it is very difficult to make generalizations about the timing of hatches, and if they will be early or late in any given year. Aquatic insects have evolved a wide range of ways to synchronize emergence, and these factors may operate in different ways at different stages in their life cycles. While this makes it very interesting to study, it also makes it very difficult to understand, and understanding the pattern for one species does not mean you know the pattern for other species. Now it is time to take a quick look at some of the factors affecting day-to-day variations in emergence.

Temperature (water and air), light intensity (cloudy or sunny) and moon phase have all been implicated in day-to-day variations of emergence. Temperature is again the dominant player, but it is not always as obvious at it might seem (so what’s new!). For example, rising daily temperature is known to trigger emergence. However, for some mayflies it is not the temperature on the morning of emergence that is important. Instead it is the temperature on the morning 24 to 48 hours before emergence that seems to determine if emergence will occur or not on any particular day. Light intensity also plays a significant, but again varying role, in daily emergence. Some species emerge when light intensity is high, such as a bright sunny day, while other species prefer low light conditions for emergence. Species of the mayfly genus Baetis, or Blue-winged Olives, are a good example of species that prefer low light conditions. Their hatches are routinely heavier on cloudy overcast days than on sunny bright ones. Species of Isoperla, or Little Yellow Stoneflies, on the other hand, often emerge best on bright sunny days instead of cloudy ones. The time of year may also influence if emergence is heaviest on sunny days or cloudy ones, with cloudy days preferred more often during the summer than during the spring or fall. This is likely related to air temperature, as hot dry conditions reduces survival of most adult aquatic insects.

Moon phase has been shown to affect the emergence of some aquatic insects. This has been most prominent for species that live near the equator where other environmental triggers, like temperature and photoperiod, change little throughout the year. The response to changing moon phases is again different for different species. A few species show peak emergence only during a full moon, while other species show peak emergence during a new moon. In temperate climates there is little evidence that changes in the moon affect emergence.

Other factors probably enter the picture as well. Changes in barometric pressure, due to changing weather patterns, seem to have, at times, dramatic effects on insect hatches and fish feeding behavior. But in my experience the effects are not consistent. I’ve seen some hatches turn on when storms approach and barometric pressure is dropping, and I’ve also seen the opposite occur. I suspect the effects of changing weather are somewhat dependent upon what conditions were like before the weather started changing. A storm bringing in cool, cloudy weather, may be just what is needed to stimulate more emergence activity during hot summer conditions, but may turn off hatches when water temperature is already cool in the spring and fall.

So, is it possible to predict if a hatch will be early or late, or heavy today and not tomorrow? Nature has woven a fascinating tapestry that in many ways defies prediction. This is both a frustration and a joy. More than helping you predict when or if a hatch might occur, I hope this column may help you appreciate the beautiful intricacies of nature that every hatch represents.

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