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Caterpillars Everywhere as We Hike Along the San Pedro River

Having lived in Arizona for over 30 years, Chuck and his wife enjoy the great outdoors of the American Southwest.

San Pedro House with Cottonwood Tree behind it that ws Planted in 1956

San Pedro House with Cottonwood Tree behind it that ws Planted in 1956

We Decide To Go Hiking Along The Nearby San Pedro River

After our hike to Romero Pools in Arizona’s Catalina State Park the first weekend in April we decided to take a less strenuous hike the second weekend. Our chosen destination was a small park-like area known as the San Pedro House which is situated in the San Pedro River Riparian Area. The riparian area is a 57,000 acre conservation area stretching roughly 40 miles along Arizona’s San Pedro River from the Mexican border to the southern Arizona town of St. David.

The entire area is managed by the Federal Bureau of Land Management (BLM) and contains numerous hiking trails as well as a couple of old ghost towns and the remains of the old eighteenth century Spanish fort, known as the Presidio Santa Cruz de Terrenate,which is perched on a bluff high above the San Pedro River.a few miles north of the San Pedro House location (See my Hub An Enjoyable Hike to the Historic Presidio Santa Cruz de Terrenate)

Information Alert Sign on Interstate-10 Alerting drivers to keep safe driving distance to avoid accidents and safe distance from people to avoid COVID-19

Information Alert Sign on Interstate-10 Alerting drivers to keep safe driving distance to avoid accidents and safe distance from people to avoid COVID-19

The San Pedro River itself starts as a small stream that combines with two other streams, the Las Nutrias and the El Sauz, about ten miles south of the U.S.-Mexican Border in the Mexican State of Sonora. The result of this merging of three streams is the San Pedro River which flows northward to the U.S. border. While the San Pedro Riparian Area only extends as far as the town of St. David the river itself continues another 124 miles northward until it merges with the Gila River near Winkelman, Arizona.

130 year old Cottonwood Tree a few yards west of the San Pedro House

130 year old Cottonwood Tree a few yards west of the San Pedro House

San Pedro House

San Pedro House sits in a mountain valley between the Dragoon Mountains to the east and Huachuca Mountains to the west. The river at this point flows between the two mountain ranges.

It wasn’t long after turning east on State Highway 90 just north e of the City of Sierra Vista until we easily spotted our destination in the distance. The area we were passing through was desert covered with mostly brownish scrub. However, in the distance was a lush green line created by the cottonwood trees and other lush vegetation that flourishes along the river.

The San Pedro House is a small historic ranch house that has been restored and is now used as an information center and gift shop. Due to the COVID-19 restrictions on gatherings the San Pedro House itself is closed and a notice on the door listed a number of scheduled activities at the site which have been cancelled for the same reason.

The surrounding area is a globally recognized birding area which attracts bird watchers throughout the year. The ramada behind the house with picnic tables has also been roped off. On a couple of trips in previous years the ramada has been the site of talks on birds or a starting point for ranger led bird watching tours. One time we visited there were a number of people in the ramada helping to put tracking bands on hummingbirds. We were invited to join but preferred to go hiking.

There are a couple of other abandoned sheds and an abandoned wooden water tower as well as a couple of very large cottonwood trees. The biggest one, located a few yards west of the house is 130 years old and is so huge that I observed on this trip that a number of large branches had recently been removed including one whose main part was three feet or more in diameter and probably would have ended up tipping the tree over if it hadn’t been removed.

My enjoying the peace and tranquility along with the lush vegetation  of this desert oasis

My enjoying the peace and tranquility along with the lush vegetation of this desert oasis

Water Was Flowing In The River

The San Pedro House sits in a meadow that is surrounded by trees and hiking trails. Like other places we have visited on weekends during social distancing, the parking lot by the house was at least three quarters full but throughout our three or four hour visit we only encountered a few other people and were able to easily maintain a safe distance. There is a wide dirt path that leads from the house to the river a couple of hundred yards or so away.

Since it is spring, there is water in the San Pedro and other rivers thanks to spring rains and mountain snow melt. In the case of the San Pedro River, numerous streams and rivers carry water from rain and snow melt in a 4,700 square mile area to the San Pedro River. However, once the mountain snow has melted and the spring rains cease, the San Pedro and other rivers and streams either shrink to a trickle or dry up altogether..

caterpillars-everywhere-as-we-hike-along-the-san-pedro-river

However, heavy summer monsoon rain storms can quickly turn a dry river and its nearby streams into a raging flash flood. While the flood water passes through quickly it can tear up land adjacent to the rivers uprooting

trees to and other brush. As we hike along the river we encounter a number of trees and other debris from past floods laying dead along the trail.

According to the area map there are a number of interconnected and intersecting trails that eventually lead in a circle back to the San Pedro House. However, the trails are not well marked and in many spots overgrown with vegetation.

Caterpillars along the shore of the San Pedro River

Caterpillars along the shore of the San Pedro River

Despite having to climb over or detour around a fallen tree we didn’t have much of a problem until we reached the point where, according to the map the river curved to the east and the trail curved northwest back to the San Pedro House.

We were able to find our way to Kingfisher pond, which is the hole left by an old rock quarry that filled with water from the river overflowing and probably ground water as well as the water table lying close to the surface in the area. It is a small, picturesque where, according to the guidebook, kingfishers nest. We only found the pond because I saw a kingfisher fly over us and we headed in the direction he came from.

Raining Caterpillars

On the trail the scenery was beautiful and the weather perfect. A dry 70 degrees with a light breeze and canopy of shade from the numerous cottonwood and other trees.

Shortly after starting on the trail I began noticing a number of caterpillars crossing the path. As I continued on to where the trees were a little denser I began noticing increasing numbers of caterpillars and started taking pictures of them.

Tent Caterpillars on a rock in the San Pedro River

Tent Caterpillars on a rock in the San Pedro River

The larger ones were close to an inch long and they were literally dropping from the trees. My wife suddenly became aware of them when she noticed some large rocks at the water’s edge literally covered with caterpillars (when she later posted some pictures, on her Facebook page, of rocks covered with caterpillars some of the viewers asked if she had Photoshopped the picture to increase the number of caterpillars on the rock - which she hadn’t).

The caterpillars were literally raining down from the trees above, My wife freaked out a bit when she noticed some on her hat and clothes but endured it long enough to give me her cell phone to take some pictures of her with the caterpillars on her hat. After that I checked her regularly and flicked caterpillars off her hat and other clothing (as well as off mine as well).

Like me she was fascinated with the photo opportunities they offered and we spent about an hour photographing them.

Western Tent Caterpillars

Doing some research after the hike I learned that these were a species of Western Tent Caterpillars (Malacosoma californicum).

Tent caterpillars are found throughout North America and Europe as well as probably other parts of the world. Three subspecies of these caterpillars, known collectively as Western Tent Caterpillars, all seem to look the same as each other as well as other subspecies with different names in other places which also look and act much the same wherever they are found.

They have a four stage life cycle starting out as eggs laid by female moths (moths being the fourth stage and eggs the first) in summer. A female moth lays a mass of up to 300 eggs together encircling a tree branch and covering the mass with a frothy substance called spumaline which helps keep the eggs moist as well as providing a protective barrier for the developing larvae against parasitoid wasps.

Larvae develop within the eggs within about 3 to 4 weeks after the laying of the eggs. However, larvae remain encased within the egg until the following spring.

Closeup view of the communal nest in a tree created by the caterpillars after they hatched.

Closeup view of the communal nest in a tree created by the caterpillars after they hatched.

View of a smaller nest in tree housing a large group of tree caterpillars

View of a smaller nest in tree housing a large group of tree caterpillars

Feeling Sorry For Caterpillars Falling Out Of The Trees

While I immediately recognized the inch or so long caterpillars crawling on the ground around me as larvae on their way to becoming butterflies or moths, I mistakenly assumed that they had just hatched from eggs in the tree branches above. Their mother moths or at least Mother Nature should have left them instructions to stay in the trees where the budding leaves were their food.

I also mistakenly assumed that the relatively large silken tents in the trees had been woven by female moths as a protective shelter for their eggs. All this thanks to having paid some attention in high school science classes.

However, I was wrong on both counts as the inch or so long caterpillars falling around us were not newly hatched ones looking for food but mature ones looking around for a good place to spin a cocoon and proceed on to the next stage of life.

Despite failing to learn or remember the finer details of biology I did learn and remember how to research and find answers to questions.

White, silky tents in trees created by tent caterpillars as a place where a group of caterpillars can retreat to after foraging along the tree's branches for food.

White, silky tents in trees created by tent caterpillars as a place where a group of caterpillars can retreat to after foraging along the tree's branches for food.

Caterpillars On the Ground Were Moving To Stages 3 And 4 Of Their Lives

When the mass of eggs laid by a female moth the previous summer hatch the caterpillars that emerge are about three millimeters long. Being social creatures, those emerging first start weaving the silken tents in which the group builds a tent to shelter themselves (sometimes they combine with another nearby group to build one tent for both). The tents are located in locations in the trees which provide exposure to the morning sun as the caterpillars cannot digest their food if their body temperature is less than 59° F (15° C)

Hatching of the larvae occurs at the same time as new leaves begin sprouting on the trees. The caterpillars rely on the newly budding leaves for food as mature leaves are too tough for them to digest. As caterpillars travel out along tree branches they leave behind pheromones which enable them to find their way back to the nest. If they find some tender young leaf buds they not only eat their fill but leave a different chemical trail on their way back to the nest for others in the nest to follow back to and dine on themselves.

Final Stage Of The Tent Caterpillar’s Life

Tent caterpillars grow quickly and complete their larval stage as worm-like creatures crawling in trees and on the ground within about eight weeks. At this point they are not only preparing to leave the larval stage but their food source, budding tree leaves, is also maturing and becoming indigestible for them.

This is when they began dropping to the ground and searching, not for food, but for a good place to attach and spin a cocoon for the transition to their fourth and final stage of life.

Following the passage of about 8 weeks the caterpillars emerge from their cocoons as moths ready to take flight.

Just before emerging from their cocoons female moths, their abdomens already containing a load of fully developed eggs, begin emitting pheromones that attract males who are already flying about in the area. As soon as a female has fully emerged from her cocoon she copulates with a waiting male and then proceeds to find a good spot where she lays her eggs and spreads the protective covering.

It takes the female tent moth 24 hours or less after emerging from her cocoon to complete her life's objective of continuing her species by laying the eggs that will give birth to the next generation of tent caterpillars the following spring. Having fulfilled her mission she dies

As for the male moths they enjoy a week or so of flying around sipping nectar and helping to pollinate plants for another week or so until they die.

(NOTE: There are a large number of subspecies of tent moths and most of the information I was able to find during my quick research dealt with eastern North American Tent Moth species the life and habits of which may vary a little from the subspecies that we encountered. Feel free to alert me in the comments section below of any factual mistakes in my description of tent moths above. Thankyou

Traveling from Tucson the the San Pedro House Hiking Area in San Pedro Riparian Area

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Chuck Nugent

Comments

Chuck Nugent (author) from Tucson, Arizona on July 28, 2020:

Peggy - thank you for your comments. I'm glad you enjoyed the article. The caterpillars did make the hike more interesting. All in all it was a great hike with some great photo opportunities. Thanks again for your comments.

Peggy Woods from Houston, Texas on July 21, 2020:

I would have felt as your wife did with caterpillars falling all over me. Yikes! The information you discovered regarding them is fascinating. Thanks for taking us along on another one of your hikes into the countryside along the San Pedro river.

Chuck Nugent (author) from Tucson, Arizona on April 19, 2020:

Liz - you're right it was a great photo opportunity and I was able to identify and learn all about the tent caterpillars with a couple of Google searches after I got home. My wife also had a great time photographing the caterpillars but got shaken up every time they got on her so I had to keep checking her and brushing off any that got on her.

Liz Westwood from UK on April 16, 2020:

Your hike turned into a great nature lesson with some amazing caterpillar photos.