Squash Bugs Come to Town

After more than twenty years of growing squash here in Eugene Oregon, I never saw a squash bug (Anasa tristis) until two or three years ago. Since then they have become much more common and there have been reports from many local gardeners having to deal with them. I knew what they were, however, because my parents had problems with hordes of the bugs when they were gardening in Grants Pass, Oregon. What has changed to make them such pests here in the last few years? Who knows for sure, but there are some hints based upon the development cycle of the squash bug.

I start all of my squash from seed planted directly in the garden. This means I’m at the mercy of spring weather for germination and early plant development. This year we had a cool early season, so the squash took their time getting big. Some of my neighbors had bigger plants earlier this year and they were the first to report squash bug infestations, so I knew to be on the lookout.

If you lie on the ground next to your squash plants and peer up underneath the leaves, besides contemplatively communing with your plants, you can quickly survey the undersides of the leaves for squash bug eggs. They are also common on the stems. Freshly laid eggs are light tan in color, getting to dark bronze shortly before hatching. (click on the images for a better view)

After the reports from my neighbors, I started surveying in mid-July after my vines were starting to size up and were beginning to flower and set fruit. I found plenty of eggs. Scraping them off the leaves with a fingernail or a stick effectively suppresses them, but you have to find them all. The gestation time for the eggs is about 10 days, give or take a couple of days depending on the temperature. If you check three or four times a week, you have a good chance of finding most of them before they hatch, but as the squash plants get bigger it gets progressively harder to search everywhere and find them all.

The squash bug nymphs are gregarious, initially forming clusters of small nymphs on the underside of the leaf where the eggs were laid. They start sucking plant juice and injecting enzymes into the leaf that causes damage and wilt. So it you miss the eggs, the next thing to look for are wilt and and brown spots on the surface of leaves.

If you turn over a leaf with a brown spot be prepared to find a cluster of squash bug nymphs. They are soft and easy to squish, but large masses of them are repulsive. If you carefully place the leaf above a bucket of soapy water and give it a sharp tap, all of the bugs will fall into your bucket and the leaf will remain intact. Those that you miss will likely be back in the same spot the next day.

As the nymphs develop and get larger you will find them more often in dead squash leaves on the ground, or attacking fruit. Eventually you are bound to find a few adults, brown true bugs that are capable of flight. This year I found the pair of adult mating bugs pictured above on August 18th, clearly grown up from eggs I missed earlier.

It turns out that you can model the rate of development of many cold blooded organisms with a “degree day” (DD) model. The metabolism, and hence the rate of development, of the organism is dependent on the ambient temperature. Most models have a “threshold” temperature, Tthrsh, below which no development occurs, and a “saturation” temperature, Tsat, over which no further increase in the rate of development is seen. Between these extremes, development is approximately linear with temperature so the duration for any particular stage of the developmental process can be expressed in “degree days.” For squash bugs, the literature suggests that the good numbers for Tthrsh = 58 °F and Tsat = 92 °F.

Degree-day accumulations required for each stage of development for the squash bug1

Host: Summer squashDegree Days (°F-day)
Nymphal instars (1-5):554
Generation time:747

The world is warming, so could it be that Eugene is suddenly much more friendly to squash bugs? I was curious so I pulled up the temperature history for Eugene, OR and calculated the squash bug developmental degree days for several years in Eugene, also for a year in Grants Pass when my parents lived there, and a year in Kansas where some of the studies I looked at were conducted. The results are shown below.

It becomes clear that there has been a significant increase in the development time available for squash bugs in recent years in Eugene. The entire season amounted to about 1000 DD in the three years I looked at around 2010, compared to >1250 DD in more recent years with last year clocking in at more than 1500 DD and closely resembling the Grants Pass back in 2010.

With the generation time of the squash bug about 750 DD, a big factor affecting the likely size of the subsequent year emergent population in the Spring will be the successful development of the second generation of bugs from the previous summer. Grants Pass in 2010 and Eugene in 2021 both surpassed 1500 DD which would allow for fully developed second-generation adults to go into diapause and be set to overwinter.

A study from Kansas in 1985 looked in detail how the various generations of bugs progressed through the year, the pattern of egg laying and lifespan of the adults2. I included one of the study years on my chart above for Kansas to show what a different place that is from Oregon. The warm nights in the Midwest allow for much more rapid development of the bugs and surely is part of the reason they are such a problem in some of these states.

The Kansas study looked at the progression of development of the various generations of bugs and provided a more complete picture about how these critters work. I was surprised to learn that the adult female squash bug will continue to lay a bunch of eggs every few days for a period of more than two months after it emerges from diapause in the spring. This helps to explain the almost continuous discovery of eggs once the season starts. The long period of egg laying guarantees that all stages of development will be in the squash patch by mid summer. The earliest maturing bugs in the Kansas study mate and become reproductive in mid-July and into August. Bugs maturing later go into diapause in preparation for winter and are not reproductive the first season.

In my garden I observed a drop-off in the number of egg masses I was seeing toward the end of July and early August, but then there seemed to be a late explosion of egg masses mid-to-late August. This is about the time I discovered the first pair of mating adult squash bugs, so one explanation for the sudden increase in eggs is the presence of the new adult reproductive bugs. This year (2022) with a cool early season, the number of development degree days is lagging significantly from last year (2021). It is unlikely that bugs from eggs laid now will have a chance to develop into adults before the end of the season. Maybe we will get a little more of a reprieve next year.

As the season progresses and the squash plants get bigger I find more eggs on stems nearer the ground than under the leaves way up on top. The butternut squash (C. moschata) does not attract the bugs nearly as much as the C. pepo and C. maxima. If they have a favorite, it is the winter squash. Once the fruits are forming, the nymphs would like to such out their juices!

The degree-day chart makes it look likely that the expansion of squash bugs into the Eugene area can be explained by climate change and warmer summers. A recent expansion of range can offer great opportunity for the invading organism because predators have yet to grow the numbers needed to control the invaders. For the time being, it looks like my manual egg scraping and bug squishing are going to be the main control mechanisms.

1 Fargo, W. S., and E. L. Bonjour. 1988. Developmental rate of the squash bug, Anasa tristis (Heteroptera: Coreidae), at constant temperatures. Environ. Entomol. 17: 926-929 (1988).

2 Nechols, James R.; Voltinism, Seasonal Reproduction, and Diapause in the Squash Bug (Heteroptera: Coreidae) in Kansas Environ. Entomol. 16: 269-273 (1987).


  1. Great article on squash bugs! We have had them in varying amounts for several years. Linda is the gardener. She tears off a piece of duct tape, wraps it around her fingers with the sticky side out, and cleans off the eggs. It’s good to know about the 10-day development period. Linda wears rubber gloves and squishes the adult bugs, which are slow-moving and easy to catch. She also sprays dishwashing soap and water on the nymphs.
    We have also had a few adult bugs cruising through our dining room from time to time. They sound like little blimps with their steady hum and slow-moving straight-line flight path. You know right away what just got into the house. You can watch them land and then squish them with a tissue or paper towel. Bleah.

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