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THE BUG MAN: Born Pregnant - Aphids

Photo courtesy James A. Bethke.
Oleander aphids on a milkweed seed pod.

ByJames A. Bethke.

Many of the insecticide trials that I performed at the University of California Riverside were against aphids, which are one of the most common pests of early bedding plants in the ornamental plant production industry. One common method of infesting plants was to carefully place five adult melon or green peach aphids on the terminal of a small chrysanthemum plant in the greenhouse. We did this to ensure roughly the same number of aphids per plant prior to pesticide application. Aphids in warm environments are usually all female and bear live young, which are also all female. In cooler environments or in the northern latitudes, parthenogenetic insects can reproduce sexually and lay dormant or diapausing eggs that will hatch in the warm spring. In our area, they will typically remain parthenogenetic.

Therefore, if the five females we placed on the plants bear five aphids per day, then each plant will potentially have 30 aphids after the first day (5 adults + 25 young), 55 after the second, and 80 after the third. After the third day, the first 25 young are now adults and can produce young, so on day four there are the current 80 aphids plus 25x5=125 for a total of 205. By now I am sure you can see where I am going with this.

Some entomologists say aphids are born pregnant, and basically, they are. That’s why some gardeners will not see aphids on their plants one day and heavily infested the next. Many times, shiny, sticky leaves or sooty mold are seen before the aphids are. Aphids are one of those insects that feed on plant sap, digesting the proteins and pooping the sugars. The sugars are the shiny, sticky substance on the leaves called honey dew, and black sooty mold is what grows on the honey dew. While sitting under our vine-covered arbor at home, my wife asks why it feels like tiny droplets of rain are hitting her head once in a while. “Oh, it’s nothing sweetie.” 😂

My milkweed at home is often covered in these beautiful, orange oleander aphids pictured above. If you look closely at the picture, you will see that the aphids have two protuberances (black hair-like tubes) at the back of the insect. Those are called cornicles. These are only found on aphids, so if you see them on an insect like this, it is definitely an aphid. Aphids can exude pheromones from their cornicles and commonly an alarm pheromone. If an aphid is threatened or harmed, the alarm pheromone warns aphids nearby, and they will bail from the plant almost immediately to avoid the threat. Under a microscope you can see the little droplet of pheromone at the tips of the cornicles.

I often get calls from my butterfly rearing friends, “How do I get rid of those aphids on my milkweed?” That’s a good question. Although there are many methods of controlling aphids, most of them are also detrimental to caterpillars and eggs. In my yard, I let the aphid’s natural enemies take care of them. There are a host of aphid natural enemies including wasps, ladybird beetles, hover flies, lace wings, etc. However, because the aphids produce honeydew, they also attract ants, which will tirelessly protect the aphids. That means that if you want to control aphids naturally, you will need to control the ants first.

For more information about aphids in the landscape and garden, see the University of Florida’s Featured Creatures: or UC’s UCIPM web page: The UC IPM web site also gives information about ant control.


Jim Bethke

James A. Bethke is the Floriculture and Nursery Farm Advisor Emeritus for San Diego and Riverside Counties.

The Floriculture and Nursery Farm Advisor Emeritus conducts applied research, education and outreach programs to improve production and viability of the floriculture and nursery industries in San Diego and Riverside Counties.

Jim's program emphasizes the integrated pest management of major pests of floriculture and nursery production. He collaborates with regulators, growers, and other scientists on advisory committees that set policy based on science.


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