2023 is shaping up to be “The Year of the Aphid.” At least, in southwest Ohio. I’ve already posted BYGL Alerts on aphids titled, Sticky, Dripping European Beech, and Sticky, Dripping Oaks. This Alert will be the third in the “Sticky, Dripping” series.
I visited tuliptrees (Liriodendron tulipifera (family Magnoliaceae)) earlier this week in a municipal park in southwest Ohio with leaves so sticky they were like fly paper. The trees had one of the highest populations that I’ve ever seen of our native Tuliptree Aphid (Illinoia liriodendri).
Why are there so many aphids this season? First, most species of aphids found on deciduous trees are “cool-season” insects. For several reasons, they do best in the spring and fall. We certainly had a long, cool spring.
Second, heavy, driving rains are the enemy of aphids. They get washed off and have a hard time climbing back up their woody Mt. Everest. Heavy rains may also wash away honeydew deposits. While we had relatively abundant rainfall this spring, we did not have many storms that packed high winds to produce driving rain.
Third, aphids have a very high “reproductive potential” meaning that aphids are very good at making more aphids. For example, tuliptree aphids that develop in the spring are parthenogenetic meaning the females don’t need to be fertilized by males. Also, the females give birth to live young. So, they don’t need to take time to mate, and no time is lost with eggs hatching.
Finally, a long, cool spring also slows the development of the “3-Ps”: Predators, Parasitoids, and Pathogens. These natural regulators help keep aphids in check. Of course, this is changing rapidly. It was easy to find predators on the hunt for the tuliptree aphids.
The handiwork of small wasps in the genus Aphidius (family Braconidae) was also on display. The wasps use their tiny ovipositors (ovi = egg) to insert a single egg inside aphids. The wasp larva consumes the aphid’s innards leaving behind a puffy, dry, husk referred to as an “aphid mummy.” A hole in the mummy indicates a new wasp has emerged. There were only a few mummies; however, this could soon change.
Other Sources of Sticky, Dripping Liquid on Tuliptrees
Our native Tuliptree Scale (Toumeyella liriodendri), which is a type of "soft scale", may also appear on tuliptrees as well as magnolias. It’s a reminder that tuliptrees and magnolias are family relatives. There’s also the possibility that tuliptrees are infested with Calico Scale (Eulecanium cerasorum). This non-native soft scale has a broad host range including tuliptrees.
Aphids and soft scales feed the same way. They insert their piercing-sucking mouthparts into phloem vessels to tap plant sap. They withdraw carbohydrates which provide energy and extract amino acids which are building blocks for proteins. However, the sap only contains trace amounts of amino acids compared to huge amounts of dissolved carbohydrates.
This means they must process a large amount of sap to extract the small amount of amino acids. They discharge the excess sugar-rich liquid from their anus in the form of a sticky, sugar fluid called "honeydew" which is actually a nice name for scale or aphid diarrhea.
The honeydew drips onto the leaves and stems of the host plant as well as understory plants. It commonly attracts a plethora of freeloading sugar-sippers including bees, wasps, flies, and ants.
The honeydew on leaves and stems may eventually become colonized by black sooty molds. Although the molds do not harm plants; they're not pathogenic, heavy leaf coverage on small plants could interfere with photosynthesis. The blackened leaves also reduce the aesthetic appeal of heavily infested trees.
Tuliptree scale produces periodical outbreaks in Ohio with the last one occurring in 2012. However, this scale has been an annual no-show ever since. This does not appear to be an outbreak year unless readers report that they’re seeing high populations.
Although tuliptrees in southwest Ohio have finished blooming, it’s important to keep in mind in the spring that the flowers may be another source of sticky, dripping liquid. The flowers are a well-known bountiful source of nectar for honeybees and other pollinators. However, the primitive flowers do not yield their sugary bribe to pollinators in the same way as more advanced flowers.
The nectar simply pools in the base of the cup-like flowers. It’s not unusual for the nectar to seep between the 6 upturned petals to drip onto the 3 downward-hanging sepals that act like a nectar sluice directing it onto leaves and branches. The clear, sweet, sticky liquid is a dead ringer for honeydew. A word of caution based on personal experience. Tipping over the flowers may release a voluminous flow of nectar to ruin your day.
An Unusual Observation: Crystallized Honeydew
On a final note, while taking pictures in the park of tuliptree aphids, I came across burr oaks with heavy deposits of honeydew on the leaves from Myzocallis oak aphids. These aphids don't infest tuliptrees and tuliptree aphids don't infest oaks.
Our rainfall has all but ceased in southwest Ohio, and temperatures are more August-like than June-like. The environmental conditions have caused the honeydew to crystallize as shown in the images below. The pattern reminded me of salt crystals that form at the edge of evaporation ponds. The crystallization may have hampered the colonization of the honeydew by black sooty molds.
This honeydew crystallization is something I’ve never observed with aphids or soft scales. However, I did observe it last season with spotted lanternfly (Lycorma delicatula) in Cincinnati during a late-season "mini-drought."