Environmental conditions coupled with certain biological properties have conspired to make this “The Year of the Aphid” with honeydew spewed onto leaves, stems, sidewalks, cars, etc., across Ohio. There have been a number of BYGL Alerts posted thus far this season that have highlighted the unusually heavy aphid populations across a wide range of plants.
The aphid parade has now been joined by White Pine Aphids (Cinara strobi) appearing on their namesake host. Dave Shetlar (OSU Entomology Professor Emeritus, the “Bug Doc”) reported during this week’s BYGL Zoom Inservice that the aphids with their characteristic white racing stripes are abundant in central Ohio.
In my BYGL Alert titled, “Sticky, Dripping Tuliptrees,” I listed those conditions and properties that have played a role in our unusually heavy aphid populations thus far this season. Here is a short refresher.
Most aphids that infest deciduous and evergreen trees and shrubs as well as herbaceous perennials are “cool-season” insects. This means they do best in the spring and fall. Our extended cool spring meant that things went off the rails for us as it was full steam ahead for aphids.
In biological parlance, aphids have a high “reproductive potential” meaning that aphids are very good at making more aphids. Some species skip the need for males and females to get acquainted with one another because the females are parthenogenetic meaning they don’t need to be fertilized by males. Some of us (males) believe this is an evolutionary dead end, but we could be biased.
Also, the females of some species give birth to live young. So, there is no time lost to eggs hatching. The females just pump out more aphids that land on their feed ready to do aphid things.
Water is an aphid’s enemy. Heavy, driving rains can send aphids on a one-way water ride of doom. Heavy rainfall events will also wash away honeydew.
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.
Hope Springs Eternal
As noted above, heavy rainfall can be an enemy to aphids. However, the jury is still out on the overall impact of the recent rains primarily owing to the hit-or-miss nature of heavy, driving rainfall events. It’s apparent that the rains washed away the accumulations of honeydew on leaf surfaces, but I’m still seeing some substantial localized aphid populations, particularly on oaks.
There is little doubt that the 3-Ps are now entering the aphid picture in Ohio. The appearance of enemies of aphids was heavily influenced by springtime temperatures. Although we continue to experience days with below-average temperatures, the average temperatures for early June are much higher than for early May. Collectively, recent temperatures have been favoring a more rapid development of predators, from eggs to adults.
The number of aphids themselves also impacts the development of the 3-Ps, particularly aphid predators. That’s because predator females are drawn to large aphid buffets.
For example, lady beetle females don’t just haphazardly lay their eggs, they lay their eggs among aphids. The same is true for the females of hover flies (order Diptera, family Syrphidae) and lacewings (Order Neuroptera).
There is a wide range of lady beetles that are beginning to appear on plants suffering from heavy aphid infestations. It’s important to identify the beetles given that some of the immatures look nothing like beneficial insects.
Participants in this week’s BYGL Zoom Inservice noted the abundance of Multicolored Asian Lady Beetles (MALB) (Harmonia axyridis) larvae and adults lurking among aphid buffets. Indeed, I’ve observed that virtually every large collection of aphids has had a cadre of alligator-like MALB larvae enjoying a bountiful feast.
Of course, there is a possible downside. MALB is notorious for invading homes in the fall. Will the abundance of aphids this season support a significant rise in MALB populations making them a significant nuisance this fall? Time will tell.
Lacewings are delicate insects named for their elaborate lace-like wing venation. Ohio is home to members of two families: green lacewings (Chrysopidae) and brown lacewings (Hemerobiidae). Eggs are laid on long stalks presumably to place them out of reach of other predators including lacewing larvae.
The larvae of all species are voracious predators. They’re equipped with wicked-looking sickle-shaped mandibles that project forward in front of the head. The hollow or grooved mandibles are used to seize, pierce, and extract the juices of their hapless prey.
Green lacewing table fare includes anything the rapacious larvae can wrap their mandibles around. However, aphids are a favorite meat item which is why the larvae of some green lacewing species are called “aphid-lions.”
The green lacewing larvae come in two forms depending on the species. So-called “naked” larvae have elongated spindle-shaped bodies covered in intricate markings which are thought to provide camouflage from other predators including birds. Other lacewing larvae carry camouflage to another level by covering themselves in debris. These so-called “debris carriers” have short bristles which are festooned with a wide range of materials.
Hover flies (= syrphid flies) provide a two-fer. The adults are significant plant pollinators which is why they are sometimes called “flower flies.” They mimic bees but the ruse is revealed by the flies only having two wings as noted in the name Diptera with “di” meaning two, and “ptera” meaning wing.
Fly larvae are called “maggots” and the maggots of most flies just look like squirming animated rice. However, hover fly maggots are active aphid hunters as illustrated in the images below.
Finally, I reported in the BYGL Alert titled, “Sticky, Dripping Tuliptrees” that our native Tuliptree Aphid (Illinoia liriodendri) was being targeted by small parasitoid wasps belonging to the genus Aphidius (family Braconidae). 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.
Don’t Shoot the Good Guys
Aphids on trees and shrubs seldom cause enough harm to the overall health of their plant host to warrant initiating control measures. However, they can be a serious nuisance pest by spewing sticky honeydew onto patios, decks, cars, lounging gardeners, etc. An unsightly mess can be created by black sooty molds taking advantage of the sugary food source.
Topical broad-spectrum insecticides that kill all insects can undo the good work of aphid predators as well as parasitoids. So, it’s important to select aphid management methods that reduce the number of aphids without reducing the number of enemies of aphids.
There are a number of topical insecticides that are considered “low-impact” relative to beneficial insects including products with the active ingredient azadirachtin. Azadiractin is extracted from the tropical neem tree and acts as an insect growth regulator as well as a feeding deterrent.
Insecticidal soaps are also somewhat selective. The active ingredient can affect hover fly larvae, but other predators such as lady beetle and lacewing larvae are generally spared. However, it’s important to purchase the products rather than concoct a DIY mix that includes such things as dishwashing detergents. Insecticidal soap products are tested for phytotoxicity on plants. Detergents aren’t tested and may have herbicidal properties. Of course, knocking the leaves off targeted plants will have an impact on the aphids.
A coarse stream of water mimics heavy, driving rainfall. This means an effective aphid management option is to hit low-growing plants with a coarse stream of water from a garden hose. Making thunder noises is optional but risks earning puzzled (uneasy?) stares from your neighbors. Speaking from personal experience.