There have been 7 BYGL Alerts this season dedicated to aphids on trees and shrubs. This will be the 8th. The unusual abundance of aphids this season caused us to declare 2023 “The Year of Aphid” in an Alert posted on June 16 [see “The Year of the Aphid. Is Help on the Way?”].
This alert focuses on White Pine Aphid (Cinara strobi). The aphid is specific to its namesake host, Eastern White Pine (Pinus strobus), and may be found throughout the range of this native tree. The unusual-looking aphids are notable for their white-on-black color motif accentuated by a white “racing stripe” running along the midline.
Despite its large size and propensity to appear in huge numbers, white pine aphids are not known to cause significant harm to the overall health of established white pines. However, they may appear at the same time white pines are losing inner needles.
The white pine aphid has multiple generations throughout the season with populations expanding with each successive generation. Consequently, the largest populations are normally seen in the fall with huge aggregations “suddenly appearing” on white pines.
A Misguided Diagnosis
The late-season appearance of hordes of white pine aphids may coincide with the normal browning and shedding of last season’s needles on white pine. Unfortunately, the convergence of the two independent events can lead to a misguided diagnosis.
I received an e-mail message yesterday with pictures of a large, healthy-looking eastern white pine showing dramatic normal inner needle browning. The owner reported that the tree was covered with aphids and had concluded that the aphids were killing the tree. They were looking for a spray recommendation to “save” their tree.
Exploding Aphid Populations
The white pine aphid’s late-season population explosions are the result of life cycle modifications that are not commonly seen with other insects. The changes allow the aphid to skip or compress several steps in its life cycle to reduce the time required to complete each generation.
The first trick this aphid has up its six sleeves starts with summer generations being composed entirely of females; there are no males. This type of asexual reproduction is called parthenogenesis and it means females don’t need to waste time going on dates and feigning interest to get acquainted with males.
The second trick is that white pine aphid females don’t lay eggs. They are viviparous meaning they give “birth” to nymphs that are capable of hitting the ground running, literally. Most insects are oviparous meaning they lay eggs. It takes time for the eggs to hatch, and the eggs are at constant risk of becoming lunch for other insects until they hatch.
The white pine aphid’s final life cycle-condensing trick is more subtle. The nymphs that are pumped out of the females already have inside their bodies the embryos of future females. They arrive armed with the essence of their granddaughters!
Aphids and other phloem sap-slurping insects insert their soda straw-like piercing-sucking mouthparts into phloem vessels to withdraw sap. Aphids are aided by a tiny pump-like organ located in their heads.
Aphids withdraw both carbohydrates for energy and amino acids to build proteins. However, carbohydrates make up a much higher percentage of the phloem sap compared to amino acids. So, the aphids must withdraw a large quantity of sap to extract the necessary quantity of amino acids required to meet their needs. The excess sap is discharged from their anus in the form of a sticky, sugary, clear liquid called honeydew which is a polite name for liquid aphid poo.
A Short Segway
On a side note, the “amino” in amino acids refers to an “amino group” that is an essential part of the amino acid molecule. The amino group consists of two hydrogen atoms attached to a nitrogen atom. Thus, nitrogen is critical to the production of amino acids. Applying nitrogen fertilizer to trees is essentially aiding and abetting aphids and other phloem sap-sucking insects by supporting a greater production of amino acids.
On another side note, many animals, both large and small, cannot fully utilize their food without the help of microorganisms living in their gut. Aphids are no exception with white pine aphids housing two bacteria: Buchnera aphidicola and Serratia symbiotica. These bacteria are obligate endosymbionts meaning they must reside inside a host to survive. In return for a cozy home inside the gut of a white pine aphid, the bacteria process the sap extract to provide essential nutrients to their host.
Back to Sticky Notes
White pine aphids are notorious for raining large quantities of sweet sticky honeydew onto the needles and stems of their host tree as well as onto understory plants, sidewalks, parked cars, hapless gardeners, etc. A dingy patina is added when the sugar coating becomes colonized by black sooty molds. The sooty mold causes no harm; however, it can affect the aesthetics of heavily infested trees as well as lawn furniture.
It’s common for the sugary honeydew produced by aphids and other sap-sucking insects to attract various stinging insects including yellowjackets (Vespula spp. and Dolichovespula spp.), baldfaced hornets (D. maculata), and paper wasps (Polistes spp.). Indeed, aphid infestations are often disclosed by the arrival of the stinging brigade. Fortunately, these wasps are seldom aggressive since they aren’t defending their nest.
Although these predators may offer some protection against other predators, you can’t change a tiger’s stripes. These predators may occasionally enjoy a little meat with their sugar by gobbling up some of the aphids.
Ants on the other hand are much better aphid shepherds. In exchange for an occasional sweet treat, ants provide security services by using their powerful mandibles to fend off predators bent on thinning their aphid herd.
Help is on the Way, Naturally
White pine aphids seldom cause significant harm to the overall health of their conifer host. Most online references note the most significant damage occurs on young trees, particularly seedlings. Established white pines are capable of hosting large numbers of aphids without being harmed.
Also, white pine aphid populations tend to rise and fall dramatically from year to year owing to the combined impact of predators, parasitoids, and pathogens (the 3-Ps). Lady beetles and lacewing larvae commonly graze on the aphids.
White pine aphids may also fall victim to the entomopathogenic (= arthropod infecting) fungus, Lecanicillium lecanii (formerly Verticillium lecanii). The aphids pick up the sticky spores of the fungus which germinate to release tendrils of fungal hyphae that invade and digest the internal workings of the aphids. Wide-spread infections are enhanced by damp weather and aphid-crowding conditions meaning the fungus is more effective when aphid populations are high.
Thanks to the combined population suppression impacts of the 3-Ps, white pine trees seldom suffer repeated sap loss from white pine aphids year after year. Indeed, many of the images in this Alert were taken a few years ago on a heavily infested white pine in southwest Ohio that has (sadly) never been re-infested. This has been unfortunate from an aphid photo-op perspective.