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Using Aphids to Manage Aphids

Adult black aphid sits on a pecan leaf.

Adult black aphid. (Photo by Ted Cottrell, USDA)

Pest management needs and challenges from season to season always change, sometimes more or less than the previous season. This change is evident with the annual variability in populations of the three species of aphids that regularly feed on pecan foliage. Some seasons are worse when greater aphid numbers cause more feeding injury and honeydew accumulation on foliage.

Natural variability in pest populations is influenced by unseen factors dictating whether intervention will be needed to prevent economic damage to the pecan crop. Although we can rarely pinpoint an exact cause for these differences, there are many abiotic and biotic conditions that can combine to generally yield a negative or positive impact on pest populations, including the pecan-feeding aphids.

Manipulation of many factors influencing pest populations is out of our control. When attempts at manipulation are enacted, they typically involve promoting biotic factors but often fail to be used at a scale needed to make a difference, or they do not fully account for the biology of the organisms involved. For example, early season orchard floor covers used to promote beneficials to feed on pecan aphids in the canopy mostly promote beneficials that stay low and out of the canopy. The exotic seven-spotted lady beetle (Coccinella septempuncta) generally dominates early season orchard floor covers, but you will not find it in the pecan canopy feeding on aphids simply because it is not an arboreal species. Even tactics such as trap cropping for stink bug management are rarely applied at a scale to prevent being swamped by the stink bug population in the surrounding environment.

When we do not purposely cause something to happen, that does not mean it does not happen. The magnitude of natural pest control that takes place in your orchard is higher than you can ever imagine. Recognizing what is already happening and making decisions that allow it to continue is likely one of the cheapest and most effective pest management tactics available. Early to mid-season aphid management is undoubtedly amenable to natural control, except possibly in the far western pecan production areas for several reasons.

The larval stage of the multicolored Asian lady beetle is a voracious predator of pecan aphids. (Photo by Ted Cottrell, USDA)

It was not that long ago that a season-long effort to control the blackmargined aphid and the yellow pecan aphid was needed throughout most growing regions. This need was partly due to the broad-spectrum insecticides used that also decimated natural enemies and allowed the faster-regenerating aphids to flourish. And did they ever, with leaves dripping honeydew and sooty mold blackening leaves. However, the replacement of these broad-spectrum chemistries with more target-specific products allowed more natural enemies to survive and assist with aphid management. At about the same time, the establishment of the multicolored Asian lady beetle (Harmonia axyridis) in the U.S. began to have a tremendous positive impact on pecan aphid management, especially during the first half of the season.

Different multicolored Asian lady beetles pinned to a board for comparison.

Some color morphs of the multicolored Asian lady beetle (Harmonia axyridis). (Photo by Ted Cottrell)

From May through mid-July, some yellow pecan aphids and blackmargined aphids on pecan can be an advantage. Yes, an advantage. For this simple reason: pecan can sustain a low population of these two aphid species with negligible impact on tree energy reserves, and at the same time, these aphids will attract natural enemies to pecan and retain them to reproduce on pecan. You can literally use the pecan trees in your orchard as banker plants to grow your own natural enemies.

Banker plants are a plant species different from the crop plant that will provide an alternative food source and reproduction site for natural enemies. The banker plants may be planted alongside the crop or within the cropping area. The intent is for the natural enemies to move from the banker plant to the crop plant to feed on crop pests. By thinking of the pecan tree—which is already in the orchard—as a banker plant, the work is already done, and there are no hiccups convincing the natural enemies to leave the banker plant to the crop plant.

The appeal of doing this is that numerous beneficial species will respond to the early season pecan aphids. Most of these many predator species feeding on aphids do not discriminate between feeding on any of the three pecan species, thus ensuring that low-level, early season black pecan aphid populations are kept low. If low levels of yellow pecan aphid and/or blackmargined aphid are not present during the early season, these low population levels are a warning to be vigilant against black pecan aphid numbers creeping higher earlier than usual because fewer natural enemies are available to reduce early season black pecan aphid populations.

Yellow pecan aphid adults and nymphs on a bright green pecan leaf.

Yellow pecan aphid adults and nymphs. (Photo by Ted Cottrell, USDA ARS)

Blackmargined aphid adults and nymphs on the underside of green pecan leaf.

Blackmargined aphid adults and nymphs. (Photo by Ted Cottrell, USDA ARS)

Previous research has demonstrated that innate levels of senescence-retarding plant bioregulators produced by many pecan cultivars during the first half of the season slow black pecan aphid’s chlorotic feeding injury. These bioregulators, along with the presence of early season predators, typically do a decent job of keeping aphid populations in check.

But beware the black pecan aphid during the latter half of the season. When the tree begins producing mostly senescence-promoting plant bioregulators, the black pecan aphid becomes more efficient and quicker at eliciting chlorotic damage. During this critical time, low levels of black pecan aphid can cause economic damage before their populations are reduced by natural enemies. Although not proven, it is likely that pecan cultivars susceptible to early season black pecan aphid damage either do not produce at all or do not produce as much of the senescence-retarding plant bioregulators. By late season, all cultivars become quite susceptible to black pecan aphid feeding damage.

As good as natural enemies are during the early season, they are not as good for black pecan aphid management during the late season. Natural enemies will not reduce emerging populations of the black pecan aphid later in the season and will not prevent serious foliar damage. This is simply because the low number of black pecan aphids that cause economic injury is lower than the level needed to attract and build natural enemy numbers to keep the black population level below an injurious level.

In summary, early season pecan aphids can be managed with the natural enemies attracted to them, especially since the establishment of the multicolored Asian lady beetle. Recognizing these natural enemies’ predation potential and allowing them to do their job during the first half of the season can provide significant control. However, during the last half of the season, natural enemies typically do not bring the black pecan aphid under control until populations far exceed economic thresholds.

Author Photo

Ted E. Cottrell

Ted E. Cottrell is a Research Entomologist in the Southeastern Fruit and Tree Nut Research Laboratory with USDA, Agricultural Research Service in Byron, Georgia.