Greenbug

Greenbugs have evolved resistance to many insecticides over the years, leading to the recognition of a series of insecticide-resistance greenbug biotypes. Just as the proportion of resistant aphids will increase with the frequency of insecticide applications, so their occurrence has greatly diminished over the past few decades as usage of pyrethroid and organophosphate insecticides has declined. This is because resistance is conferred by unique versions of specific genes (alleles) that can be costly in the absence of selection imposed by the insecticide. Insecticide-resistant aphids are often less competitive than those with the alternative susceptible alleles and are gradually replaced when insecticide use declines. Furthermore, the availability of newer insecticides with novel active ingredients and modes of action (e.g., flonicamid, sulfoxaflor, flupyradifurone) provide farmers with more reliable control alternatives to which greenbugs have no history of repeated exposure or resistance evolution. Responsible use of these products (e.g., spot-treatment of infested areas, treatment only after economic thresholds are surpassed, and rotation of materials with different modes of action) will ensure their continued effectiveness in the isolated circumstances where treatment may be justified.

Resistant hybrids can reduce the susceptibility of sorghum to aphids, and many greenbug-resistant sorghum cultivars were developed over the decades when greenbug was a major pest lacking effective natural control. Unfortunately, the same 'single gene' sources of resistance were employed in multiple varieties, one at a time. Since these traits provided only partial resistance, greenbugs could still feed and reproduce on the plants, albeit at much slower rates, which allowed biotypes to evolve and adapt to them. Finally, around the year 2000, region-wide biological control of greenbug became established across the High Plains, and large greenbug populations with the numerical abundance required for rapid biotype evolution became a thing of the past. The lesson learned was that, with sufficient support from biological control, resistance traits can remain effective indefinitely without selecting for new aphid biotypes. At one time, all new sorghum hybrids were screened for susceptibility to greenbug damage, but this is no longer the case. However, hybrids with various levels of resistance to biotype I, which is still the predominant greenbug biotype, are still available, although yield potential is likely to be a more important consideration in most years.

Most sorghum seed is now treated with neonicotinoid insecticides (clothinadin, thiamethoxam, imidicloprid) that are systemic within seedlings and provide good early season protection against greenbugs and all other aphids. Seed treatments are an attractive option for regions where greenbugs and chinch bugs tend to be perennial problems shortly after crop emergence. However, their use is now so widespread that concerns are growing about their secondary impacts on the insect community and broader environmental impacts. When all seed is treated with insecticide, it creates a 'green desert' in which no prey are available for beneficial species during the first few weeks of crop development, meaning that parasitoids and predators will not colonize the crop or be able to reproduce and increase their numbers in that field. This creates a risk that pests can arrive later, when the seed treatment wears off, and quickly produce damaging populations in the absence of any natural enemies. For example, seed treatments will prevent the colonization of seedling sorghum by corn leaf aphids which are known to foster the recruitment and reproduction of natural enemies that help control more damaging aphid species arriving in later growth stages.

Natural control of greenbug and other aphid populations is mostly provided by four families of beneficial insects – lady beetles, lacewings, hoverflies and parasitoid wasps. See KSRE publication MF2222: 'Biological Control of Agricultural Pests in Kansas' for more details on these insects. Several species of lady beetles feed on greenbugs, but the most important in Kansas is the convergent lady beetle, Hippodamia convergens. The adults have two convergent white lines on the pronotum (area behind the head), and orange wing covers that have variable numbers of black spots. The bright orange eggs are laid in clusters on the undersides of leaves near aphid colonies. The larvae resemble little alligators with orange markings. The larvae of lacewings have large, sickle-shaped mandibles that they use to impale aphids and drink their body fluids. Larve of syphid flies are legless maggots that are often found voraciously consuming aphids within their colonies. Activity of the aphid parasitoid Lysiphlebus testaceipes can be assessed by looking for brown, swollen, dried-up aphids (mummies) on the surfaces of greenbug-infested leaves. Take note of these insects when scouting for greenbugs, as control measures will not be required if they are present on most infested plants. Note that there is no evidence that biological control can be improved by purchasing beneficial insects from mail order sources and releasing them to augment naturally occurring populations in open-field situations. Large numbers of predators and parasitoids will migrate naturally into sorghum fields as populations of other aphids decline and earlier crops such as wheat mature and dry down.