Aphid Troubles

Sugarcane Aphid
Photo: http://www.nbaii.res.in

Entomologists are concerned when insects build large enough populations to cause crop damage. Why does it happen? There are many factors involved that may interact in ways that confound analysis. Pest problems can appear suddenly, leaving entomologists with a mystery to be solved in a short time. Many people need the mystery to be solved quickly for economic success.

The sugarcane aphid, Melanaphis sacchari, an exotic insect that has been in the US since 1997, suddenly developed large populations on Sorghum in Texas, in some cases causing losses of over 50 percent. For over a decade, the sugarcane aphid was present, but not a pest of note. The sugarcane aphid has a history in Asia and Africa where it is a sporadic pest of Sorghum. Some locations have problems, others do not. A number of factors are potentially relevant.

Host Resistance: There is evidence that Melanaphis sacchari, develop better on some varieties of sorghum. Has an insect resistance trait been inadvertently bred out of sorghum planted in Texas? This is one possibility but without knowledge of the basis of resistance and good markers the resistance hypothesis is difficult to test.

Adaptation: Aphids must avoid plant defense and be able get all their nutritional needs while feeding on a single plant. Aphids that feed on multiple species have a conditioning period when moving from one species to another. Aphid genes and aphid “biotypes” (aphids that have an adaptive set of genes) may be important. One biotype may thrive on sugarcane but not sorghum. Another may thrive on sorghum. Is a biotype involved? This hypothesis is difficult to prove without markers.

Symbionts: Aphids cannot survive without bacterial symbionts that colonize the aphid gut. Symbionts may metabolize plant toxins and synthesize important nutrients such as vitamins and amino acids. Some symbionts make substances that are secreted in aphid saliva and trigger a plant defensive response. The wrong symbiont strain can inhibit aphid colonization of a plant. A symbiont may thrive if the aphids is on one plant host but not another. A mutation in the symbiont or acquisition of a new symbiont (or strain) may adapt an aphid to a plant. Is a symbiont involved? Without good markers and knowledge of symbiont biology, this hypothesis is difficult to test.

Natural enemies: Beneficial insects including predators (such as lady beetles) and parasitoids can keep a population in check. Their effect is often unknown until they are inadvertently removed from the population. Is a key natural enemy involved?

Weather: Weather has a large effect on insect populations. Insects have an optimal temperature range where development is maximum. Too cool, too hot, too wet, too dry can all affect populations. Weather also affects natural enemies and can indirectly affect pest populations. Under some conditions, a natural enemy population could grow too slow to provide effective pest control.

Is the problem a one time perfect storm of weather conditions in 2013 that will on appear sporadically? Or has a change occurred to produce a more persistent problem?

The damage can result in significant losses, but pesticides are not a viable solution. The expense of pesticide applications subtracts from the profitability of growing, plus pesticides create other problems. The amount of money spent on identifying problems and developing solutions is dwarfed by the potential economic losses. Production of some crops has been abandoned in the past when insect problems could not be solved.

Entomologists will never run out of problems to address. The ability to find solutions depends on our knowledge base. This is one reason why we study insects.