Aphids - The Newest Worst Pest

For many years, thrips took the honours as the pest problem most growers could do without. Whiteflies have had their turn at the top as well, especially in certain crops such as poinsettia and during the gaps between the availability of effective pesticides to control them. In the last year or two, spider mites have been the subject of more phone calls than I can recall, as growers battled to control them with products that were not as effective as they had once been.

The most recent bane of growers' lives (at least from a pest point of view) is aphids. Aphids have in the past been considered one of the easier pests to control. We have a variety of effective pest management tools, including a number of biocontrol compatible products, and biocontrol options are quite extensive and effective as well.

So what has happened? In simpler times (20 years ago), one aphid species was pretty much all that growers had to worry about. The green peach aphid (GPA) spent most of its time on the growing point of the plant, and was therefore very exposed to any pesticides that were applied. Certainly our pesticide arsenal was smaller than now, but pirimicarb (Pirimor then, Pirliss now) was very effective and remained very effective against this aphid for many years. As growers started to show interest in biological control, aphids were one of the poster pests demonstrating the effectiveness of this strategy. A very specific and effective parasitic wasp (Aphidius matricariae) was available commercially, or would come in naturally from outside and provided long-term control.

In the early 1990's, another aphid species entered the picture and began to give an indication of the complications that would follow. In most cases, growers differentiated the two based on colour; the green aphid (GPA) or the black aphid (the melon or cotton aphid - MA). In most cases the description was apt, although it was not unusual to find 'green' aphids that were pink or yellow, or 'black' aphids that were green, yellow or mottled. Growers soon discovered other differences between these two. Pirimicarb, which had always been so effective against green peach aphid was water off a duck's back to the new species. Other older products such as Thiodan and Trumpet were very effective however. This discrepancy meant that for the first time, growers needed to think about insect identification when aphid problems arose. And as for biocontrol; there too, the picture changed. A. matricariae was very effective against GPA but not nearly as useful against MA. Fortunately the commercial development of another parasitic wasp made life easier. Aphidius colemani is closely related to A. matricariae, but was effective against both aphid species, and became widely used.

In the late 1990's two more species began to make sporadic appearances. The potato aphid (PA) and the foxglove aphid (FGA) are larger than the GPA which is in turn larger than MA. Other characteristics can be used to distinguish between the various species, but in the main it is best left to someone with experience to be certain. What we have seen over the last few years is that these last two species of aphids are becoming more and more common, and currently, the species most often found in ornamentals crops (at least in Ontario) seems to be FGA. The range of pesticides registered for aphid control is much greater than it was 20 years ago and we probably have a better choice of products for this pest than for any other. However, for biological control, we run into more difficulty by the entry of these new pests into the picture. The parasitic wasp, A. colemani remains as an effective control for GPA and MA, however it is not effective against the other two species. For that we turn to another wasp called Aphidius ervi.

There is also increasing use of another aphid natural enemy that is effective against all species. The predatory midge Aphidoletes has been commercially available for many years and widely used especially in the greenhouse vegetable industry. However, for whatever reason (perhaps because Aphidius wasps worked so well early on), Aphidoletes has not been used as often in ornamentals. It had always been promoted as a natural enemy that worked best when it had an existing population of aphids on which it could establish. For obvious reasons, most flower growers were not anxious to wait for the aphids to build up before introducing control measures. However, recent work by Applied Bionomics in BC has demonstrated that it can also be very effective when used preventatively, before aphids build up. Given the difficulties that many growers have had with aphids in recent years, we can expect to see Aphidoletes use increase even more.

Aphids can be one of the most frequent, and damaging of pests, affecting a wide range of crops. The evolving nature of different species as pests keeps all growers on their toes, and allows none of us to become complacent. Today, they are many growers' worst pest. Tomorrow?