Factors to Consider in using Biocontrol Agents for Aphids

Table of Contents

• Parasitic Wasps
  • Aphidius species/Banker Plants
  • Aphelinus abdominalis
    
• Aphid Predators
  • Aphidoletes aphidimyza
  • Hippodamia convergens
  • Harmonia axyridis
  • Lacewings (Chrysoperla spp.)
    
• Parasitoids versus Predators

Greenhouse vegetable growers, particularly pepper growers, commonly contend with several aphid species which often include the green peach aphid (Myzus persicae), potato aphid (Macrosiphum euphorbiae), cotton aphid (Aphis gossypii), and the foxglove aphid (Aulacorthum solani). Fortunately, to manage these aphid pests, commercial insectaries are able to supply an array of biological control (biocontrol) agents including several parasitic wasps such as Aphidius colemani, Aphidius matricariae, Aphidius ervi, and Aphelinus abdominalis, a predatory midge, Aphidoletes aphidimyza, ladybird beetles (Hippodamia convergens sp. and Harmonia axyridis), and lacewings (Chrysoperla sp.). To optimally use each of these biocontrol agents, growers need to consider a couple of key questions such as - would all these biocontrol agents perform equally well for all the aphid species, and where should they be released for best results?

To address these questions, aspects of each of the major biocontrol agents will be discussed below.

Parasitic Wasps

Aphidius species/Banker Plants

Aphidius colemani attacks smaller aphid species and is therefore most useful against cotton and green peach aphids. Studies indicate that it does not accept potato and foxglove aphids as hosts. Aphidius matricariae is effective against green peach aphid, not very effective against cotton aphid, and like A. colemani, does not parasitize potato and foxglove aphids. Although Aphdius ervi is very similar in appearance and biology to the other Aphidius species, there are some differences. For instance, Aphdius ervi takes nearly 19 days at 21°C to complete its life cycle, whereas A. colemani takes 14 days at the same temperature. Aphdius ervi also prefers to parasitize larger aphid species such as potato and foxglove aphids.

Aphidius species are best used when aphid numbers are very low. To facilitate continuous release of low numbers of these species, most pepper growers use "banker plants" (Fig. 1) which essentially consist of seedlings of a cereal species e.g. rye. These seedlings are host to cereal aphid species which do not attack vegetable crops, and the cereal aphids in turn are hosts or food for the parasitic wasps. Despite use of such banker plants, growers often experience inadequate control of aphid infestations. One possible reason for this is location of "banker plants" or release points for the wasps.

Figure 1: Banker plant located at end of row for release of biological control agents against aphids

Banker plants are most frequently placed along walkways at the ends of rows which are often about 90 m long. Dispersal studies carried out in the field with A. colemani indicate that although this wasp can move at least 16 m within 24 hr after release, the majority move only 1 to 2 m from the point of release within the same time, and that 88% of their eggs are laid within the first 2 days after emergence. Such data indicate that the majority of parasitization by A. colemani occurs within a few metres from their points of release. Such results also indicate that placement of banker plants should be more evenly distributed throughout the greenhouse, and that the distance between banker plants should be closer to 40 m for improved parasitization.

Another point to consider is quantity to order. Observations from research in the UK showed that adult emergence from "mummies" varied from 19 to 95%, or averaged around 60%. Therefore, it is best to order about 30% more than estimated requirements. However, commercial insectaries may already have factored in the expected shortfall in percentage emergence so that growers obtain the appropriate numbers ordered.

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Aphelinus abdominalis

The main aphids that Aphelinus abdominalis attacks in the greenhouse are potato and foxglove aphids. This wasp prefers to parasitize the 2nd and 3rd nymphal stages while the 1st and small 2nd nymphal stages are used for host-feeding i.e. as food by adults. To feed on an aphid, the wasp first pierces the aphid with its stinger or egg-laying body part, and then feeds on the aphid's body fluid through the tiny opening(s).

In contrast to A. colemani, egg laying activity is low during the first few days of this wasp's life. And then by the 4th day after emergence, an adult female can lay 10-15 eggs per day for the rest of its life of 15 to 27 days. During this time, an adult female may parasitize more than 200 aphids and kill about 40 by host-feeding. Because adults prefer to walk or hop rather than fly over the crop, they tend to remain localized. Studies have indeed shown that dispersal by this wasp is poor in the greenhouse, and that most remain close to their points of release. This means that these wasps should be released as close as possible to aphid infestations for best results. Note that aphids parasitized by A. abdominalis appear black while those parasitized by Aphidius species are bronze (Fig. 2).

Figure 2. Aphids parasitized by Aphelinus abdominalis (black) and Aphidius sp. (bronze)

For most efficient suppression of aphid populations in greenhouse crops, it is not only important to know where and when aphid infestations arise, but also which species comprise these infestations. In addition, we need to pay more attention to how and where releases of biocontrols are made.

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Aphid Predators

Aphid predators are generally less discriminating than parasitoids and often feed on a wide range of species other than aphids. Another general feature of aphid predators is that they thrive under high aphid densities and are therefore not usually suited for use when aphid numbers are low. Some specifics that affect use of these predators are discussed below.

Aphidoletes aphidimyza

Aphidoletes is a good general predator with many favourable characteristics. Larvae of this predatory midge (Fig. 3) prey on all aphid species commonly found in greenhouse vegetable crops. Aphidoletes is a very efficient searcher and discriminates between aphid-infested and non-infested plants. It will not lay eggs on plants without aphids. Aphidoletes actually prefers large aphid colonies for egg-laying. Laying of eggs close to the food source is required for their survival because newly hatched larvae will starve if they are more than 63 mm from their food. Most eggs are laid during the first 2 to 3 days after female emergence and optimum conditions for egg hatch are 75% RH and temperatures above 10°C.

Figure 3. Larvae of Aphidoletes aphidimyza feeding on aphids

Another important characteristic of Aphidoletes is that the number of eggs it lays correlates with the size of the aphid colony. So, the larger the aphid infestation, the more eggs it lays. Furthermore, when aphids are plentiful, Aphidoletes larvae kill more aphids than are necessary to satisfy their nutritional requirements. Larvae consume on average 5-10 aphids per day, and under laboratory conditions can consume as much as 80-100 larvae per day.

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A further positive characteristic of Aphidoletes is that unlike parasitoids, it causes little disturbance in colonies. Because of its furtive behaviour, it triggers little defensive reaction by aphids. This means that aphids attacked by Aphidoletes are less likely to disperse, escape predation, and start new colonies. When aphids are attacked by parasitoids, they defend themselves by kicking and producing alarm pheromones (chemicals used for communication within a species), resulting in their own escape, as well as many other members of their colony.

Aphidoletes are best used during early spring to early fall because larvae require at least 15.5 hr of light to prevent the pupae from diapausing. Fortunately, only very low light intensities are sufficient to prevent diapause. Adults are nocturnal and require a period of darkness for mating and egg-laying. Therefore, continuous lighting from a bright source will prevent reproduction. It is also important to note that larvae drop to the ground and use grains of sand and possibly soil debris to form cocoons. If the larvae fall on plastic or concrete that is dry and free of debris, mortality of this predator will be high. Repeated or continuous release using banker plants is necessary under such situations to achieve acceptable suppression of aphids.

Hippodamia convergens

Hippodamia is the most commonly used lady beetle in commercial crops. Both adults and larvae (Fig. 4) are general predators that feed on aphids, mites, mealy bugs, and scales. This beetle's ability to consume 50-60 aphids per day enables it to respond to high aphid populations. Females are very reproductive, being able to produce up to 1000 eggs at a rate of 10-50 per day over a 1 to 3-month period. These beetles can either be mass-reared or collected from wild populations by vacuuming beetles from their over-wintering sites. This latter practice is frowned upon for various reasons, and in fact, their use in the UK is prohibited because they do not allow release of wild harvested organisms. When wild collected beetles are released in the greenhouse, they tend to fly off through the vents after 1 to 3 days in a spring migratory flight. This tendency can be reduced by pre-conditioning or pre-feeding them prior to release. Insectary-reared beetles tend to disperse more slowly.

Figure 4. Ladybeetle larva and aphid corpses

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Harmonia axyridis

Harmonia has several favourable characteristics. Larvae and adults feed on a wide range of aphid species and may eat about 150 aphids per day. This beetle can tolerate low temperatures and therefore can be useful in "cool" greenhouses. Adults can live for months and lay more than 3000 eggs during their life. On the downside, some regard this lady beetle as a nuisance because of its aggregation behaviour in the fall when they enter buildings in large numbers to overwinter. In addition, they have caused losses in the wine industry because they contaminate grapes with materials that they excrete, giving the wine a significant off-flavour or odour.

Lacewings (Chrysoperla spp.)

Lacewings also feed on a variety of prey including aphids, thrips, spider mites, young caterpillars and moth eggs, mealybugs, scales, whitefly larvae, and pupae. However, they do have a preference for aphids over thrips, and then spider mites. Only larval stages of lacewings are predatory. Older larvae (3rd instar) are particularly voracious, and can eat unhatched eggs, other larvae, and even adults if food is scarce. A larva can consume 300-400 aphids and are usually best suited for high aphid population situations. If there is excess food, more prey will be killed than consumed. Adults, on the other hand, feed only on honeydew, nectar and pollen.

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Parasitoids versus Predators

Studies indicate that combined use of parasitoids with predators is more effective than use of either alone. Parasitoids are described as "specialists" because they feed on one or a narrow range of prey species, and this close relationship results in better control of the prey population. However, because a delay corresponding to at least one generation time must elapse before aphid populations begin to decline, parasitoids are slow to respond initially. On the other hand, predators can potentially cause a more rapid decline. However, because of their "generalist" feeding habit as discussed above, they are less specific in their choice of prey. This lack of specificity can result in fluctuations of aphid populations and inadequate control. In conclusion, studies involving these two types of biocontrol agents indicate that despite some interference between them, their combined use is additive and the end result is best when both types are present.

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