Pest control is important in all stages of the crop, but especially in propagation. Clients expect to receive a plant that is free of pests and pesticides, leaving biological control as the main means of controlling pests in prop houses. Some pests that are common to greenhouse vegetables and ornamentals are fungus gnats and shore flies, thrips, aphids, white fly and spider mite. All of these pests can be controlled biologically at the propagation stage. The pest and size of pest infestations is highly dependant on the location of the prop house, and can often be predicted by the propagation growers. Therefore, it is important to note that not all pests will affect every prop house.

Let’s begin with a tool that can be used to make biological control more effective; scouting. Scouting is extremely important as it identifies when there is a problem, where the problem is occurring and how bad it is. Scouting of small plants should be conducted weekly to make sure nothing gets out of hand too quickly. Scouting should include the set-up and weekly monitoring of sticky cards to get an assessment of the flying pest population. In propagation, sticky cards should be placed at a rate of approximately 20 per acre and hung no more than 6 inches above the tops of the plants. When checking the cards, pests should be crossed off with a marker or pen to avoid counting them multiple times. Once a good scouting regime is in place, we can start to consider which biological control agents should be used for which pest and when. Although, preventative introductions are always recommended before pests are seen.

Shore flies and fungus gnats are the most common pest in propagation. Both pests are a nuisance and can spread fungal spores; however, only fungus gnats attack plant tissue. Fungus gnat larvae feed on root tissues in the blocks or soil and provide an entry wound for many common diseases and viruses, such as Pythium spp. in cucmber. To control these pests you can use Gaeolaelaps gillespieii (Hypoaspis gillespieii), Stratiolaelaps scimitus (Hypoaspis miles) and/or Atheta coriaria on the bags and/or on the floor of the greenhouse area. They are all soil dwelling insects that have general feeding habits, meaning that they feed on a variety of pests and pest stages in the soil or media. These biologicals are capable of surviving and controlling pests under moist soil conditions, however if the media or soil is saturated they will drown. For prevention, a small amount of these biologicals should be added to the tops of the blocks, near any cracks or holes in concrete, along the perimeter walls and around posts. Additional blanket applications of the soil dwelling insects can be added to areas and/or houses that have a higher amount of fungus gnats and/or shore flies as part of a curative plan. Nematodes (Steinernema feltiae) can also be used for both fungus gnat and shore fly control and can be applied to the crop through a spray or drench application. When using nematodes it is important to remove screens from your sprayers and to keep the aqueous solution agitated.

Thrips are another common pest of many greenhouse plant species, especially in the spring and summer, and can be controlled using a combination of products, including Amblyseius swirskii, Amblyseius cucmeris, Stratiolaelaps scimitus (Hypoaspis miles) or Gaeolaelaps gillespieii (Hypoaspis gillespieii), and Atheta coriaria. For control of thrips stages on the plants, swirskii or cucumeris can be sprinkled over the tops of the plants. These predatory mites targets the newly hatched larval stages of thrips and in high enough numbers can break the life cycle of the thrips, significantly reducing their number. The hypoaspis species are ‘soil dwelling’ predatory mites that feed on the thrips pupae that fall off of the plants. They can be added to the base of the blocks or the ground in the prop house to help in thrips control. Atheta is a rove-beetle that also lives in the soil and feeds on thrips pupae. They can also be added to the base of the blocks or to the ground in the prop house to aid in thrips control.

Aphids are common to many crops at all stages and can increase their population very rapidly. This means that aphid prevention is extremely important, because by the time you see a population it could already be too large to handle economically. There are quite a few products that can be used for aphid prevention, including various Aphidius spp. and Aphelinus abdominalis. Depending on the crop, you may be able to predict what type of aphids will be found, for example, cucumber crops tend to have smaller species of aphids like Green peach and Cotton/Melon aphid. In this case you can pick an aphidius species that targets those types of aphids, like Aphidius colemani or Aphidius matricariae. Tomatoes generally get Potato aphid, meaning that a species that targets larger aphids should be chosen (e.g. Aphidius ervi). In some cases both large and small aphids can attack the same crop, which is the case for bell peppers. For this situation, the best way to proceed would be to choose a mixture of parasitoids to protect against a much wider variety of aphids

White fly are a pest common in tomatoes, cucumbers, eggplants and some ornamentals that can cause sticky leaves and fruit. They are commonly found in greenhouse propagation, but rarely become an issue. However, it is always good to know what to do if they do pose a threat. The products most commonly used in white fly prevention are Encarsia formosa and Eretmocerus eremicus or Eretmocerus mundus. These beneficial wasps lay their eggs inside of white fly nymphs, known as scales. The young parasitoid growing inside eventually eats the scale rendering it inactive and preventing it from causing further damage. The scale also cannot progress to adult hood, therefore decreasing the number of eggs laid in the next generation. As a preventative, these products should be in the form of hanging cards containing 50 or 100 viable parasitized scales. As curative, these wasps can be added in loose form to hotspots, but do not expect to see an increase in parasitized scales for three weeks. Even though the scales are parasitized, they will not change colour (black for Encarsia, yellow for Eretmocerus) for three weeks afterwards, therefore it will be very difficult to determine if the wasps are working before this time period.

The final pest sometimes found in greenhouse propagation is spider mite. Spider mite is a common pest for many greenhouse plants and can cause a significant amount of leaf damage. To help with the prevention and early control of spider mite populations there are a few options, Amblyseius andersoni, Amblyseius californicus and Amblyseius fallacis. All of these predatory mites target the juvenile spider mite before the eggs and adults, and are capable of surviving for a short period of time without pests present. However, they can only control spider mite when populations are low. If populations increase and large spots are being observed, Phytoseiulus persimilis should be added in these areas for clean-up. Persimilis prefers spider mite adults, over juveniles and eggs, and have a voracious appetite. However, their sole food source is spider mite and they should not be added to the crop until spider mite is observed. If populations increase rapidly, chemical intervention may be required to help knock down the spider mite population, but should not be used as the main means of controlling the pest. Also, propagators and their clients should remain in contact and have honest conversations about what is happening with the crop regularly.

There are many options when it comes to biological pest control in greenhouse crops, to choose the ones best for you and your particular situation, call your local biological control companies and ask for their expert opinions. When planned and conducted properly biological control can minimize or even eliminate the use of chemical pesticides which can stress the plants and decrease production. Chemical pesticides may still be required in situations where large influxes of pests come into the greenhouse, but there are many bio-friendly products that can help knock down pest populations without destroying your entire biological program. Biological control is a common strategy in many stages of production and should be integrated into propagation. A clean start to the crop can make pest control easier and more economical in later stages.

For more information:
Crop Defenders Ltd.
Amanda Eyraud, MSc