Greenhouse Tomatoes: SHEET 410
Guidelines for Biological Control
Biological controls are widely used in commercial greenhouse tomatoes in North America. As growers shift from using chemicals to using biological controls for pests, conditions have become favourable for using bumble bees for pollination. Biological control are most effective when used in an Integrated Pest Management (IPM) program (see Sheet 160). As in any IPM program, success depends upon correct identification of pest problems, regular monitoring, careful timing and integration of complementary control measures. It also depends on good clean-up, sanitation and other measures that remove breeding sites for pests and prevent them from entering the greenhouse.
The primary pests of greenhouse tomatoes grown in rockwool or hydroponic systems are greenhouse whiteflies (Trialeurodes vaporariorum), fungus gnats (Bradysia spp.) and two-spotted mites (Tetranychus urticae). Where ornamental crops are grown nearby, or were grown in the preceding crop, sweet potato whitefly (Bemesia tabaci) and western flower thrips (Frankliniella occidentalis) may be a problem; the latter is more likely to occur in soil cultures. Aphids can be an occasional problem.
Most biological controls are used after the target pest is found to be present, but some can be established ahead of time. The two main ways to use biological controls in tomatoes are:
Start with early releases as much as possible because this increases the chance of achieving successful control. This is especially important on tomatoes, because the toxic, glandular hairs on leaves and stems hinder the movement of some biological controls.
The following guidelines contain practical tips for achieving good results against pests in greenhouse tomatoes. Recommended release rates for each biological control are shown in Table 1. More detailed information on pests and biological controls can be found in the separate information sheets for each species.
There are two main ways to monitor for common tomato pests:
The main pest in greenhouse tomatoes is the greenhouse whitefly (for more on whiteflies, see Sheet 310). Sweet potato whitefly is occasionally a problem, but usually only in tomatoes grown near ornamentals, such as poinsettias. Whiteflies damage greenhouse tomatoes by covering fruit and leaves with the sticky honeydew as they feed. High populations also reduce the vigour of plants. The sweet potato whitefly can cause a net-vein symptom in fruit. Both species of whiteflies can be controlled on tomato by the parasitic wasp, ‘Encarsia’, by following steps described below.
Correct identification is important because treatment recommendations differ for each species. It is difficult to identify whiteflies trapped on sticky cards, therefore it is advisable to examine adults and immature whiteflies on leaves (for descriptions, see Sheet 310).
Release Biological Controls
‘Encarsia’: Encarsia formosa is a tiny, 1 mm (1/20 inch) long, wasp that parasitizes immature stages of whitefly. It is sold as parasitized scales glued to cards, from which the adult wasps emerge. As the wasp develops inside, greenhouse whitefly scales gradually turn black; parasitized sweet potato whitefly scales turn a tan colour.
Best results are achieved when Encarsia are introduced at low rates before whiteflies are found on monitoring traps. Releases continue, usually weekly and are maintained until 80% of whitefly pupae appear parasitized. (For more on Encarsia, see Sheet 210).
‘Dicyphus’: The predatory bug Dicyphus hesperus is a new and experimental addition to the list of commercial biological controls (for more on Dicyphus, see Sheet 280). It is best used along with other biological controls in greenhouse crops that have—or because of past history, are expected to have—whitefly, spider mite, or thrips problems.
Fungus gnats can cause significant damage in soil-less cultures. In tomatoes, most damage is caused by the larvae feeding on tender roots. As root area is lost, tomato plants become more susceptible to drought stress and less tolerant of root infections. Adult fungus gnats can transmit root rot and other diseases (for more on fungus gnats, see Sheet 320).
It is important to distinguish between fungus gnats and shore flies because biological controls for fungus gnats do not work on shore flies (for descriptions, see Sheet 310).
Release Biological Controls
The following three species of biological controls are compatible and may be used together.
‘Hypoaspis’. This soil-dwelling predatory mite feeds on fungus gnat larvae. The best way to use Hypoaspis is to establish it in the greenhouse before fungus gnats appear. Apply to tomato seedlings in flats or cubes at the start of the growing season, and then again when planting out. By feeding on other soil organisms, Hypoaspis populations can build up to high numbers that are effective in keep fungus gnat populations low (for more on Hypoaspis, see Sheet 230).
Supplement Hypoaspis with other biocontrols (below) if fungus gnat populations are high.
Insect Parasitic Nematodes. Steinernema carpocapsae, S. feltiae, and Heterorhabditis spp. are beneficial nematodes sold to control fungus gnats and other insects. They can be applied to the soil through conventional sprayers or through the irrigation system. They are effective against high populations of fungus gnats.
Note: The actual growing area for rockwool cultures with plant densities of 2-3 plants/m² (10 ft²), is typically 1/3 to 1/4 of the total floor space.
Bacillus thuringiensis israelensis (Bti): A strain of this bacteria that infects fungus gnat larvae is available (VectobacÒ ). It is applied in water to the soil or growing media after fungus gnats are established (follow instructions on the product label).
TWO-SPOTTED MITES (TSM)
Two-spotted mites are increasingly important pests of greenhouse tomatoes. They are difficult to control with biological controls or with chemicals (for more on two-spotted mite, see Sheet 300). Strains of TSM adapted to tomatoes survive well despite the toxic, glandular hairs on tomato leaves and stems, and can cause severe damage.
Controls should be used at the first sign of TSM infestation, even if it is minimal. An early sign of a TSM infestation is speckled leaves. Later, infested tomato plants become yellowish, stressed and may develop only 3-4 flowers per truss.
Release Biological Controls
‘Persimilis’: The predatory mite, Phytoseiulus persimilis, can be used to control TSM in tomatoes. The predators develop twice as fast as the pest at moderate greenhouse temperatures. Unfortunately, biological control using Persimilis is not as reliable on tomato as in other crops because:
For best results:
‘Feltiella’: Where humidity above 60% RH can be maintained, the predator Feltiella acarisuga can also be used with Persimilis. This predator is does best at high mite densities and in high humidity (for more on Feltiella, see Sheet 280).
WESTERN FLOWER THRIPS
Western flower thrips (WFT) is more likely to be a problem in soil cultures, in greenhouses recently converted to rockwool culture, or in greenhouse complexes with ornamental plants on site. WFT cause "ghost rings" on fruit, some feeding damage to leaves, and can transmit Tomato Spotted Wilt Virus, which is lethal to tomatoes. Biological control of thrips in tomatoes using predatory mites is difficult because the toxic, glandular hairs on the plants impede the mites, but other measures described below can sufficiently suppress thrips on tomato.
Release Biological Controls
‘Dicyphus’: The predatory bugs will feed on thrips and can survive on tomato if there are other pests present as well (for more on Dicyphus, see Sheet 280).
‘Cucumeris’: The predatory mites can be used on tomato to control flower thrips (for more on Cucumeris, see Sheet 220). Because the hairs on the plant impede the movement of mites, use the bulk Cucumeris product in bran and sprinkle it on to the top of each infested plant.
Aphids (such as the green peach aphid, Myzus persicae) may appear during the spring or summer. They are relatively easy to control on tomatoes using biological controls so seldom cause major problems. New infestations are usually detected first on the lower stem of tomato plants in the course of routine crop management. Aphid populations can build-up rapidly, however, so do not delay treatment (for more on aphids, see Sheet 340)
The potato aphid, Macrosiphum euphorbiae, can be a more serious problem because its toxic saliva causes leaf distortions and stunted growth. It is larger than the green peach aphid, has long slender cornicles pointing back from the abdomen (see Figure 1, Sheet 242), and tends to drop quickly from the plant when disturbed.
‘Aphidoletes’: When aphids are found, release aphid midges Aphidoletes aphidimyza (see Sheet 240), twice, 1-2 weeks apart.
‘Aphidius’: As soon as green peach aphids are detected, apply the parasite Aphidius matricariae , (see Sheet 242) weekly until established.
If the potato aphid is detected, use Aphidius ervi as well as Aphidoletes for control.
This microscopic mite (Aculops lycopersici) infests tomato stems and spreads slowly through the greenhouse. There are currently no adequate biological controls available. Most growers spot spray affected and nearby plants with dicofol (KelthaneÒ ).
Summary of IPM Guidelines Greenhouse Tomatoes
At start of crop:
When pests are detected: