Reducing the use and dependence on chemicals can be achieved by creating a ‘standing army’ of biological control agents protecting the crop. In Tomato predators of the tribe Dicyphini such as Nesidiocoris tenuis and Macrolophus pygmaeus, are most commonly used to build such a standing army. These Dicyphini bugs are capable of surviving on Tomato alone in the absence of prey. However, to quickly build up a population an alternative food source is needed. In practice the decapsulated cysts of the brine shrimp Artemia spp. are frequently used as an alternative food source.
Dicyphini bugs are generalist predatory bugs that are capable of controlling Tuta absoluta and Whiteflies, while also reducing several other pests. A downside of Dicyphini bugs is that they may cause significant crop damage at high predator population densities, which would force tomatoes growers into the application of chemical insecticides. Some Dicyphini species are more prone to cause crop damage than others. Another difference between Dicyphini species is their strata preference. Generally, some species are believed to be more abundant in upper part of the plant, while other species spreads over the whole plant. Yet, this assumption is not officially recorded. As biological control agents, all predators have their respective advantages and disadvantages. Therefore, this trail was conducted to identify which of these predators is the best biological control agent and having potential to be use by tomato growers in the Kingdom.
- The impact of using biological predators on controlling Tuta absoluta in comparison to chemical pesticides.
- Studying the behavior of biological predators when applied on tomatoes crops in greenhouses.
- Studying the impact of using fungicides on the numbers of biological predators.
- Behavior of biological predators when applied on tomatoes crops in greenhouses
- Impact of using fungicides on the numbers of biological predators.
Materials and Research Methodology:
Three tomato cultivars were used: Tone Guitar from Seminis, Falouro from Rijk Zwaan and Quaresma from Syngenta in two high-tech greenhouses with 880 plants in each compartment. The cultivated area: 350 m2, including 11 rows at 160cm distance and 25cm between plants, with plant density of 2.5/m2. Every greenhouse had three rows of each cultivar, under soilless culture using rock wool. Each row was divided into two parts, with an experimental unit respectively containing 20 plants, comprising 6 plant frequencies for each cultivar. The experimental unit was used to monitor the distribution of two species of Dicyphini bugs) over plants which were divided into two levels: 1st started from plant top up to one meter down; 2nd was from the lower leaf up to one meter high. Biological control agents were released according to table 1. Numbers of biological control agents were inventoried once every 20 days.
Table No. 1: Date of biological control agents release and rate per m2.
|
Releasing Date* |
|
21-Jan |
7-May |
11-Jun |
18-Jun |
4-Jul |
10-Aug |
5-Sept |
24-Oct |
|---|---|---|---|---|---|---|---|---|---|
|
Biological Predator |
Biological Predator Status |
Releasing rate (number/m2) |
|||||||
|
BUG A |
Adult |
3.7 |
0 |
4.9 |
4.9 |
4.3 |
2.9 |
8.6 |
8.6 |
|
Larvae |
3.1 |
0 |
2 |
0 |
0 |
0 |
0 |
0 |
|
|
BUG B |
Adult |
0 |
1.4 |
0 |
0 |
1.4 |
0 |
0 |
0 |
Due to the current circumstances in the region because of Coronavirus the release of Dicyphini bug Bwas delayed.
Two ways were used to detect Tuta absoluta infection: the first by calculating the number of infected leaves per experimental unit, second, by calculating the total number of males on the pheromone (Ten traps per greenhouse, five traps in the front and 5 at the back).
1. Number of biological control agents in the greenhouses
Dicyphini bug A was introduced 6 times during the season, whereas Dicyphini bug B was introduced only twice according as shown in figure (1). Bug B was introduced twice to protect plants as it is known for its rapid population growth. To maintain the biological control agents, Artemia food was used as an alternative food for Bug B at the rate of 80 g/greenhouse.
At the beginning of this trial, predators reproduced successfully in the biocontrol greenhouse (Figure 1). By the end of March, bug A decreased. While in the end of September, both the population of bug A and B increased again.
Figure 1: Average number of adult predators of both bugs in the biogreenhouse.
The reduction in the adult of both bio-predators was found to be associated with the frequent use of fungicides against powdery mildew and Botrytis (Figure 2). By mid of March, powdery mildew started to appear and requiring fungicides application. Later, Botrytis aggravated the problem. Fungicides were applied first by the end of March, then regularly. This led to a significant decline in the predator populations (Figure 2). However, when stopping fungicides application, the predator population increased significantly again (Figure 2).
Figure 2: Average number of adult predators in the biocontrol greenhouse. Red marks indicate the number of fungicides applications.
2. Biocontrol predators’ distribution on the plant
Numbers of both bug A and bug B adults were monitored in both upper and lower parts of tomato plants. It was observed that both biocontrol predators prefer the upper part compared to the lower one (Figure 3). It was also observed that these biocontrol predators developed better on Valouro RZ tomato cultivar as compared to other ones (Figure 3).
Figure 3: Predators’ preference in tomato plant parts
3 A and B Strata preference of the Dicyphini bugs in different Tomato varieties
3. Monitoring Tuta absoluta population
The number of Tuta absoluta larvae were monitored in both greenhouses, however, the number of Tuta-infected leaves calculated during the evaluation period was low. This due to the regular manual removal of infected leaves by the workers; thus, no leaf was left for calculation during the evaluation. Therefore, the number of weekly inventories of adult Tuta caught by pheromone traps was adopted (Figure 4). No chemical interventions were required to control Tuta in the biocontrol greenhouse. In fact, both Dicyphini bugs, in addition to regular removal of infected leaves, effectively prevented spreading of Tuta infection. However, chemical intervention was needed 7 times to control Tuta population in the chemical control greenhouse (Figure 4).
Figure 4: Number of Tuta males in the pheromone traps in both biocontrol and chemical control greenhouse. Red crosses indicated when the insecticide application took place in chemical control greenhouse
No insecticides were needed during the cultivation season in the biocontrol greenhouse. However, the number of fungicide applications against Powdery mildew and Botrytis was higher in the biocontrol greenhouse than in the chemical control greenhouse (Figure 5).Most of the chemical application in the chemically controlled greenhouse were against Tuta, closely followed by Spidermites (Figure 5). Spider mite outbreaks also occurred in the biocontrol greenhouse but were controlled by mass-release of the predatory mites Neoseiulus califirnicus and Phytoseiulus persimilis.
Figure 5: Frequency and types of chemical pest used during the season (Jan-Dec. 2020) in the biocontrol and chemical control greenhouse.
4. Tomato production
The accumulated yield of good fruit for the three cultivars in both greenhouses are presented in figure (6). Tomato harvest was started from March,2020. Our results showed that yield of both Feisty red and Valouro cultivars was higher in the biocontrol greenhouse as compared to the chemical control greenhouse, while yield of Tone Guitar cultivar was higher in the chemical control greenhouse as compared with biocontrol greenhouse. Good (marketable) yield in the biocontrol greenhouse ranged from 79.3 to 85.2kg/m2, whereas good yield in the chemical greenhouse ranged from 77.3 to 87.4kg/m2.
Figure 6: Accumulated good yield of the three tomato cultivars in both biocontrol and chemical control greenhouses.
Recommendation
Reproduction of Dicyphini bug A was significantly reduced due to frequent use of fungicides in the biocontrol greenhouse, while releasing bug B during this period has less effect on the population development. However, when stopping the application of fungicides, both bug A and bug B populations increased. This could be due to frequent application of fungicides in biocontrol greenhouse compared to the chemical control greenhouse because of limited application of sulfur in biological control greenhouse to reduce the impact and protect predators from sulfur as far as possible. Fungicides were often applied directly as spot treatment to the infected plant or the infected area. Although this approach protects the bio-predators, it made difficult to eliminate fungal infections. Yet, the full application of fungicides in biocontrol greenhouse enabled better control of fungal infections.
A striking result is that the three-weekly assessments show that there is no difference in strata preference between bug A and bug B. Both seem to prefer the higher parts of the plant. Dicyphini bugs lay their eggs and feed on soft plant tissue, which is more present in the top of the plant, which can explain their preference for the higher plant parts. At the end of the growing season there were higher numbers of bug B than bug B adults in the crop. This, while bug A was introduced 6 times and bug B only 2 times. The faster development of bug B, together with their overlap in strata preference probably causes that bug A is ultimately outcompeted by Nesi.
No chemical interventions were needed in H2 against Tuta, while being present in the greenhouse since the beginning of the growing season. A population of predatory bugs, in combination with the manual removal of infected leaves, ensured a sufficient protection of a Tomato crop against Tuta.
The most troublesome pest turned out to be both Tuta and Spider mites. In the biocontrol greenhouse Spider mite outbreaks were only controlled by frequently mass-releasing Neoseiulus californicus and Phytoseiulus persimilis. The downside of these predatory mites is that they are expensive and do not build up a population in the crop. Both die out in the absence of prey and the sticky trichomes of Tomato limits their mobility. Controlling spider mites in Tomatoes remains a matter of intensive weekly scouting and early interventions with predatory mites. Predatory mites can also be released in low numbers on a regular basis to prevent an outbreak of Spider mites.
This trial has proven that tomatoes can be cultivated for a whole season (up to 12 months) with good yield without using chemical insecticide in the Kingdom. The use of fungicides did not eliminate bio-predators. Yet it reduced their population. Therefore, suitable control of temperature and relative humidity should be maintained to prevent fungal infections and enhance bio-predators efficiency.
