Trap crops are grown as a control measure to lure pests away from the cash crop to protect it from attack. Pests are either prevented from reaching the crop or concentrated in certain parts of the field away from the main crop. The principle of trap cropping relies on pest preference for certain plant species, cultivars or a certain stage of crop development. Plants produce chemicals, or volatiles, that attract insects for pollination and repel pest insects. Different species and cultivars produce varying degrees of unique volatiles, allowing certain species or varieties to repel insect pests more strongly than others, making them suitable for selection as a trap crop. The two primary techniques utilized in trap cropping are:
Selection of a more preferred plant species or cultivar grown at the same time as the main crop.
Planting of the same species and cultivar as the main crop timed to be at the most preferred stage of development before the main crop. Whether using the same or different species, it is essential that the trap crop be more attractive than the main crop.
Trap cropping offers several benefits in a pest management system. When trap crops successfully attract pest populations, damage to the main crops is limited; therefore, main crops seldom require treatment with insecticides. When insect pests are at high concentrations in trap crops, they can be treated in a localized area instead of treating the entire field. Savings resulting from reduced pest attack and insecticide use may substantially outweigh the cost of maintaining crops that do not provide economic income. Reduced damage to main crops also increases their expected marketable yield. Further, a variety of plantings and increased concentration of insect pests may attract natural enemies, enhancing naturally occurring biocontrol.
The design and arrangement of trap and main crops depends largely upon the target pest. Knowledge of target insect behavior is therefore necessary when creating a field design. For example, a perimeter trap crop may be sufficient for reducing damage of the main crop by Colorado potato beetle, but intermittent plantings may be required for more mobile species such as the striped cucumber beetle. Trap crops may also be designed for nematodes and fungi that cause plant diseases. The required size of the trap crop is a function of the number of pests expected and the mobility of the species, but the proportion of the trap crop is typically 10-20 percent of the main crop. The primary key for effective trap cropping is the successful establishment and management of the trap crop stand; more desirable plants within the trap crop stand will have a greater impact on luring pests away from the main crop. For enhanced control, the use of trap crops can be combined with other pest management strategies, such as crop rotations, to reduce the number of expected pests, and pheromone traps, to attract pests to desired areas away from the main crop.
Despite the benefits of using trap crops, there are several concerns. First, trap cropping is only beneficial when fields are likely to be invaded with high numbers of pests. Improper management of pests on trap crops may create “pest nurseries,” facilitating a more rapid or widespread pest outbreak than may otherwise have occurred. Treatment of trap crops with insecticide may lead to increased evolution of pesticide resistance and destruction of natural enemies. Further complications may arise when trying to manage multiple pests with different behaviors. Application may be limited for certain crops.
Trap Cropping - Spider Mites
One of the most commonly used trap plants is for the early detection of two-spotted spider mite (Tetranychus urticae). Since mites are not able to fly, they are often missed by scouts who are focused on using sticky cards for monitoring pest populations.
Bean plants are most commonly used as trap plants for two-spotted spider mite. Bean plants are one of the mite’s favorite food sources.
Regular bush bean plants do very well at attracting mites. Bean seed is cheap and most ornamental plant growers have containers and growing media available so it is easy to implement a detection system.
Bean plants usually show signs of two-spotted spider mite damage much sooner than most other crops. For example, three or four adult mites on a bean leaf show the characteristic yellow speckling feeding damage on top of the leaf (Figure 1). Bean plants can be used as indicator plants in traditional pest management programs that incorporate pesticide applications.
For growers who are using biological controls, bean plants can act as both trap crops and banker plants. Banker plants are plants that are different from the primary crop being grown. Banker plants can assist in establishing, supporting and maintaining a population of one or more biological control agents. Predatory mites such as Phytoseiulus persimilis and Amblyseius californicus, and the predatory midge Feltiella acarisuga can be released as soon as spider mites are detected on bean plants.
Another benefit of trap plants is that for many crops the spider mites don’t migrate to surrounding plants until the bean plants show signs of over population. By the time this occurs, the bean plants have started to deteriorate from the damage.
When using bean plants as trap plants growers need to manage the spider mite population. Otherwise the bean plants can become excellent mite producers. This is why biological control producers often use bean plants for the production of biological control agents. If bean plants are used to monitor spider mites and the plants are not managed properly, they can produce large numbers of mites. It is important to use bean plants as part of a monitoring system. The plants need to be checked on a regular basis.
Bean plants have not been fully tested on cannabis in a formal study. Cannabis is extremely attractive to two-spotted spider mites and might be comparable to bean plants in their attraction. In these situations where the commercial plant is very attractive to spider mites, growers should consider implementing preventive releases of biological control agents.
Bean plants are also very attractive to thrips and possibly aphids. Growers who are using biological control agents should release the biologicals on both the bean plants as well as on the cannabis crop being produced.
The number of bean plants placed in a greenhouse can vary. It depends on the crop and how much production space a grower is willing to give up in order to detect the spider mites earlier in the crop cycle. The earlier the detection of spider mites occurs, the better the chance of control and the less chance of plant loss.
In spider mite susceptible crops 40 bean plants per acre is considered the minimum. Better detection results usually occur when 60-80 bean plants per acre are used.
Case Study (2017) - Trap Cropping Aphids
This study began Fall 2017 on a Nov/Dec harvest in a 300 square foot greenhouse in Humboldt County with propane heating and 600 watt hps light for supplementing additional light. There were 4 raised beds 4'x8' that were initially seeded to cycle nutrients in the raised beds. Approximately 28 plants in 1 gallon pots were transplanted on top of each bed. The initial intention was to see how the cover crops interact with the flowering cannabis plants and how well plants grow in wintertime flowering cycles.
The cover crops would soon overtake the cannabis plants and all the cover crops were damped down to ground level as well as harvested and placed around the base of each cannabis plant as a mulch layer. It soon became apparent that experiment became more about intercropping then cover cropping. As the cannabis began flowering hard, the cover crops begin to grow again, this time much slower. At this point signs of aphids began to really emerge. The intention of this garden was to observe and no pest sprays were used during this entire cycle. Being in an enclosed greenhouse with no access to beneficial insects, the aphid population began to rapidly multiply.
For quite some time the aphids did exclusively feed and reproduce on the buckwheat and peas while leaving the cannabis alone. Eventually as aphid populations rose, the density spilled over onto the cannabis plants, most of them getting stuck in the resinous flowers of the lower branches. It became apparent that population densities this high could potentially impact the microbial counts if the aphids could not be trimmed out of those flowers.
The flowers were harvested at 56 days, cured and processed over the next 2 weeks. Roughly 5% of the total flowers were disposed of (composted) due to an unmanageable level of aphid residue left in the flowers. The rest of the plants, showed excellent trichome production and budset for such a late winter crop with minimal lighting. Overall, it became very apparent that improper management of intercropping would lead to pest problems. Species selection then became a very important factor to focus on. Please visit the other pages on intercropping to learn more about more appropriate species.
Ultimately, the utilization of cover crops or trap crops in a greenhouse or indoor environments come with many factors to consider. Temperature, relative humidity and air exchange were all well managed. A simple release of lady bugs would have most likely solved this problem. It is important to have a clear understanding of which cover crops and trap crops attract each type of pest, clover being a species that attracts spider mites. If this experiment had been performed in the middle of the summer in an outdoors environment, the natural predator insects might have also kept the aphids in check. It is only when we try to artificially replicate the surrounding biome do we then realize how many other environmental factors we overlooked. Overall, with only a 5% loss of product and no expenditures on pest control measures, the experiment was relatively successful on staying below the Economic Injury Level (EIL).