- Malheur Experiment Station
Pests are a major cause of crop loss when it comes to onions. Specific pests decrease onion yield because they take over fields, kill plants, cause rotting to occur, and ruin land. In order to control pests, many pesticides have been designed to control some of the more detrimental insects. One of the important issues for sustainability in regards to pest management is resistance management. In resistance management, pesticides are used judiciously and in rotation with different modes of action so that pests do not rapidly become resistant to the pesticide. Also, natural deterrents can reduce the amount of crop damage done by insects.
One of the main insects attacking onion fields in the Pacific Northwest is the onion thrip. Thrips live on the newly opening onion leaves and rasp the leaf surfaces. They are known to reduce onion yields, increase the amount of onions affected by rot, and kill seedlings. Thrips are mainly active throughout the warm, dry summer months. Another insect negatively affecting onion quality is the onion maggot. Maggots eat the bolts off of plants, causing major damage to seedlings and rot. They can also spread pathogens that may infect numerous plants within the field.
Another anthropod pest that growers must protect their onions from is the bulb mite. Bulb mites cut off the radicle root of the plant before it becomes fully grown and established in the ground. Without a healthy radicle root, the plant has difficulty obtaining water and nutrients from the soil, causing slow and limited growth. Although not as large a concern in the Pacific Northwest, cutworms (caterpillars) are also problematic in onion crops. These insects eat both the underground and above ground portions of the plant, causing eventual death (Pests of the Northeastern US).
To sustain the viability of pesticides (and to reduce inputs), applying pesticides (for insects, pathogens, and weeds) is a good practice (Reitz, S., 2013). There are numerous pesticides registered to use on onion crops to control insects. The most common pesticides are decent at killing insects but also may harm the environment, and all side effects must be considered. It is important to understand that pest control methods will not always make a difference. They must be applied in correct amounts and at the correct time. There will come a time when fighting insects will no longer make a difference in the quality and yield of the crop. Used precisely and intelligently, best management practices can be very beneficial. Furthermore, they are very useful at reducing pollution. Monitoring pest population is a valuable IPM technique, as is field sanitation. Trapping and vacuuming for onion pests are not so practical and effective.
Many insects receive their food from crop residues located on the farm. If the grower can determine where the pests are obtaining their food and eliminate the source, then the number of insects will decrease. Harborage reduction means removing storage items that pests live and breed in. By removing any storage and unnecessary clutter, the grower is reducing places for pests to live. All of these methods can be summarized by “environmental modifications”, or to change conditions on fields so they are as unsuitable to harmful pest life as possible (PCOC Best Management Practices). Trap crops and barriers are also natural ways to reduce pest pressures, but more research is needed on their effectiveness with onion crops. Special fertilizers, such as those including seaweed and beneficial insects, can also counteract the negative effects pests have on crops; for example, nematodes are commonly used to stabilize the garden against cutworms.
Another management practice that can be extremely beneficial is crop rotation. By rotating crops season from season, disease and insect pressures are decreased, creating a healthier soil for plants to grow in. Pests specific to a given crop have a difficult time growing in fields that are constantly rotated. Also, because they cannot stay in the same place year after year, insects have to move around before finding another field to colonize.