Alfalfa is an ideal environment to foster the population development of beneficial and pest insects.
Large numbers of natural enemies can be found in alfalfa including lady-bird beetles, green lacewings, big-eyed bugs, damsel bugs, syrphid flies, and numerous parasitic wasps.
A healthy alfalfa crop with a diverse array of natural enemies can help suppress pest populations below economic thresholds.
Fostering the use of natural enemies in an integrated pest management (IPM) program allows alfalfa growers to better manage pest outbreaks. IPM practices include the use of biological control, cutting schedule modifications, strip or border cutting, more resistant alfalfa varieties, and pesticides when needed.
The cornerstone of IPM is a good knowledge of the pest attacking a crop and an understanding of the relationship of pest density to crop damage. It is important to properly identify insects when sampling alfalfa fields.
Many insect species, especially in the immature stages, are similar in appearance and can be easily confused. For example, lygus nymphs can be confused with general predator big-eyed bug nymphs.
The blue alfalfa aphid and pea aphid are similar and should be distinguished with a careful examination of the antenna with a hand lens. Improper identification can lead to incorrect management decisions.
The failure to properly identify natural enemies can lead to unnecessary pesticide applications if the natural enemy populations are sufficient to maintain pest numbers below economic treatment levels.
It is important that a scouting program include an assessment of pests and natural enemies including the specific life stage.
Routine field sampling
Regular scouting of alfalfa fields is necessary to determine pest density and when treatment is warranted. The action threshold is defined as the level of pest populations where control action should be implemented to avoid significant damage to the crop.
Action thresholds help determine the level of control action and the proper timing of the action, taking into account the relative pest density, distribution, and the plant’s growth stage.
How to monitor
The use of efficient sampling methods is necessary especially when scouting large alfalfa fields. The sampling method depends on the insect.
The most common sampling method in alfalfa is a 15-inch-wide sweep net. How the sweep net is used can greatly influence the effectiveness for collecting insects and the treatment decisions based on the number of insects caught.
The following is a standard method for sampling. Swing the sweep net in a 180-degree arc so the rim of the net strikes the top 6-8 inches of the alfalfa. Hold the net slightly less than vertical so the bottom edge strikes the alfalfa before the top edge. This helps move the insects into the net.
Each 180 degree side-to-side sweep counts as one. A common practice is to sweep from right to left, walk a step, and then sweep from left to right. After taking the desired 10 sweeps at each location, quickly pull the net through the air to force the insects into the bottom of the net bag.
Grab the net bag with a hand at about the mid-point. Count the insects and divide the total by 10 to get the average number of insects per sweep. To gain a good insect count, take sweep net samples in four different areas of the field. Refer to the UC-Davis IPM Web site for specific sweep net sampling guidelines for each pest.
For shorter re-growth alfalfa, do not rely on sweep net sampling to determine the population levels. Examine plant stems for insects and recently damaged foliage. Randomly choose five stems from four areas per field. Place each stem sample over a white pan and tap. This will dislodge the insects into the pan for assessment.
Yellow sticky traps can be used for estimating relative densities of flying insects. Sticky traps can capture significantly more insects including lady-bird beetles. Sweeps or stem counts are more useful to determine adult density changes over time.
Proper pesticide selection
A problem in implementing an IPM program in alfalfa is the lack of information on the compatibility of insecticides with natural enemies. It is important to evaluate the relative toxicity of selected pesticides to natural enemies including parasites, aphid predators, Lepidoptera, and the Egyptian alfalfa weevil.
Some pesticides are highly toxic to some life stages of natural enemies. Broad spectrum insecticides including carbaryl should be avoided since it is highly toxic to all life stages.
Products including Bacillus thuringiensis and cyromazine are relatively non-toxic to all life stages of natural enemies.
The conservation of natural enemies through the use of timed applications of selective insecticides for targeted pest management is feasible.