Alfalfa insects: What to look for, how and when
Most entomologists enjoy insects and many smile whenever they see an alfalfa field. Why? An alfalfa crop is usually home to large numbers of insects and a great diversity of species. Most of these insects are harmless or beneficial; lady beetles are an example. The pest species are generally found in numbers too few to reduce alfalfa yield. Unfortunately, in some fields, in some years, populations of an insect pest reach levels that could cause economic yield loss . Because of these occasional bad actors, alfalfa should be scouted from green up through the last cutting.
Alfalfa insect populations and the pest species present will vary with weather and season. For example, the migrant potato leafhopper is Minnesota alfalfa's most consistent insect pest but rarely reaches damaging levels before the first cutting. The insects that most often cause first crop concerns in Minnesota include alfalfa weevil, pea aphid, and plant bugs. Biological information, preferred scouting/sampling methods, and a scouting calendar for common Minnesota alfalfa pests insects are shown in Tables 1 and 2.
A 15–inch diameter, heavy–duty, sweep net is a necessary tool for scouting insects in alfalfa. Sweep nets are needed to quantify insect numbers when using economic thresholds for several key alfalfa insects and for the initial detection of most alfalfa insects. The sweep net can allow you to find insect pest populations early, before yield loss or obvious symptoms happen.
Figure 1. Suggested sampling plan for obtaining
representative estimates of alfalfa insect popula–
tions. Circular or "W" shaped patterns may also
Sweep nets can be ordered from several on–line suppliers. When purchasing a sweep net, consider the following:
- Avoid the lightweight aerial nets designed to catch butterflies.
- Specify heavy–duty.
- A long–handled net is better and marking the handle with inch increments will allow you to measure crop height with the same tool.
- Aluminum handles are durable and light but metal can rub off on your hands.
- Purchase a net that you can get purchase replacement parts for; if your net doesn't wear out, you're not using it right.
A 10X–15X hand lens, containers for saving specimens for later identification and identification guides are other important tools for alfalfa scouting.
Management and insecticides
Investments in scouting and basing any insecticide applications on economic thresholds pays. Although it might seem an easier approach, insurance insecticide applications do not always save crop yield but they often do waste crop input dollars. Insurance insecticides might actually make your alfalfa insect problems worse. They kill beneficial insects and the loss of biological control could flare other insect problems. Finally, insecticide overuse can lead to insecticide resistant insect populations. Always read and follow the insecticide label. Many insecticides are harmful to honey bees. Avoid applications to blooming alfalfa or when honeybees are active! Notify beekeepers in the area if you must treat flowering alfalfa.
Some on–line suppliers of insect sweep nets and hand lenses follow:
- BioQuip Products http://www.bioquip.com
- Ben Meadows http://www.benmeadows.com
- Forestry Suppliers http://www.forestry-suppliers.com
- Great Lakes IPM, Inc. http://www.greatlakesipm.com
- Gempler's http://www.gemplers.com
Alfalfa insect scouting and insect sampling techniques
The following alfalfa insect scouting guidelines are derived from Bill Hutchison's Alfalfa IPM: Sampling alfalfa insects. 1993. University of Minnesota Extension. They thoroughly explain why and how to scout.
Goals of sampling
Correct identification and efficient sampling methods for beneficial and pest insects are two critical steps toward implementation of integrated pest management (IPM) programs. IPM includes the use of all feasible control tactics (e.g., crop rotation, resistant hybrids and chemical control) to manage pests within a profitable, yet environmentally sound production system. Efficient sampling methods are necessary for making accurate and timely evaluations of insect population (infestation) levels. These estimates can then be used for comparison with economic thresholds and consideration of appropriate management tactics.
Early detection of economic insect populations
Insect mortality can result from starvation, desiccation and exposure (conditions which often occur when alfalfa is cut), as well as from predators, parasites, disease and insecticide applications. Most insects are more vulnerable to each of these factors when they are young. The lowest labeled rates of insecticides can often provide effective control when treating young (immature) insects. Hence, early detection is advantageous. Nevertheless, control action is still appropriate only when yield and quality savings justify control costs.
For production of high yielding, high quality alfalfa, the crop should be checked weekly. In cooler weather, sampling can be done less frequently. In hot weather, however, sampling frequency should be increased (insects develop, feed and reproduce faster under warmer conditions). Shorter sampling intervals are also necessary as insect populations and/or damage approach economically damaging levels.
Table 1. Biology and crop damage for Minnesota alfalfa insects.
|Insect Pest*||Overwinter stage in MN||Damaging stage||Mouthparts||Feeding site||Crop damage and symptoms|
|Aphid (pea)||Egg||Adult/ Nymph||Piercing– sucking||Phloem sap, leaves & stems||Saliva toxins cause stunting/ distortion/ yellowing /sooty mold|
|Aphid (Spotted alfalfa)||?||Adult/ Nymph||Piercing– sucking||Phloem sap, leaves & stems||Saliva toxins cause stunting /yellowing /sooty mold|
|Aphid (Cowpea)||?||Adult/ Nymph||Piercing– sucking||Phloem sap, leaves & stems||Saliva toxins cause stunting /sooty mold|
|Alfalfa blotch leafminer||Adult||Larvae/ adult||Hooks/ scraping, sucking||Larval leaf mines||Leaf drop, adult feeding "pinholes"|
|Alfalfa caterpillar||Pupa||Larvae||Chewing||Foliage||General defoliation|
|Blister beetle spp.||Larvae on grasshopper eggs||Adult body (toxin)||Chewing||Foliage||Minimal injury to alfalfa/ toxic to horses: live and dried beetles contain cantharadin toxin|
|Cutworm (Variegated)||Migrant||Larvae||Chewing||Foliage||Defoliation, especially on regrowth|
|Grasshopper spp.||Egg||Adult/ nymph||Chewing||Foliage||General defoliation|
|Plant bug (Alfalfa)||Egg within plant stem||Adult/ nymph||Piercing– sucking||Phloem sap of leaf veins||Stunting/ distorted leaflet tips|
|Plant bug (Tarnished)||Adult in leaf litter||Adult/ nymph||Piercing– sucking||Phloem sap of leaf veins||Stunting/ distorted leaflet tips|
|Potato leafhopper||Migrant||Adult/ nymph||Piercing– sucking||Phloem sap of leaves||Phloem damage/ stunting/ hopperburn|
|Meadow spittlebug||Egg||Nymph||Piercing– sucking||Sap feeding on stems||Distinctive spittle|
|Alfalfa weevil||Adult in leaf litter||Adult/ larvae||Chewing||New growth defoliation||Delayed 2nd crop growth|
|Weevil (Clover leaf)||Larvae/ egg||Adult/ larvae||Chewing||Lower canopy||Defoliation|
|*Most common Minnesota pest species in bold|
Table 2. Scouting calendar for Minnesota alfalfa insects.
|Timing of damage|
|Insect Pest*||Threshold sample method**||1st crop||2nd crop||3rd crop||Late season|
|Aphid (Spotted alfalfa)||/stem||X||X|
|Alfalfa blotch leafminer||% leaflets damaged||X||X|
|Blister beetle spp.||Detect with sweeps||X||X||X|
|Plant bug (Alfalfa)||/sweep||X||X||X||X|
|Plant bug (Tarnished)||/sweep||X||X||X||X|
|Alfalfa weevil||%terminals/ stem||X||X|
|Weevil (Clover leaf)||/plant (crown)||X|
|*Most common Minnesota pest species in bold|
**Sweep nets can effectively be used for detection and identification of most alfalfa pest and beneficial insects