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Extension Entomology

Category: Alfalfa

Alfalfa—-Alfalfa weevil, damage, aphids, lady beetle

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Alfalfa weevil larval feeding damage seems to be almost finished (fig 1.) throughout north central Kansas, for one of the following reasons: some larvae were killed naturally a few weeks ago due to the freezing temperatures, some were killed by insecticide applications, and some have just developed  to the pupal stage and thus quit feeding. There are still a few active larvae (fig. 2) however, most remaining weevils are mature larvae, but there are also a very few younger ones (i.e. note the tiny larva that just hatched, at the tip of the pencil), plus a few pupae and adults.

Figure 1. AW feeding

 

Figure 2 Various stages AW larvae (Cayden Wyckoff)

Aphids are still present (fig. 3), but in tremendously reduced numbers, also probably for the same reasons as the factors that decimated the weevils, but plus–there are significant numbers of lady beetle pupae (fig. 4) and adults (fig. 5). This means there were significant numbers of lady beetle larvae previously feeding on those aphids. Hopefully, these ladybeetles will survive and move to any fields wherever they can find pests to feed on during the rest of the growing season.

Figure 3. Aphid (Cayden Wyckoff)

 

Figure 4. Lady Beetle pupae (Cayden Wyckoff)

Figure 5. Adult Lady Beetle (Cayden Wyckoff)

Many alfalfa fields have already just been/or are currently being, swathed throughout south central and north central Kansas. This should take care of any remaining alfalfa weevil larvae. However, it may allow some stem feeding, often called “barking”, by adults especially under the windrows and may continue until temperatures warm up into the 80’s.

Alfalfa

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Much of the alfalfa throughout north central Kansas was significantly affected by last week’s freezing temperatures, as previously noted. This is especially true of older, less robust stands, as indicated by the plants in the lower left portion of fig.1. Sampling these freeze-affected

Figure 1 Alfalfa affected by the freeze (by Cayden Wyckoff)

areas with a sweep net revealed only very few live alfalfa weevil larvae, i.e. an average of 1 live larvae/10 sweeps. However, the less freeze-affected plants (upper right portion of fig. 1) had a much more significant infestation, i.e. these areas averaged 26 live larvae/10 sweeps. The vast majority of alfalfa weevil larvae detected this week were mature larvae, and many were actually on the ground, see Fig. 2. probably preparing to pupate, as a few new adults were also detected, again, see Fig 2.

Figure 2 Mature Alfalfa weevil larvae, new adult AW plus aphids and Lady beetle larva (by Cayden Wyckoff)

Figure 3 Aphids and lady beetle larvae (by Cayden Wyckoff)

 

Aphid populations, both pea, see Fig. 3, and cowpea, seemed to have dramatically declined also. This is probably a combination of the freezing temperatures coupled with a healthy population of lady beetle larvae, see Fig. 3, which have been voracious feeders on these aphids.

 

Pea Aphids/Cowpea Aphids

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

The recent cold weather also played havoc with these aphids (Fig. 3). However, many aphids, in untreated fields, were just “knocked off” or left the plants and are still alive, but in the leaf litter or residue under the foliage. There are also lady beetle larvae feeding on these aphids so, hopefully, these aphid populations won’t come back as dense as they were previous to the cold weather.

Figure 3. Pea and Cowpea aphids (picture by Cayden Wyckoff)

 

Alfalfa Weevil

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Alfalfa weevils have been, and are still, very active throughout south central and north central Kansas during the last few days, even through the cold weather. However, just as the alfalfa plants have been variously affected by the recent cold weather, so have the alfalfa weevil larvae. The plants shown towards the middle of this picture (Fig. 1),

Figure 1Alfalfa freeze damage (picture by Cayden Wyckoff)

had the upper part of the foliage killed by the recent cold temperatures, as were the larvae in that foliage (Fig. 2).

Figure 2 Dead and live AW larvae (KSU Extension Entomology)

The adjacent plants were not as seriously affected and thus, neither were the larvae in those plants.

Thus, each field needs to be monitored at least weekly, even those fields already treated. Nevertheless, please — always remember, to follow all label directions for whatever product applied, especially as far as reentry, PHI, etc.

 

 

Alfalfa Weevils

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Alfalfa weevil activity has increased significantly in the last week throughout North Central Kansas. The recent warm weather has really stimulated egg hatch thus there are large numbers of very small 1st instar larvae just starting to feed, plus older, larger larvae (fig. 1) that have been feeding for a week or so. However, some fields have very low infestation levels, while others have already reached the 100% infested level. Recent freezing temperatures appeared to have killed some top growth in some fields, which can be mistaken for insect damage.

Thus, sampling each field is always prudent, but even more so this year. We have gotten many questions recently about the predicted cold weather and its effect on the weevils, but remember, alfalfa weevils are cool weather insects. Temperatures into the mid 20’s for a couple hours may kill small larvae, as we saw in 2018, but probably won’t affect the eggs or adults. Then, anytime the temperatures are over about 45°F, the larvae feed and do so 24/7 as long as temperatures are above 45°F. Many fields were treated this week and probably should be whenever the treatment threshold is reached as the predicted temperatures for the next 10 days looks like it will slow the feeding activity down but probably not be cold enough to kill very many larvae. Have also gotten the question about spraying for army cutworms and that effect on alfalfa weevils. If both occur at threshold or one or the other does and you make an application of an insecticide you should get pretty good control of both. However, remember, cooler weather will slow down the effect of the insecticide.

 

Also, check this super neat picture of biological control at work (fig. 2). A turkey harvested by a Kansas youth hunter on 6 April had a crop completely filled with large army cutworm larvae.

 

Alfalfa Update – aphids, potato leafhoppers

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Pea aphids (see pic) are and have been returning, or at least increasing in numbers, to many alfalfa fields throughout north central Kansas. These are primarily cool weather aphids and are usually the last ones still feeding in the fall on alfalfa and the 1st ones in the spring. However, with the advent of cool/cold weather this late fall feeding should be negligible. Potato leafhoppers (see pic), for the most part, have emigrated, or at least are not present in easily detectable numbers, so “hopper burn” (see pic) and its consequences, should not be problematic this fall/winter.

 

Pea aphids

 

Potato leafhopper

 

Hopper burn

 

Alfalfa and Soybeans

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

Green cloverworm adults (see pic) have been very common in soybean and alfalfa fields the last couple of weeks, and this has caused concern about potential green cloverworm infestations next year. However, green cloverworm adults are, or have been, migrating to the southern US for overwintering. Thus, since they do not overwinter in Kansas, infestations next year will depend on wherever the adults come back to, so predicting future infestations after overwintering adults return from the southern US are not possible.

Adult green cloverworm

 

 

 

Crop Update – Soybean & Alfalfa

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth

There doesn’t seem to be a lot of pest activity, at least at the present time, in any of the crops throughout north central Kansas. However, there does seem to be a relatively large number of green cloverworms of different stages (see picture)

 

in both conventionally planted double cropped soybeans. There is enough foliage, however, in all fields that these larvae should not cause any problems, although in some fields they are chewing multiple ragged holes in leaves. There are also a few thistle caterpillars and webworms but again, all defoliators together should not cause enough damage to impact yield. There are also large numbers of potato leafhoppers in both soybeans and alfalfa. They should not cause problems in soybeans. However, they will cause problems in alfalfa if they continue to feed and inject a toxin into the leaves as they do this feeding. Swathing should help mitigate this problem, but monitoring the stubble for leafhoppers should continue to ensure adequate regrowth.

Alfalfa Update

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth and Dr. Holly Davis

Potato leafhoppers are rapidly increasing throughout alfalfa fields in north central Kansas for two reasons:  1) potato leafhopper adults are still migrating in and 2) the eggs are hatching and nymphs seem to be everywhere.  These nymphs are very small and very shy – which means they are easily under counted as they hop to the underside of leaves, or even off the leaves, at the least disturbance.

Alfalfa weevils mating—(photo by T. Sexton)

Parasitized Alfalfa Caterpillar

Alfalfa weevil adults have mostly migrated out of alfalfa fields in north central Kansas, however there are a few that pupated late and that are just emerging out of their pupal cells.  Interesting, at least to us, was that some of these adults were mating (see picture).  Most of the literature reports alfalfa weevils mating in the late summer, fall/winter  —  not soon after becoming adults.

Alfalfa or garden webworms are also relatively common in alfalfa, where they may cause a problem in new alfalfa, and soybeans.  The next generation will probably be more problematic in small soybeans because there will probably be more webworms as this generation is more of a “spring board” generation.

Alfalfa caterpillars (see picture of larva with attached parasitoid eggs) are also quite common in alfalfa fields as are the white and/or yellow butterflies that they develop into.  However, they have not ever been found in densities great enough to cause any negative impact on yield.

 

Alfalfa Update – Potato Leafhoppers and Pea Aphids

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa seems to be growing very well and many fields around north central Kansas have finally dried out enough to swath and remove the hay from the field.  However, potato leafhoppers continue to migrate into the state and will continue to for about another month.  Most are still adults and have been/are now depositing eggs in stems and the tiny nymphs are just starting to emerge.  Thus, potato leafhopper feeding will become more evident as “hopper burn”, the yellowing of leaves which can reduce the health of the plants and the nutritive value of the foliage.  Therefore, if fields were just recently cut, or will be in the near future, while potato leafhoppers are still migrating into the state, they will be very vulnerable to potato leafhopper feeding damage.

Pea aphids are still plentiful throughout alfalfa fields in north central Kansas.  Populations should not reach treatable levels this late in the year, and they are a good host for many beneficial insects.

For more information regarding these and other alfalfa pests, please see the KSU Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf