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Extension Entomology

Category: Alfalfa

Alfalfa—pea aphids, adult green lacewings, lady beetles, potato leaf hopper

–by Dr. Jeff Whitworth

Many alfalfa fields were sampled throughout north central Kansas over the last few days. Very few pea aphids were detected (fig 2.) – but those that remain are providing nutrition for many beneficials in alfalfa fields i.e., adult green lacewings and lady beetles (many) and lady beetle larvae (fig 3.). No alfalfa weevils were observed.

 

Figure 2 Pea aphids

 

Figure 3a Adult green lacewing

Figure 3b Adult ladybug

Figure 3c Lady beetle larva

However, potato leafhoppers, all adults, were collected in every field sampled (fig 4.). This indicates potato leafhoppers are immigrating into the state and will soon, if not already, be depositing eggs in stems, which soon hatch and thus increase populations. Potato leafhoppers remove plant juice and while so doing may inject a toxin into the plant. This feeding alone, may stress plants, especially in the often hot/dry conditions in July/August in Kansas. However, the introduction of a toxin may also negatively affect the plants. One characteristic symptom of potato leafhopper feeding starts out with yellowing from the tip of the leaves and travels through the leaf to the stem. This is often called “hopper burn” and can then negatively impact alfalfa production, both in quantity and quality by lowering the nutritional value.

 

Figure 4 Potato leafhopper

 

Alfalfa Weevil Situation

–by Sarah Zukoff

 

I wanted to update you on the alfalfa weevil situation I’ve been dealing with. I’ve gotten many different calls from farmers and agents across Kansas the last few weeks. After working with Romulo Lollato in Ag; Judy O’Mara and Erick DeWolf in Plant path; Frank Pairs from CSU and Kelly Seuhs from OSU I think we have an idea of what’s going on. 

 

I received calls of extensive damage in alfalfa fields and multiple field failures of alfalfa weevil sprays-(see pic’s) in May. After visiting over 30 fields across the state and doing lab assays on collected larvae we’ve determined pretty wide spread lambda-cyhalothrin resistance with suspected resistance to Indoxicarb and even Chlorpyrifos. We’re setting up assays to test these later this year. The weevil damage was extensive and after multiple weevil sprays had little mortality. The frost hit after this damage and these very stressed plants died back pretty severely. These plants then experienced soggy spongy roots with some rot that further damaged plants. Larvae seemingly occurred in multiple rounds as the frost interrupted normal development.  I haven’t determined if the different weevil strains play into this story yet. So lots and lots of larvae that are literally STILL active in fields now with many sprays not working. After ten days, the plants are slowly bouncing back with yellowed regrowth. Farmers are struggling to find anything that works in some of these fields, consultants are being blamed for not doing their job, and agents are being pressured to find SOMETHING to help the farmers with… Therefore, I now need to quantify resistance to the other AI’s and ponder this delayed expanded hatching over two months.

 

 

ALFALFA—Pea Aphids, Lady Beetles, Syrphid Fly

–by Dr. Jeff Whitworth

 

Pea aphids are still quite active throughout north central Kansas. However, most populations have been considerably diminished for several reasons, but now are mainly just a food source for various beneficial insects (fig, 1) as seen here, two species of lady beetles actively feeding on pea aphids, and (fig. 2) a syrphid fly larva that was also actively feeding on pea aphids.

 

Figure 1 Lady Beetle feeding on pea aphid  — (Cody Wyckoff)

Figure 2 Syrphid fly larva feeding on pea aphid (Cody Wyckoff)

Pea aphids are usually considered a pest when populations approach or exceed a treatment threshold, they can also be useful as a food source for beneficials until other aphid species increase, if they do, in other crops. Even though these aphids are still plentiful in most north central Kansas alfalfa fields, there was NONE that came anywhere close to a treatment threshold. Also, a very few alfalfa weevil larvae (fig. 3) can also be found, this is not unusual, or a cause for concern.

Figure 3 Alfalfa weevil larva (Cody Wyckoff)

 

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

–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

–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

–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

–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

–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

–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

–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

 

 

 

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