Thank you for your question.

Unfortunately at this time I cannot provide you with a positive identification of this insect.  However I doubt that it is a weevil.  For the dahlia, the insect is some type of beetle.  The zinnia may be showing larval forms of this beetle. Unless I am able to come up with an ID, it will be difficult for me to suggest remedial action.

Could you please provide answers for the following:

1). How close together are your dahlias and zinnias?

2). Do you have any of these growing nearby:  corn, cucumbers, melons, squash?

3).  Is this the first summer that you have observed them?

4).  On which type of plant did you first notice these critters?  The zinnias or the dahlias?

5).  For both the dahlias and the zinnias, what type of damage did you observe to the plants?

6.  For how many seasons have you been growing these two plants?

7.  For the zinnias, did you start these from seeds or did you start them as transplants from some garden center? 

While we may be momentarily puzzled, we are persistent.

Looking forward to your responses.  Thanks for consulting us.

One additional item.

This is what I think this insect is, but there are aspects associated with this insect that don’t make complete sense with the information in your question. That is why I asked for further information.

https://bugguide.net/node/view/2027787

Besides responses to my previous questions, would appreciate your thinking as whether the beetle in the above site looks anything like that which you observed last summer.

Thanks for consulting us.

Thanks for getting back to me.

Was it last spring (2023) that you purchased the dahlias or was it in 2022 and then you stored the tubers over the winter of 2022-23?

Am I correct in my thinking that you first noticed these green insects on the dahlias and then only after that did you see the infestation on your zinnias?

Although it rather looks like a weevil, I don't think that it is.  But a precise identification is still eluding me.

Thanks again for consulting us.

Sorry to keep bothering you but I need one more piece of information.

When you purchased the dahlias, were these bare tubers or were they plants with foliage on them?

Many thanks

Thank you for your latest response and your kind patience.  I appreciate your attending to my numerous questions which, at times, were somewhat redundant.  In this process, I kept encountering things that did not make complete sense and so was hesitant to suggest remedial actions.  As I will describe, the most significant piece of information from you was that you purchased dahlia tubers in spring 2023 FROM COSTCO.  This finally enabled me to gain a full understanding of your situation.  I think that I can now suggest remedial actions so that 2024 will not be a repeat of 2023.  Most importantly, this will not require the use of any chemical pesticides by you.

You are NOT dealing with a weevil.  I suspected this from the start as the insect on your dahlia just did not look like a weevil.  Check out the pictures of weevils from the following and you can see what I mean:

https://extension.umn.edu/nuisance-insects/home-invading-weevils

Note the very pointed heads and enlarged abdomens of weevils in the images at this site.  The adult shown in your dahlia picture does not show these features.  I ran the insect in the dahlia picture through two different electronic keys.  Both did not indicate a weevil but rather something called a rootworm.  Finally, the insect that you are encountering is affecting your dahlias, zinnias, and your squash/pumpkins.  Weevils typically have a rather narrow range of plants they attack that does not  include several different plants.  I now believe that the insect you are dealing with is the Northern corn rootworm (Diabrotica barberi) or something very similar to it.  This was the insect whose image I sent to you a while back but when I did, I said that something did not make complete sense to me.  Besides corn, this insect can also attack cucumbers/squash, members of the aster family (zinnias are in this family), and dahlias.  Here is some more information about this insect:

https://bugguide.net/node/view/144447

https://en.wikipedia.org/wiki/Diabrotica_barberi

https://cornrootworm.extension.iastate.edu/basics/life-cycle - :~:text=Adults emerge from the soil,hard frosts in the fall.

This insect has a life cycle that encompasses two years.  In year one, adults lay eggs on the leaves and flowers of the plant.  These eggs hatch into larvae.  These larvae bore into the plant’s stems and roots and begin a pupal stage.  They overwinter in this stage.  In year two, adults emerge from these pupae and burrow out of the roots and stems.  The adults lay eggs and the fun begins all over again.

The only problem with all of this is that this species of rootworm is not commonly found in Minnesota.  That was a stumbling block for me and why things were not making complete sense to me.  However, when you said you purchased the tubers from Costco in 2023, I realized that the tubers you got from Costco most likely did not come from Minnesota but from elsewhere.  Costco gets most of their plants from southern states where this rootworm is quite common.

So here is what I think happened for your plants:

1).  The dahlia tubers you bought in spring 2023 contained pupal stages of this rootworm.

2).  When you planted these tubers, adults emerged from the emerging plant stems.  This is what you are observing in your dahlia picture.  While these adults are susceptible to various insecticides such as Sevin, the pupae still in the stems are not.  So after you applied Sevin, additional adults emerged from the stems, and this is why you observed only short-term effects of the Sevin.

3). The emergence of adults on your dahlias explains why you first noticed the dahlias to be affected. It also explains why 2023 was the first year that you encountered this in your plants.

4).  These emerging adults (think June 2023) then attacked your zinnias and pumpkins/squash.  They laid eggs from which larvae quickly developed.  This is what you are seeing in your zinnia picture.  Notice the yellow larvae present.  These larvae subsequently bored into the stems and roots of the zinnias, pumpkins, and squash and will overwinter there.

To prevent a recurrence of things in 2024, you will need to break the life cycle of the insect.  I suggest the following:

1).  Put all your existing dahlia tubers in tightly sealed plastic bags and dispose in the trash as soon as possible.  These tubers most likely contain pupal forms of this rootworm.  There is no way that you will be able to eliminate them.

2).  For 2024, purchase new dahlia tubers.  Do NOT get them from a big-box store.  Locate a reputable garden center to make your purchase.  It is possible that you may need to travel to Willmar to do this.

3).  If you did not do so last fall, this spring remove from your garden all plant debris from the zinnias, pumpkins, and squash.  Use a rake to get as much of this plant material as possible.  Lurking within such debris are most likely pupae of this rootworm.  Do NOT compost this material.  Instead, bag it up and throw out with the trash.

4).  Start your zinnia seeds inside as you have done in previous years.

5).  For 2024, plant your dahlias, zinnias, pumpkins, and squash at sites at least 10 feet away from where your planted them in 2023.  You want to create a physical barrier between any insects still lurking at the 2023 site and your new 2024 site.

All things considered, I now feel quite confident in my assessment of your situation and how to go forward into 2024.  It is consistent with all the information you provided.  Please read over what I have written and compare it to any other thoughts you might have regarding what happened in 2023.  I’m going to make special note of your situation and want to keep track of things.  For this reason, I would very much appreciate hearing what happens during the upcoming season.

Thanks for using our forum and especially for bringing your situation to my attention.  I learned much is researching it.  Again, my apologies for taking so long to resolve things.    Get back to us if you have any further questions.  GOOD LUCK.

Glad to have been of assistance.

Thanks again for consulting us.

The tubers have been bagged and thrown out. The tubers went absolutely crazy last summer, so pitching them was sad, but happy to do it to get rid of them!  I will take care of the plant material left.  (Looks like I might have lost my weather window this week.  My plan was to go out and do that on Friday.  Is there anything I can treat the soil with?  On the off chance this was a weavil, is there anything further I can do to eliminate this possibility? 

Thanks for the update and hope things go better in 2024 than in 2023.

I think that using new tubers for 2024 is a good idea. Likewise I would like to stress a previous suggestion that you put these new tubers in an entirely different location from that which you used in 2023. This would be preferable to your applying any insecticide to the 2023 location.

Your point about what precautions you should take in 2024 if weevils were involved in 2023 is well taken. You prior suggestion that the insects present in 2023 could be pale green weevils (Polydrusus impressifrons) also has merit. In the off chance that you haven’t seen information about this weevil, look at the following (note that this weevil is sometimes found in greenhouses):

http://www.minnesotaseasons.com/Insects/pale_green_weevil.html

https://fruitedge.umn.edu/insect-pests/polydrusus-weevils

https://www.greentimbers.ca/vegetation-wildlife/wildlife/insects/weevils/

If the 2023 critters were weevils, the adults died in 2023 after laying eggs. These eggs would then have hatched in 2023. The larvae that emerged would have overwintered (2023-2024) in plant debris and/or the soil. What this suggests is that a precautionary procedure you could take for 2024 is to thoroughly remove any plant debris from the 2023 site and discard in the trash. It sounds as if you might have already done this. Stressing my earlier suggestion and if possible, put any new tubers in a location several feet away from the 2023 location. Adult weevils are unable to fly but move by walking. This would be another plus in creating a significant distance from the 2023 site and any 2024 site.

Good luck and please feel free to get back to me if you have further questions or thoughts. Thanks again for consulting us.