Monarch Watch Blog

Tagging wild and reared monarchs: Best practices

Tuesday, July 9th, 2019 at 11:11 am by Monarch Watch
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Greetings taggers and welcome to the 2019 monarch tagging season. This year marks Monarch Watch’s 28th season! Over the years, thousands of taggers have contributed to our tagging database. It is an enormous record and a veritable gold mine of information about how the migration functions. The record represents at least 1.8 million tagged butterflies and lists where, when and by whom each butterfly was tagged. The sex of each butterfly and whether the butterfly was wild–caught or reared, tagged and released is also recorded. These data have told us a lot about the migration. Yet, this record could be improved but we need your help. Diving into the data has revealed a number of surprises such as the difference between the probability that a reared monarch will reach Mexico and the probability that a wild–tagged monarch will do so. The recovery rate is higher for wild caught monarchs (0.9% vs 0.5%) and it is the data from the wild–caught butterflies that tell us the most about the migration. Frankly, for some analyses, we have to set the reared monarch data aside. That doesn’t mean it is not valuable, but its uses are limited.

Given the difference between the recovery rates of wild–caught and reared monarchs, what are our goals going forward? For wild–caught monarchs, we have several goals. First, we need to increase the number of taggers from western Minnesota and Iowa westward into Nebraska and the Dakotas. This region is known to produce large numbers of monarchs and those tagged have high recovery rates. Increased tagging in this area will give us a more complete understanding of dynamics of the migration. Second, we need to increase the number of wild monarchs that are tagged since these provide the most valuable data. Third, we need to increase the number of taggers who tag from the beginning of the tagging season in early August until the migration ends. Tagging records for the entire season will help us establish the proportion of the late–season monarchs that reach the overwintering sites. When tagging wild–caught monarchs, many taggers run out of tags well before the season ends. That’s great, but it would help us to know when all tags had been used by indicating this via the online tagging data submission form.

For those of you who prefer to rear, tag and release, we have a few suggestions as to how you might improve the odds that your reared monarchs will reach the overwintering sites in Mexico. But, it is more than reaching Mexico. The best outcome for a wild monarch is to survive the migration, the winter conditions, the flight north in the spring and to successfully reproduce in Texas in the spring. It seems reasonable that this should be the goal for those who choose to rear, tag and release. To reach that goal, we have to know something about the wild monarchs that allows them to survive. The migration is a strong selective force. It eliminates the weak, those with diseases, the undersized and those with genetic and other deficiencies. It also eliminates those that have not received the environmental cues that properly trigger diapause and the orientation and directional flight characteristics of the migration. One way to increase the success rate for reared monarchs is to rear monarchs in a way that maximizes their exposure to environmental changes (day/night temperatures, changing photoperiod with the ability to sense sunup and sundown, etc.) that occur in the fall. In other words, rearing outdoors, on porches, in pole barns, open garages, etc., would likely produce better results than rearing in an air–conditioned kitchen, spare bedroom or similar space.

As to the actual rearing, raising the monarchs on living plants–potted or in the ground–is likely to produce the largest monarchs, provided that the monarch larvae have an abundance of foliage to feed on at all times. Cut foliage in the form of leaves also works well, but the leaves have to be fresh and abundant relative to the numbers of larvae in each container. Containers should be cleaned each day once the larvae reach the 4th instar. To avoid passing the monarch disease Ophryocystis elektroscirrha (O.e. or OE) from outdoor monarchs to reared monarchs, both the living and cut foliage can be sanitized using a 10% bleach solution with a drop or two of liquid soap added. After soaking in the bleach solution for two minutes, the leaves should be rinsed thoroughly with clean water and patted dry before being fed to larvae. Living plants can be sprayed with the bleach solution and then rinsed. If you are using cut stems with leaves intact, they can be cleaned the same way. In that case, be sure to cut the stems under warm water before placing them in vases, etc. The warm water keeps the latex vesicles from closing down the transport of water to the leaves. Cut stems work to feed larvae, but they can go limp and be less suitable as a food source than cut leaves.

We are offering these suggestions since reared butterflies tend to average smaller than wild migrants. That difference can be reduced significantly if careful attention is given to rearing larvae under the best possible conditions. Large monarchs have the best chance of reaching Mexico, surviving the winter and reproducing in Texas. There are several reasons for this: better glide ratio, better lift with cross or quartering winds, larger fat bodies, more resistance to stress, etc. There are very few small monarchs among those that return in the spring.

To summarize:
1. Rear larvae under the most natural conditions possible.

2. Provide an abundance of living or fresh–picked and sanitized foliage to larvae.

3. Provide clean rearing conditions.

4. Plan the rearing so that the newly–emerged monarchs can be tagged early in the migratory season (10 days before to 10 days after the expected date of arrival of the leading edge of the migration in your area*).

5. Tag the butterflies once the wings have hardened and release them the day after emergence if possible.

6. When it comes to tagging, tag only the largest** and most–fit monarchs. Records of tags applied to monarchs that have little chance of reaching Mexico add to the mass of tagging data, but do not help us learn which monarchs reach Mexico – unless the measurements, weight and condition of every monarch tagged and released is recorded. There are a few taggers who keep such detailed records and those data can be very informative. If you collect such data and are willing to share it please contact us; do not add this information to the standard tagging datasheet.

As a final note, this text is not a directive. We are not telling you what to do; rather, we are simply providing suggestions that may lead to more successful rearing and tagging efforts.

*Migrations seem to move in concert with the declining sun angle at solar noon (SASN). The fastest migrations have a leading edge of 57 degrees. Therefore, “early” is defined as 10 days either side of when the sun angle at solar noon reaches 57 at your latitude. To determine SASN for your latitude, visit A table of SASN values by latitude and/or further information will be posted at a later date.

**The easiest way to judge the size of your monarchs is to measure the forewing from the base to the apex of the wing (Figure 1). These measures range from 46–52 mm with most migratory monarchs measuring 49–51 mm. After some experience with both wild and reared monarchs, it is relatively easy to judge those that are below 49 mm.

Figure 1. Measuring monarch forewing.

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