Monarch Watch Update - July 17, 2003



1) Welcome!

2) Virus Attack

3) Status of the Population

4) Mid Summer Conditions

5) When does the migration northward end?

6) The Pre-migration Migration

7) 2003 Membership/Tagging Kits

8) Tachinids: Monarch Parasitoids

9) Upcoming Monarch Events

10) How to Unsubscribe from this Update


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2) Virus Attack

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3) Status of the Population – by Chip Taylor

Last month I wrote:

"I’m optimistic about the reports from the rest of the country. The drought has diminished (see below) and the milkweeds appear to be in good condition in most locations. Moderately good numbers of monarchs have been reported from nearly all of the core areas of the summer breeding range over the last three weeks. It is definitely looking good for large fall migration – assuming normal rainfall and temperatures from now to the end of August."

Nothing has happened over the last month to lead me to alter this assessment. Reports indicate monarchs have reached nearly all of the breeding areas and have been found at several locations in Canada near and beyond the known limits of milkweeds. Generally, there are more reports from these outlying sites in good monarch years so these accounts are a good sign that there will be a strong fall migration. The monarchs reported in May and early June at the northern limits of the breeding range originate in the southern states, especially Texas, and the numbers observed probably reflect: 1) the size of the first generation; 2) conditions favoring the movement of these butterflies northward; 3) suitable conditions in the breeding areas; and 4) the number of observers who report their findings. Those of us who are attempting to monitor the monarch population are highly dependent on the accounts posted to Dplex-L or Journey North by observers throughout the area east of the Rockies. Please continue with these reports. Even though they may be anecdotal, these accounts give us a general picture of the distribution and abundance of monarchs.


4) Mid Summer Conditions

Generally, the mid summer conditions appear to be favorable for the buildup of the monarch population. Although rainfall has been excessive in some parts of Indiana and Ohio, the overall weather pattern and temperatures seem to be favorable for monarch breeding. The drought of last year continues to linger in a few areas, e.g. northern Maine, New Hampshire, Vermont, the upper peninsula of Michigan and western Michigan, eastern Kansas and northwestern Missouri, and parts of western Nebraska, South Dakota, and southwestern North Dakota as well as Wyoming, Montana and Colorado (see Aside from the areas in Michigan, none of the drought-affected areas are known to be major breeding areas for monarchs. It is probably safe to say that good to excellent conditions persist for more than 90% of the monarch breeding population at this time.


5) When does the migration northward end - or does it? – by Chip Taylor

Last month in a paragraph with the above title I proposed that monarchs stopped migrating as the date approached the summer equinox of 21 June. Specifically, I said "My guess is that the migration stops at each degree of latitude northward at a particular date, that these dates can be predicted, and that all directional migration stops before the 21st of June at all latitudes." Well, what did you see and what are you seeing now? Did monarchs stop migrating and have they become local in their movements?


6) The Pre-migration Migration - by Chip Taylor

About 15 years ago, before the inception of Monarch Watch, while I was collecting monarchs in an alfalfa field for an upcoming class project, I noticed a number of monarchs flying over the field in a southwesterly direction 2-4 meters above ground as if they were migrating. The date was mid August, about a month too early for the fall migration, and I dismissed the idea that these monarchs could be migrating or moving south. Subsequently, I have observed this behavior most years and it’s especially evident in years when the population numbers are high. I have never observed this behavior before 1 August nor after the 20th of the month. The actual window for these flights is probably from the 4th to the 18th. Colleagues and friends driving I-70 across Kansas have also reported seeing monarchs moving north to south during this period so I know I’m not imagining things. However, in spite of posting observations about this to Dplex-L, writing this up in the Season Summary, and mentioning these observations to other monarch biologists, confirming or refuting accounts have not come to my attention. Kansas can’t be the only place this is happening.

Monarchs begin to reappear in the southern states in late August and early September about a month before the true migration arrives. Whether this increase is due to reproduction by the few local monarchs that have survived the summer in the south or due to a flow of pre-migration monarchs from the north is not yet clear.

Based on my Kansas observation and using the 4-18 August window for Lawrence, KS (38:57N), if this behavior occurs over all latitudes, it should be possible to predict the window of such flight for each region and therefore the arrival time of pre-migration monarchs from the north across all latitudes.

Predicted windows for a pre-migration migration:

Latitude / Dates
45N / 1-29 July
40N / 29 July - 15 August
35N / 15-31 August
30N / 31 August - 13 September
25N / 13-26 September

I should point out the physical environment during these periods is nearly a mirror image of those encountered by the newly emerged first generation butterflies as they move north in May and early June. As to why they move south at this time, it may be that butterflies emerging at northern latitudes after a certain date are less likely to reproduce successfully if they stay in the north than if they leave the area. This is not a conscious decision by the butterflies of course but selection may have fine-tuned them to respond to the changing conditions in a manner that has the effect of maximizing their ability to replicate.


7) 2003 Membership/Tagging Kits

Monarch tags are going fast! The number of Tagging Membership orders we are processing has dramatically increased since the last update. To make sure you receive all of the tags that you’ll need for the upcoming tagging season you should place your order soon. We will begin mailing the kits the first week of August. 2003 Memberships will include 25 tags, the pre-migration newsletter, the 2003 Season Summary (mailed summer 2004) and one or two additional mailings. Orders may be placed online via Gulliver’s Gift Shop at

and offline orders may be called, faxed, or mailed to:
3515 Silverside Road, Suite 203
Wilmington, DE 19810
toll-free phone - (800) 780-9986
toll-free fax - (877) 687-4878

For your convenience, an abbreviated order form is available at


8) Tachinids: Monarch Parasitoids - by Chip Taylor

Shortly after writing the update last month (11 June), I began to discover 5th instar monarch larvae as I walked my golden retriever, Chugaah, down the hill to my milkweed restoration plot early in the morning. As we walked around the property each morning, usually from 5:45-6:45AM, I found a number of 4th and 5th instar larvae (mostly 5th) at the tops of four different species of milkweeds, A. viridis, A. sulivantii, A. viridiflora and A. syriaca. This was the first time I had found 5th instar larvae on A. sullivantii and all the larvae were found feeding on new tender leaves at the top of the plants. A. sullivantii is notable for the amount of latex it produces and copious amounts of this milky substance are known to inhibit feeding by monarch larvae. Previously, I had observed considerable egg laying by monarch females on sullivantii but the larvae seemed to abandon the plant. These plants were more mature than those observed this past month. Nevertheless, it was surprising to find these 5th instar larvae. On a whim, I decided to rear the larvae found on all plants to the adult stage. Two of the first three larvae I collected turned out to be parasitized by tachinid flies and I decided to rear all the 3-5th instar larvae I found on these walks to determine the rate of parasitism.

A few words about finding larvae and then I’ll get to the tachinids and the project.

In my experience, the best time to find monarch larvae, especially 5th instars, is during the first hour of light on a morning in the 60s following an evening in the 80s or 70s. In this region (eastern Kansas), monarch larvae frequently leave the plants during the day and "hide" in the vegetation near the base of the milkweeds. The larvae could be leaving the plants to avoid heat stress but they may also be avoiding insects such as paper-wasps, yellow jackets, tachinid flies and other insects that search vegetation for soft-bodied insects. On warm evenings the larvae resume feeding at the tops of the plants but as the temperature drops into the 60s toward morning the larvae become sluggish and, in effect, get stuck at the top of the plants where they are quite visible. Fortunately, for the larvae, the predatory and parasitic insects are also too cold to be active under these conditions. As the sun rises, the larvae quickly heat up and move down the plants. The trick to easy pickings is to get to the milkweed patch before this happens. The early bird gets the monarchs so to speak.

The two most frequently asked questions we receive at Monarch Watch are "Why are monarch larvae eating my dill, fennel, parsley, etc., when they are supposed to be eating milkweed? and "What are the white worm-like things that emerged from my J caterpillar, or chrysalis? Well, you know the answer to the first question - the larvae aren’t monarchs, they are those of the black swallowtail, Papilio polyxenes. It is often difficult to convince people of this fact but that’s another story. When we get the second question, we send out this standard reply:

"Your monarch larvae has been parasitized by tachinid flies, probably Lespesia archippivora. The female flies seek out and oviposit on the larvae. The tiny brown casings that you may also observe are fly puparia. The flies are smaller than house flies and are gray in color. The female flies are quite persistent and are difficult to keep out of cages made from netting. They may also have the ability to lay eggs on larvae through the netting e.g., when the larvae are crawling on the netting rather than the plants. Fortunately, unlike most Lepidoptera, the monarch has relatively few fly and wasp parasites to contend with. One reference indicates that 5 tachinid species have been reared from monarch larvae and pupae. In some locations the flies kill more than 90% of the larvae not found and harvested by spiders, wasps, earwigs, lady bugs (beetles), lacewings, ants, stink bugs and other predators."

People don’t argue with us about this reply but there is much more to tell and, of course, much more to learn about the relationship of tachinids and monarchs.

The term "tachinids" is short for a group of flies in the family Tachinidae. This is one of the most diverse groups of flies in North America and many are similar in appearance to house flies. However, a close look shows that they have more abundant and longer bristles and more robust antennae than house flies. They are also slower moving than house flies, especially when searching for hosts. Members of this group are parasitoids of soft-bodied insects, spiders and scorpions. Parasitoids are insects that lay eggs on or within other insects. The parasitoid larvae develop within the host usually feeding on "non-essential tissues" until they near maturity, when their increased feeding and growth kills the host. Lespesia archippivora, the most common tachinid reared from monarch larvae and pupae, is native to North America, occurring from British Colombia to Ontario and all of the United States and Mexico. It is known from Costa Rica and Brazil, suggesting it may be widespread in South America as well. It is now considered to be an important species for the control of a number of economically important insects. It is known to parasitize 60 species of Lepidoptera in 13 families. In one study, over 40% of the monarchs in Hawaii were parasitized by Lespesia archippivora; however, late in the season the percent of monarch larvae parasitized has exceeded 90% in our milkweed patches on West Campus. For several photos of L. archippivora see

Although 5 tachinid species have been reared from monarchs, and the number may be much higher. The impact of these parasitoids on the monarch population is not known. There are many complicating factors. The monarch population is distributed over the eastern half of the continent in a great variety of habitats and rates of parasitism may vary from region to region and from one field site to another over a relatively short distance due to such factors as the availability of alternative hosts, local climates and year-to-year variations in local conditions. It will take some time to derive a comprehensive picture of this relationship.

Here are my general impressions. The following are not facts, just observations, and a closer look at the relationships may show these impressions to be incorrect.

1) The rate of parasitism increases through the season and can vary substantially from year to year.
2) The rate of parasitism decreases with increasing latitude.
3) The diversity of tachinids attacking monarchs is higher at lower latitudes.
4) The availability of alternative hosts plays an important role in the level of parasitism on monarchs.
5) Parasitism increases with the density of the larval hosts and declines when larvae are hyper dispersed.

The current rate of emergence of tachinid parasites from monarchs in my area is quite low, 15.7% (see below), perhaps the lowest I’ve seen for this location. The explanation may be last summer’s drought and the dry winter. The drought last summer reduced virtually all Lepidoptera, leaving the tachinids with few hosts. The dry winter may have also reduced the survival of the overwintering puparia leaving relatively few adult flies to start this year’s population.

It is not clear how the parasitic flies locate the hosts. Odor from the caterpillar, perhaps from the frass (feces), may be a cue. The flies are seldom seen on plants without larvae. Once near larvae, the flies vigorously search the surface of the leaves for their quarry. Yet, they move slowly enough so that they can be captured if one is persistent. When a larva is located, the fly hesitates a second or two and then moves in to lay an egg, usually on the anterior third of the caterpillar Many tachinids lay incubated eggs on their prey. "Incubated" in this case means that the larva within the fly egg is well developed and hatches shortly after it is has been laid. This is the case in Lespesia archippivora. I have looked closely for eggs after witnessing oviposition by the fly but have been unable to find them. According to John Stireman of Iowa State University, the fly eggs are small and translucent. In one study, the larvae hatched within 20 minutes after the egg was laid. Evidently the larvae are quick to find a good point on the caterpillar to burrow in. In other tachinid species, e.g. Winthermia diversa from Australia (see photo p. 29 1998 Monarch Watch Season Summary), eggs - which are sometimes quite visible - are affixed to the outside of the cuticle and the hatching fly larva bores into the caterpillar. Monarch larvae that have been oviposited on by a fly drop from the plant and curl into a C shaped position and begin to turn the C inside out rolling over and over. This writhing can continue for several minutes and may be an attempt to "rub off" the egg or perhaps the fly larva. At some point the monarch larva stops rolling and climbs the plant and continues feeding. The flies evidently do not oviposit in first, second and early third instar larvae but direct their attention to larger larvae. They do not oviposit in pupae. The tachinid larvae usually reach maturity when the monarch larva is in the J stage prior to pupation. Late in the J stage, or during the first two days of pupal development, the fly larvae eat their way through the dorsum of the caterpillar, just behind the head, or through the wing pads in the case of the pupa/chrysalis and slide down long strings of silk or tissue eventually dropping to the ground. The fly larvae wiggle and writhe to get beneath the duff on the ground and then their cuticle begins to harden and tan forming the brown puparium within which the larva undergoes complete metamorphosis to become an adult fly in 2-3 weeks. L. archippivora usually overwinters in the larva or pupa of the host but this can’t be the case for monarchs since the larvae and pupae are not freeze tolerant and the population leaves the area 2-4 weeks before frost at most latitudes.

I’m going to try to record the rate of parasitism of monarch larvae for each month. Here is the record so far. The July record goes through the 12th although I haven’t found a larva since the 5th.

Rate of parasitism of 3-5th instar larvae collected in the wild, Berryton, KS

Date / Parasitized / Normal / Deformed / Dead-other causes* / Proportion Parasitized**

June ’03 / 3 / 10 / 1 / 7 / .214

July ’03 / 0 / 5 / 0 / 0 / .000

Total / 3 / 15 / 1 / 7 / .157

* Disease or pesticide – symptoms typical of non-inclusion virus or pesticide poisoning.

** Proportion of all J (pre-pupation) or pupal stage immatures from which tachinid larvae emerged.

My thanks to John Stireman of Iowa State University and Jim O’Hara of Agriculture and Agri-Food Canada in Ottawa for providing some of the information used in this account.


9) Upcoming Monarch Events

Please send us information on tagging events, butterfly festivals and other events associated with monarchs that are open to the public. The following are public events we are aware of to date. Additional information will be sent in next month's update and posted online when available.

15-17 August 9am-6pm daily - Powell Gardens Festival of Butterflies - Kingsville, MO - Monarch Watch will provide hands on programs - Festival admission applies: $7 adults; $6 seniors; $2.50 children ages 5-12. More info:

22-24 August 9am-6pm daily - Powell Gardens Festival of Butterflies - Kingsville, MO - Monarch Watch will provide hands on programs - Festival admission applies: $7 adults; $6 seniors; $2.50 children ages 5-12. More info:

6 September 8am-Noon - Monarch Watch Open House - Foley Hall, KU West Campus, Lawrence, KS - Come visit us in our new space!

13 September 7-11am - Monarch Tagging Event - Baker Wetlands, Lawrence, KS - Annual monarch tagging event with Jayhawk Audubon Club.

19 September Early PM - Monarch Watch in Central Park - Central Park (The castle), New York, NY - Monarch conservation and tagging activities.

18 October TBA - Grapevine Butterfly Festival - Grapevine, TX - Butterfly festival & monarch tagging


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