Hi Harshad,

If you know well the life cycle history of the species you study, it will help a lot to predict that transition phase. It means it is a specific species, cause there are so many known species of Lepidoptera. 

Just in the Indo-Malayan and Australian region, you have an estimated 47,286 known species of Lepidoptera. 

It is hard to say if they all behave the same way. As to your point that they would leave the host plant to enter pupal stage, that needs a serious verification. 

Larvae, usually feed on leaves spots and then move to others spots. They persist in that feeding behavior until they will enter into another instar stage; then they will stop feeding and it is possible to observe a total absence of movements as in total contrast of the feeding period.

That "wandering phase" is not only before the transition phase of passing to larval form to pupal form. It does exist at the end of each instar (I to II and etc). It might be less in duration compare to the passing of a last larval instar to pupation. Once you have identified that such and such larvae are entering the final days of the final instar, you can monitor more closely this event.

The larvae will stop moving, having already chosen a pupation site. It stays just there until the final metamorphosis to become an adult.  

You need to well know the different morphology, the size of the larval instar, particularly the last one.  You need to be able to identify this either in the laboratory or the field. 

Notice that life cycle history period will have some variations between lab rearing and field condition. 

This is necessary for precision and time-saving. If you assist to an outbreak of those defoliators, you will multiply the chance of seeing them entering into that "wandering phase" just before pupation. 

Some tips: 

To see that a larvae is not moving, does not mean they are in that phase. A simple way to check this is by touching them: they will never change their position if they are already well-entering pupation; else at the opposite, they would move. 

This needs time and patience, cause it is always interesting to assist to a phenomenon several times, not only once. 

Pierre (Martinique Island)

Thanks Mr. Exelis. That is very helpful. I do have an idea of the larval in star and its corresponding size for the species I am studying. Due to this I have an idea when they are likely to wander off. You are right in the observation that larvae remain relatively unperturbed once they have finalized the pupation site.

 Thanks Mr. Monyer for the reference. However I could not access the book. Is the link working ?

Mr Exélis Moïse Pierre answered well, you can follow it. Dear friend, I observed that the larvae of silkworm prefer to pupate during night hours rarely/hardly it pupate during day time. Similarly, the adult emergence is also occurs during night hours only.

Thanks Mr. Lalit Kumar.  

On top of that, larvae foraging and feeding behavior may increase dramatically before any molting stage to sustain the much higher demand in energy cost during that "wandering' period: energy sufficient stock. However, during the hot period of the day, this does happen as well.

Usually, insect entering into the ecdysis time as mentioned by Mr. Lalitkumar V Ghetiya does not occur during daytime.  Some Lepidopteran species manage to adopt a camouflage strategy by efficient mimicry with leaves color &  to blend with the environment during pupal stage deemed as vulnerable: detection avoidance from enemies. 

Before attaining the pupal stage, it is often preceded of the "pre-pupa" corresponding effectively to a dormant larval instar.; you called that "the beginning of wandering stage". 

Which species of Lepidoptera are you studying ? Perhaps If you give us the scientific name of these lepidoptera  species and the locality and climatic conditions ( temperature, relative humidity, altitud, rainfall ) we can help you.

Thanks Mr. Luis. The species is Mycalesis mineus, commonly called Dark branded bush-brown (western coast of India: tropical climate, temperature of around 27 degrees and humidity around 70-80%. 

Hello Harshad:  

Thanks for the information. About the Dark brand bush brown  satyrid Mycalesis mineus, the life cycle was studied by Horace Tan (2014). He points out that  toward the end of the 5th instar for both colour forms of larvae, the body gradually shrinks in length and turn mostly yellowish green. Typically, the caterpillar will seek out a spot on the underside of a leaf blade to spin a silk pad. It then anchors itself there via its anal end, and assumes its upside-down pre-pupatory pose. After one day as a pre-pupa, pupation takes place. The smooth pupa is mostly green throughout. It is somewhat angular in appearance, with a dorsal keel on the thorax and ridges defining the dorsal wing margins. There are a few small black spots dotting the wing pad margin. Dorso-lateral pairs of small whitish spots can be observed on abdominal segments 2-6.  Check out the excellent photos in this link

http://butterflycircle.blogspot.com.co/2014/12/life-history-of-dark-brand-bush-brown.html

Regards,

Luis Miguel

Thanks Mr. Luis for being prompt in providing a link. The snaps of larval instars are indeed clear.  My observations in this regard match the description in the blog.

However, the life cycle of many insects species could be influenced and witness variations upon environmental conditions fluctuations and food source changes (Norman Kamarudin et al., 2004; Cheong et al., 2010 & Gibbs et al., 2006).

The differences can go as much as 5 to 10 days depending on laboratory or field rearing conditions. 

Pierre (Martinique Island)

When the larva is fully grown, hormones such as prothoracicotropic hormone (PTTH) are produced. At this point the larva stops feeding, and begins "wandering"  for a suitable pupation site, often the underside of a leaf or other concealed location.So, detection of the above mentioned PTTH hormone is a good way to determine the wandering stage.

You might be able to determine the answer by rearing and sacrificing caterpillars for dissection (or cross sections?) Whereas the other instars of lepidoptera form another similar caterpillar within the old exuvae, the last instar is actually starting the pupation process and forming the beginning of the pupae under the exuvae. You might be able to correlate the stage of this process (perhaps the degree of breakdown of the layers) with the behavioral changed that leads to wandering.

I am sure that you are aware that unusually extreme wandering may indicate that a parasite is nearing pupation inside the host rather than the host itself. 

 Thanks Mr. Exelis for all the references. Dr. Sitli for the suggestion.  Mr. Lewis I have not tried the dissections so far. Thanks for the suggestion about the parasitic attack.  I was only aware that parasitic attacks could cause wandering of larvae to off plant substrates for pupation. However I do not much about he parasitic development leading to extreme wandering. 

Exélis Moïse Pierre, do you have sources for your initial answer where you say ""wandering phase" is not only before the transition phase of passing to larval form to pupal form. It does exist at the end of each instar (I to II and etc)"??

Thank you,

Kelsey

Kelsey Fisher, My comments are solely based on my personal field & laboratory insect rearing work. So it would be like "Unpublished data" or personal observation. Yes it does exist at the end of each instar. The larvae would pinned itself hard to the leaf surface in total immobility for a lapse of time depending on the instar change. It seems that it takes more time with the bigger size instar. Once will not be confused with the suction activity of defoliators since they would not stay on the same spot but rather move on cm by cm.

So once the larvae stayed idle, that is the phase that i referred to upon numerous observations and testing more in the field than lab. To enforce my assumption I did remove by force the larvae (bagworms) and it did not move either (lethargic state). I can not assume it is similar for all species. I did that testing on indigenous bagworms species (Pteroma pendula-Metisa plana) in Malaysia preying on oil palm leaves.

Welcome

Pierre @ Ali

Thanks! Just wanted to know if I could read more about it - I did some lit searches and nothing came up. Cool observation.