Bag Worm Moth

Very small things amuse a man of my age. A hummingbird, the extraordinary way a strellitzia bloom opens. Why at a certain point in its growth does the bud turn at right angles? Why does the bud split so gradually and why do the flowers emerge always in a certain exact order, so that the sharp unopened end of the bud looks like a bird’s beak and the blue and orange petals make a bird of paradise? What strange deity made such a complicated world when presumably he could have made a simple one?

My mature bird of paradise plants were disfigured. The damage was holes in the leaves. The flowers were not affected.

The holes had become familiar, I had become use to living with them. Noticeably, on the surrounding scrumptious ti, ginger, and gardenia, no holes.

Bird of paradise (Strelitzia reginae) is a tough plant. From experience I knew that it takes a pretty big creature to cause it any damage, such as caterpillar, snail, grasshopper, or goat, and this damage usually is a missing hunk from the edge of the leaf. But in the case of my Birds there were multiple holes in the center of the leaf.

The one thing I knew for sure was that I wasn’t going to spray or drench with insecticide. There was very effective biological control going on. Overall, without the help of any insecticide, the plants were super clean: no “white fly,” mealybug, or scale – just holes.

If I chose the option of treating the plants with chemicals, I was sure to kill the beneficial predators which were keeping the plants healthy if not hole proof.

The holes really weren’t a critical problem, the plants were generally doing fine. I really didn’t care much about the appearance, because there were plenty of undamaged flowers.

Suddenly, just in passing, I noticed this ugly 3/8-inch bag hanging from a thread below a hole in a leaf. Looking further I saw more ugly bags, each associated with its own hole.


Bagworm Husk hanging from a Bird of Paradise leaf

By “ugly” I mean the bag is scary ugly! “Are those poisonous spines that shaggy thing is covered with” was my last thought just as I pinched one between my thumb and forefinger. It went squish. There had been something alive inside, now gone with a smear of green ooze. The fingers were still good so I continued manually picking and squishing until the plants were free of cases. Many were dry husks.

Bagworm Husk hanging from a Bird of Paradise leaf

Bagworm Moth Pupil Case off of White Pineapple Plant, no holes

I decided this survival strategy was marvelous:

  • If the bag is noticed nothing would want to eat it because it’s so dangerous looking.
  • Even though it's nasty looking, the bag covering is effective camouflage greatly reducing the animal’s visibility.
  • The bag hangs off the bottom of leaves, which keeps it out of plain sight of birds.

After eradicating the bags I had no specimens to photograph to help identify what they were. But wait, there’s one on that five gallon bucket. And wait, there’s one the sliding door glass, on the Puakenekene (no holes), on the White Pineapple (no holes) -- a veritable stampede of the little things everywhere, but no apparent damage except to the Strellitzia.

Norman Nagata, the Maui County Extension Agent immediately recognized them from a snapshot: they were the caterpillar/larvae of the Bag Worm Moth (Brachycyttarus griseus de Joannis, 1929).

A little research revealed that these pests are relatively fresh off the boat, arriving most likely from Vietnam or Guam and first identified in Hawaii on the windward side of Oahu at Haiku in 1984.

Despite their presence on damaged birds of paradise the larvae are actually grass eaters that sometimes cause noticeable damage to lawns as they drag the larval case (bag) behind them before they ascend to some place, attaching themselves by a thread and pupate.

The case gets decorated somehow in a noticeable pattern, not random, with pieces of dry grass. Since it’s hard to believe that this accidently happens from being dragged, I wondered if the caterpillar could squeeze out of the case far enough to decorate its own case or if it gets another bagworm pal to help as in, "decorate my bag and I'll do your ironing".

Only the male escapes the case and turns into a moth. Tragically, the female is locked in the case and somehow attracts a male, is fertilized, lays eggs, then dies.

Now I knew all about the bagworm moth except for the answer to the main questions. If they are grass eaters, why the holes in the Bird of Paradise? Is the damage a bagworm's graffiti tag? Or is bird of paradise a bagworm salad?

To support the graffiti theory:

This bagworm is abundant in lawns, but is not considered to be a significant pest. However, it also feeds on various ornamental plants such as canna lilies and anthuriums, where it scrapes holes in the leaves. The caterpillars pupate in exposed positions and hundreds of bags are often seen hanging on house walls.

Using the word “scrapes” makes this activity seem more like vandalism rather than grazing. Again, why climb a wall? Why climb a five gallon bucket? There isn't much food for an insect up there. Why climb a pineapple and not eat, if that's what they are doing on the bird of paradise, eating holes?

Exploring the topic further: What is included in the group of plants beyond grasses that suffer from bag worm damage, besides bird of paradise, canna lilies and anthuriums?

This is a new pest to Hawaii, and the host range is not well known. The larvae have been recorded feeding on bermudagrass, Hilograss (Paspalum conjugatum), foxtail (Setaria spp.), Natal redtop (Rhynchelytrum repens (Wild.) C. E. Hubb.), Desmodium sp., sensitive plant, (Mimosa pudica L.), green kyllinga (Kyllinga brevifolia Rottb.), and purple nutsedge (Cyperus rotundus L.). It has also been able to complete its life cycle on Citrus spp., Nishiki juniper, lima beans and Spanish needle (Bidens pilosa L.).

Mitchell's description leaves unexplained how the animal can "complete its life cycle" on a 5-gallon plastic bucket or wall. The failure to report what kind of damage it does to Citrus spp. is also confusing. Do the bagworms make holes in Citrus or do they just use the plant for a platform to disperse eggs?

Auntie Jeanne's entertaining and informative article "Just Hanging Out - Bagworm Moth" mentions basil and lemongrass as being damaged by the bagworm and the commenters claim that, besides grass, ohia, Queen Emma rose bush and iris are at least scraped if not eaten.

Why does the bag worm moth larvae climb stuff it can't scrape? Why doesn't it scrape everything it climbs that is scrapeable? And does it eat as well as scrape? All unanswered questions.

So my solution to protect my Birds is to hire a security guard.

Philip Marlowe, owner of Gold Dust Day Gecko Security Systems Inc.
on Bag Worm patrol

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