Evidence of Gypsy Moth Population

In March 2003 a live adult gypsy moth was caught in an early warning trap in Hamilton, a major entry point for international air freight. No more moths have been caught since then but experts believe it was unlikely to have been alone. MAF’s experts agree that there is overwhelming evidence that:

• The trapped moth entered the country in an egg mass.
• The egg mass was laid in Japan between late May and early September and hatched here in January.
• Other moths hatched, mated, and laid eggs.
• The moth population is likely to be near where the moth was trapped.
• A gypsy moth population is capable of establishing and expanding from a single egg mass.
• The absence of another moth catch does not mean the trapped moth was alone.

The trapped moth entered the country in an egg mass

MAF has established a long history of intercepting gypsy moth life stages on used vehicle imports, ships, containers and even passenger belongings.

Data on used vehicle imports from Japan for the years 2000 to 2002 record 428 interceptions of Lymantriid life stages. From August 2000 to June 2002 a sample of 152 Lymantriid interceptions were identified using DNA techniques at Lincoln University. Of these 116 were egg masses, 16 larvae, 18 pupae and 2.

A total of 137 samples were identified to species level and 124 of these were gypsy moth (107 egg masses, 9 larvae, 7 pupae and one adult). This data shows that we are more than 15 times more likely to intercept egg masses than pupae and 11 times more likely to intercept eggs masses than larvae.

MAF Quarantine Service has intercepted 66 viable egg masses and 57 egg masses for which viability was not determined in last two years. In contrast, they have not intercepted a single pupae or larvae during that same period.

In the vicinity of the moth trapped in Hamilton there are a number of imported car yards and industrial sites processing goods and receiving sea freight containers imported from Japan. These risk items (imported vehicles and imported goods and sea freight containers) have a history gypsy moth border interceptions.

Diagnostic work carried out using morphological characters and DNA analysis of the gypsy moth caught in Hamilton has confirmed that it is Asian strain and its most likely origin is.

Gypsy moth egg masses are a resilient, easily transported life stage often affixed to containers, vehicles and ships. Since the egg stage lasts almost nine months it provides an ideal pathway for long distance dispersal. The egg masses are often deposited by the female on inanimate objects and under structures or in crevices making them very difficult to detect during inspection. Gypsy moth females of Asian strain are attracted to lights which increases a chance of eggs being laid on the imported cars, containers, ships and other inanimate objects awaiting loading in port in Japan. While every effort is made to intercept the gypsy moth, no inspection guarantees 100% total detection of life stages on vehicles, containers and other goods.

Summary

1. There is a history of gypsy moth interceptions on used vehicles, ships and containers from Japan.
2. Egg masses are 15 times more likely to be intercepted than pupae.
3. Egg masses are 11 times more likely to be intercepted than larvae.
4. Imported risk goods known capable of being contaminated with gypsy moth lifestages are entering the Hamilton area.
5. Japan is the most likely origin of the gypsy moth caught at Hamilton and that pupal stage of GM is not present in Japan until mid April at the earliest.
6. The egg stage lasts about nine months providing a pathway for long distance dispersal.
7. Viable life stages are able to enter New Zealand despite border protection measures.

The egg mass was laid in Japan between late May and early September and hatched here in January

The life span of gypsy moth adults (moth) is 10 to 14 days and adults are present in summer through early autumn (May to September in Japan). Flight times of gypsy moths in Japan are:

• Okinawa - mid to late May.
• Fukokua/Kyushu - mid June.
• Tokyo/Yokohama - late June.
• Morioka/Sendai - early July.
• Sapporo/Hokkaido - late July to late Aug.

The developmental period of gypsy moth pupa is up to 2 weeks, and in the wild pupae are present in late spring and summer.

The developmental period of gypsy moth larvae is 8 to 10 weeks and larvae are present in spring and early summer.

The developmental period of gypsy moth egg stage is about 9 months, including diapause and incubation period. Eggs are laid in summer and autumn and hatch in spring. Flight data of gypsy moth from Japan indicate that egg masses would be laid from mid May (southernmost part of Japan) to late August and early September (northernmost part of Japan). An egg mass most commonly contains 500 to 1000 eggs covered in female scales.

If the moth caught in Hamilton entered New Zealand as an adult moth it would have had to travel from Japan and reach the trap within two weeks. In March, when the moth would have had to leave Japan it is late winter and early spring and there are no adult moths present. The moth caught in Hamilton was still alive and in very good condition indicating very short flight before being caught in trap. It is therefore highly unlikely the catch represents the entry of an adult male moth from Japan.

The flight data from Japan indicates that pupae (lasting 10 – 14 days) could have been present in Japan in mid April at earliest (in southernmost part of the range), well after the NZ trap catch on 26 March in Hamilton. It is therefore highly unlikely that trap catch in Hamilton was a result of a pupa arriving from Japan.

If we assume the moth caught entered as a larva (caterpillar) it would have had to come from Japan in December to March. There are no larval stages present in Japan from December to February. However, there is a possibility that some early instar larvae could be present in Japan in March. If we assume this stage as the possible entry to NZ, these larvae will not be able to develop to adult stage by March under the weather conditions present in Hamilton or elsewhere in New Zealand. It is therefore highly unlikely the catch resulted from the entry of gypsy moth larvae from Japan.

It is most likely that an egg mass arrived in Hamilton from Japan around December 2002 to January 2003 resulting in a male caught in late March 2003. Such an egg mass would have all preconditions for development satisfied as it would have been exposed to a period of low temperatures (vernalisation) in Japan before being shipped to NZ and exposed to higher temperatures that triggered egg development.

There is a remote possibility that the moth originated from within New Zealand and arrived in Hamilton as an egg mass, larvae or pupa from somewhere else in New Zealand. MAF operates a national trapping programme for gypsy moth (the original Hamilton trap is a part of that grid) and Hamilton was the only location where a moth has been captured. Also, MAF operates a number of other surveillance programmes and relies on public to report suspect exotic organisms like gypsy moth. No gypsy moths have been reported elsewhere. It is therefore unlikely that there is a well established population of gypsy moth outside of the Hamilton infestation.

Summary

1. No adult moth present in Japan in March 2003.
2. No pupae present in Japan in February and early March 2003.
3. Only very early instar larvae potentially present in Japan in early March.
4. Egg masses laid in Japan from late May to early September.
5. Based on Hamilton temperature data, trapped male moth would have resulted from and egg mass hatched in early/late January 2003, arriving to New Zealand 4-6 weeks beforehand.

Other moths hatched, mated, and laid eggs

Given that it is most probable the male moth captured came from an egg mass and given that in an egg mass contains 500-1000 eggs, it is reasonable to expect that number of eggs also would have hatched (very high hatch rates are recorded in vivo).

First and second instar larvae of the Asian strain are capable of "ballooning" (catching the wind on silken threads) as a mean of dispersal and way to reach the food source if eggs were laid on an inanimate object (e.g. car, container etc.).

Under Hamilton climatic conditions and in the absence of natural predators and parasitoids quite a number of larvae could have developed to adulthood in an area close to the original egg mass. It is likely that there were other surviving male and female moths from the same egg mass. It is likely that they could have mated and the females laid new egg-masses. Based on literature, in low density populations more females will be developing from one egg mass so one male caught in the trap may be representing a building population.

Summary

1. Egg mass contains 500-1000 eggs.
2. Early instar larvae disperse in search of suitable food sources.
3. Most larvae disperse short distance.
4. Absence of natural predators and parasitoids in New Zealand indicating potential for high survival rate of larvae to adulthood.
5. Very likely that other male and female moths hatched and mated.

The moth population is likely to be near where the moth was trapped

The captured moth was in good condition when found, indicating that it had flown only a short distance, probably originating from host trees in close proximity to the trap.

Dispersal in the first year is usually within several hundred meters from the egg mass (most ballooning larvae do not disperse further than 800m although much longer distances have been recorded). There are quite a number of favourable hosts in the vicinity of the trap that caught the moth.

Although capable of long distance dispersal, it is known that the male gypsy moth seldom flies more than 800 metres.

Summary

1. Trapped moth was in good condition indicating it hadn’t flown far.
2. Normal flight and ballooning distances indicate strong likelihood that the moth originated in close proximity of the trap.

A gypsy moth population is capable of establishing and expanding from a single egg mass

A model that has been developed using trap catch data from the European strain of gypsy moth in North America (Liebhold and Bascompte 2003; Leibhold 2003 personal communication – report to MAF) suggests that even based on one moth trap catch there is a significant chance of trap catches (ie. a persisting, surviving population) in the second year in the absence of treatment (about 17%). The Asian strain of gypsy moth, that was caught in Hamilton, has significantly higher reproductive potential and dispersal capability than European strain. This, combined with the absence of natural predators and parasitoids in New Zealand is expected to result in significantly higher survival rate of gypsy moth in New Zealand than in USA.

Summary

1. Even based on one male moth trapped there is a significant likelihood of population of gypsy moth surviving in second year (European strain, USDA data) without treatment.
2. Survival rates higher than in USA are very likely under Hamilton conditions.

The absence of another moth catch does not mean the trapped moth was alone

The male moth in Hamilton was caught very late in the season (compared to the similar latitude overseas) indicating the very end of the flight season. Additional traps to form a dense trapping grid were deployed a week and more after the trap catch. It is likely these were deployed after the end of flight activity.

Data from literature indicate trap efficacy of 1-4% at trap densities applied in the Hamilton early warning gypsy moth grid (800 metres and more apart) at the time of the capture (MAF internal report 2003). There were a total of 17 traps in Hamilton. With this low trap density we could at best only expect to catch only four out of every hundred moths present in the area.

At low population levels gypsy moth is very difficult to detect due to the relatively low trap efficacy at the densities applied in Hamilton. The purpose of the national trapping grid is detection of a gypsy moth population early enough to be able to attempt eradication, rather than to trap high numbers of moth. Much higher densities of traps are needed to achieve higher levels of trapping grid efficacy (80 metres spacing for up to 18% efficacy).

Summary

1. The Hamilton moth was caught very late in the season.
2. The dense trapping grid deployed a week and more after the trap catch was likely after the end of flight activity.
3. The national trapping grid has low trapping efficacy due to the relatively low density of traps.
4. Even at high trap densities trap efficacy is quite low.

Conclusion

All of the above provides significant evidence of a potential established population of hundreds of individuals. MAF considers this to be "overwhelming evidence" of a population and of the need to take eradication action.

The advisory group of experts MAF consulted agree that one male gypsy moth in Hamilton presents a significant risk of a population establishing and sufficient reason to require prompt and decisive action to prevent further spread and establishment of the gypsy moth over a wider area.

Page last updated: 7 August 2008