Mesquite Biological Control: Leaf-tier, Evippe sp. #1

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Summary | Taxonomy and Origin | Lifecycle | Host Specificity | Mass-rearing and Release | Establishment, Damage and Impact | Current Research | References

Summary
Evippe is from Argentina and was released by CSIRO and collaborators throughout Australia from 1998 to 2000. It has established in all regions and is having dramatic impacts in the Pilbara region in Western Australia. Research is currently underway to better quantify impacts there, but available evidence suggests a dramatic reduction in growth rates and seed production.

Taxonomy and origin
The genus Evippe has 24 described species. Evippe sp. #1 is an undescribed leaf-tying moth from north-west Argentina that is most closely related to E. omphalopa. It was selected as a potential biological control agent because gelechiid moths had been observed to cause widespread defoliation of mesquite in Argentina, and because it was relatively easily reared. Evippe sp. #1 has only ever been recorded from central-northern Argentina, where it has been bred from three species of Prosopis. It is the most abundant leaf tier to be bred from Prosopis samples that CSIRO obtained.

Lifecycle
Adults do not need to feed and are probably nocturnal. Females lack a sclerotised ovipositor and most eggs are therefore pushed into existing cracks and spaces in the plant, predominantly under bark, but occasionally into old leaf-mines, under vacated lerps, in immature foliage, etc. Females will also glue egg batches onto plant foliage, typically onto mature pinnules. They may also be oviposited into gauze and other inorganic material (at least in the laboratory), but this is rare, even in the absence of any plant material.

Eggs hatch 8-10 days after oviposition. The first instars mine mature pinnules, leaving a webbed entry hole, which is also used by larvae to rid the mine of frass. Leaf mines appear externally as a pale blister. Larvae emerge from their leaf mines within 3-4 days and make leaf ties (opposing and adjacent pinnules tightly webbed together to form a capsule) in which the larvae feed. New leaf ties are made when food runs out and for pupation. Both leaf mines and leaf ties are occupied by single larva or pupa. In Australia the last instar larvae enter diapause in late April or early May, exiting in July.

Adults emerge from about 35 days after oviposition (at ca 27oC, 60% RH). In the laboratory they lived for up to 20 days, with 50% still alive after 9-13 days. Oviposition begins within a day of emergence and continued through most of the life of the female.

Host-specificity
Host-specificity testing by CSIRO from 1996-98 showed that Evippe sp. #1 is restricted in host-range to members of the genus Prosopis. Oviposition may occur on non-targets under laboratory conditions, but probably results from cages disrupting oviposition and dispersal behaviours. Limited leaf mining and preliminary leaf tying did occur on the introduced fodder tree Leucaena leucocephala, but all larvae died within four weeks of oviposition. No larval feeding occurred on the other 58 plant species tested. In its native range Evippe sp. #1 has only been recorded from Prosopis species.

Mass-rearing and release
Evippe is relatively easy to mass-rear and was reared in a similar way to Prosopidopsylla flava. Approximately 40-200 adults were placed into gauze cages together three to six potted mesquite plants (150 mm diameter pots or equivalent). Additional plants were added as required. Evippe was mostly shipped as late instar larvae because adults were relatively fragile and short-lived. Boxes with leaf-ties were placed directly in the field, generally within 48 hours of harvesting.

Releases were made on mesquite in six regions throughout northern Australia. Approximately 62 000 individuals were released between March 1998 and February 2000 with an average release size of approximately 1100.

Establishment, damage and impact
Evippe has established in all regions, with the possible exception of north-western New South Wales. Its abundance is correlated with temperature. The moth is most abundant in the hottest region (Pilbara), least in the coolest region (north-west New South Wales and south-west Queensland), and intermediate elsewhere (north Queensland, and Barkly Tablelands). Whether this is just a function of shorter generation times in hotter regions is not known.

High levels of prolonged defoliation in the Pilbara is almost certainly having a major impact on mesquite populations there, through reduced growth rates, and dramatically reduced seed production. However, the only plant deaths observed there are in parts of the infestation where drought-related salt intrusion was also occurring and is therefore likely to be the result of an interaction between salt intrusion and prolonged defoliation. Ongoing research in the Pilbara is better quantifying damage and impact there (see below). Impact has not been observed elsewhere, but is likely to be negligible unless leaf-tier abundance increases dramatically and becomes more prolonged.

Evippe has attracted a relatively diverse parasitoid fauna, although it is surprisingly similar throughout Australia. However parasitism rates are very low, at least within three years of releases, and almost certainly have no significant impact on the moth populations. A species of Brachymeria (Chalcididae) occurs in all regions and is by far the most abundant. Further surveys will be required into the future to determine whether parasitism rates increase to a point where it might threaten impact.

Current research
Research is currently underway in the Pilbara region to quantify the impact the leaf-tier is having on mesquite populations. Survey work has been ongoing since 1998 to quantify season level of defoliation and reproduction through the years. Defoliation has been very high and prolonged since 1999 and adult plants have had an average of less then 6 pods/tree since reproduction was first recorded in 2000. Seed banks are also very low, although no seed bank data is available from prior to the leaf-tiers release and therefore we cannot be absolutely sure that this is a direct result of biological control.

An insect exclusion trial has been underway since 2003 to better quantify the impact of the moth, by comparing plant growth and reproduction under natural conditions with and without the leaf-tier.

References

1. van Klinken, R.D. and Heard, T.A. (2000). Estimating fundamental host range: a host-specificity study of a biocontrol agent for Prosopis species (Leguminosae). Biocontrol Science and Technology 10, 331-342.

2. van Klinken, R.D., Fichera, G. and Cordo, H. (2003). Targeting biological control across diverse landscapes: the release, establishment and early success of two insects on mesquite (Prosopis) in rangeland Australia. Biological Control 26, 8-20.

3. van Klinken, R.D. (subm.). Semi-concealed Lepidopteran biological control agents may not be at risk from parasitism in rangeland Australia: evidence from a gelechiid leaf-tier on mesquite (Mimosaceae: Prosopis).

Summary | Taxonomy and Origin | Lifecycle | Host Specificity | Mass-rearing and Release | Establishment, Damage and Impact | Current Research | References