Egg parasitoids of heliothis
On this page:
- Trichogramma and Telenomus wasps are important biological control agents
- What are parasitoids?
- Egg parasitoids
- Detecting egg parasitoids
- Normal and parasitised heliothis eggs
- Releasing Trichogramma wasps
- Conserving Trichogramma
- Nursery crops
- Monitoring levels of egg parasitism
- Tools for collecting eggs and assessing the levels of egg parasitism
- Looking for trends in crop scout data
- Identifying egg parasitoids
Trichogramma and Telenomus wasps are important biological control agents
Heliothis are serious pests of a range of crops in Australia. There are two pest species: the native budworm, Helicoverpa punctigera, and the cotton bollworm or corn earworm, Helicoverpa armigera. Both species are commonly called heliothis. Helicoverpa armigera has developed resistance to a range of chemical insecticides and can be difficult to control.
There are a number of beneficial insects and spiders that attack heliothis. It is important to recognise these beneficials so that they can be conserved. Parasitoids are very important beneficials and can cause high mortality of heliothis, particularly in crops that have not been sprayed with broad-spectrum insecticides.
What are parasitoids?
Parasitoids are specialised forms of parasites that kill their hosts. This is in contrast to true parasites, e.g. fleas or ticks, that do not usually kill their hosts. Parasitoids are very important biological control agents. The most common parasitoids of heliothis are species of wasps and flies.
Parasitoids are usually described according to the life stage of the host that they attack. The tiny Trichogramma wasp is called an egg parasitoid because it attacks the egg stage, and Microplitis wasps are called larval parasitoids because they attack the larval stage. Both of these wasps can significantly reduce the numbers of heliothis larvae that develop in crops.
Egg parasitoids
The Trichogramma wasp is very small, about 0.5 mm long, and difficult to see in crops. The female Trichogramma wasp lays her eggs inside a heliothis egg. The wasp larvae feed on the developing heliothis larva, killing it before it hatches. Trichogramma are brown or yellow wasps with red eyes. Another egg parasitoid called Telenomus also attacks heliothis eggs. Telenomus is a very small black wasp with black eyes. Usually two to four Trichogramma or one Telenomus develop inside a heliothis egg. Egg parasitoids are active during daylight hours, and prefer fine days to rainy days.
Detecting egg parasitoids
Egg parasitoids can be detected by examining the colour of heliothis eggs. Eggs that have been attacked by Trichogramma turn a distinctive jet black colour 3-4 days after being attacked. The presence of a lot of black eggs in your crop indicates activity by Trichogramma. It is important not to confuse black-parasitised eggs with black-head eggs. Black-head eggs are three day old eggs where the black head capsule of the developing larva is visible through the clear egg shell. This gives the egg a black appearance to the naked eye. By contrast, black-parasitised eggs have a jet black egg shell that is not transparent. Black parasitised eggs will not produce caterpillars, but will eventually produce one or more wasps.
Normal and parasitised heliothis eggs
A) Normal one day old eggs. These eggs are pearly white in colour
B) Normal two day old eggs. These eggs are called brown, coloured or 'red-ring'.
C) Normal three day old eggs. This is the black-head stage of egg development. the black head capsule of the developing larva is visible through the clear egg shell.
D) Three day old parasitised eggs. These eggs are typically fawn or brown.
C) Four day old parasitised eggs. these eggs have a black egg shell.
Releasing Trichogramma wasps
Trichogramma can be purchased and released into crops to kill heliothis eggs. They are available from 'Bugs for Bugs', Mundubbera Qld., and the company should be contacted with respect to costs and recommended release rates. The Department of Primary Industries has been releasing Trichogramma pretiosum in various cropping regions of Queensland over the past five years. This wasp is common in Kununurra (WA). It is now established in some parts of Queensland (e.g. the Lockyer Valley) where it is an effective natural biocontrol agent of heliothis.
Conserving Trichogramma
Farmers can encourage natural populations of Trichogramma to build up in their crops by avoiding the use of broad-spectrum chemical insecticides such as synthetic pyrethroids and organophosphates. There are 11 species of Trichogramma reported from Queensland. High natural levels of heliothis egg parasitism due to these wasps are common in some crops. One way to conserve Trichogramma is to use selective biopesticides, such as DipelŪ and GemstarŪ. Most chemical insecticides are very toxic to Trichogramma.
Nursery crops
Some unsprayed crops act as nurseries for Trichogramma and other natural enemies of heliothis. Sorghum and maize are particularly good sources of Trichogramma wasps, and they can be conserved by using GemstarŪ against heliothis larvae. Trichogramma wasps are rarely active in chickpea or pigeonpea. Consequently, chickpea or pigeonpea trap crops do NOT act as nurseries for Trichogramma.
If you are interested in planting a nursery crop for these parasitoids consider using a mixed hybrid or sequential planting of sorghum so that it flowers for a longer period of time (heliothis prefer to lay in pre-flowering sorghum heads).
Monitoring levels of egg parasitism
The best way to monitor egg parasitism is to collect eggs and hold them at room temperature until they either hatch or turn black. It is best to collect brown eggs (eggs that are about two days old). Collecting white eggs gives an under-estimate of egg parasitism because they may have just been laid and not had sufficient time to be found by Trichogramma. Conversely, collecting black eggs over-estimates parasitism because all eggs that have been parasitised eventually turn jet black.
Brown eggs can be collected using a leaf punch, a very fine paint brush dipped in water, or by using fine forceps to collect plant structures containing eggs (e.g. maize silks). Each brown egg should be placed into a cell in a plastic microtitre tray, and the tray covered with strips of wide sticky tape. This prevents larvae that hatch from unparasitised eggs moving into nearby cells and eating eggs. Each tray should be labelled with the date, collection locality, and crop description. It is important to keep the eggs cool in an esky when transferring them from the field to the office or laboratory because exposure to high temperatures, e.g. in a car glovebox, or on a dash board, may kill the eggs.
The level of egg parasitism can be determined by counting the number of eggs that turn jet black and dividing it by the total number of eggs collected. For example, if you collect 100 eggs and 50 turn jet black, then the level of egg parasitism is 0.5 or 50%. Trichogramma wasps, or other egg parasitoids, will eventually emerge from the jet black eggs.
Tools for collecting eggs and assessing the levels of egg parasitism
A leaf punch is used to cut out a small circular piece of leaf around an egg. Each leaf disc and egg is individually placed into a well in a plastic mictotitre tray, with each tray capable of holding 96 leaf discs. The tray is then covered with sticky tape or glad wrap to prevent hatching larvae from moving to adjacent cells and eating eggs.
Looking for trends in crop scout data
Crop scout data may also indicate that egg parasitoids are active in your crop. It is important to consider the previous scouting data when making a spray decision-not just the current set of counts. Consecutive sets of data may show a trend. For example the presence of a lot of brown eggs followed by a lot of black eggs and very few larvae suggests that egg parasitoids are active. Ideally brown eggs should be collected and held to record percentage egg parasitism. If this is not possible and egg parasitoids are thought to be active, then the crop should be inspected more regularly for black eggs or very small larvae. If there are a lot of black eggs and very few larvae, then Trichogramma are active. Avoid spraying or use a biopesticide so that the egg parasitoids are not significantly disrupted.
Once you start spraying with broad-spectrum chemicals you are generally 'locked-in' to that method of pest management because you will kill most of your parasitoids and predators. Some insecticides, like DipelŪ and GemstarŪ, will not harm Trichogramma.
Egg parasitoids are beneficial insects to have in crops because they kill pests before they hatch. As selective insecticides gradually replace the older, broad-spectrum products, egg parasitoids are likely to become more abundant throughout agricultural regions. Every effort should be made to conserve them and to incorporate them into integrated pest management programs.
The life cycle, from egg to adult, of a Trichogramma wasp normally takes 10 days. Every ten days a new generation of Trichogramma wasps will emerge and start searching for heliothis eggs.
Adult Trichogramma wasps rarely live for more than a week.
Do NOT spray the sorghum with chemical insecticides or you will kill the wasps.
GemstarŪ will not harm Trichogramma and should be used against heliothis larvae if necessary.
Identifying egg parasitoids
The identification of species of egg parasitoids is difficult, and best left to specialist taxonomists. However there are three genera of egg parasitoids commonly found in heliothis eggs that can be readily identified using the following key.
1. Small yellow or brown wasp with red eyes (go to 2).
Small black wasp with black eyes then Telenomus
2. Forewing with long fringe hairs.
Radial vein absent then Trichogrammatoidea
3 Forewing with short fringe hairs.
Radial vein present then Trichogramma
Suppliers of Trichogramma
Bugs for Bugs (Integrated Pest Management Pty. Ltd.). 28 Orton Street, Mundubbera, Qld. 4626.Telephone: (07) 4165 4663 Facsimile:(07) 4165 4626
Acknowledgement
The Cotton Research and Development Corporation (CRDC) provided funding toward the cost of the research reported.
Further information
- The good bug book. Complied by the Australasian Biological Control Group. 1995.
- The cotton pest and beneficial guide. By B.A. Pyke and E.H. Brown. 1996.
- Sweet corn insect pests and their natural enemies: An IPM field guide. R. Llewellyn. 2000
For more information, please contact: Brad Scholz, Entomologist, DPI's Agency for Food and Fibre Sciences, 203 Tor Street, PO Box 102, TOOWOOMBA Q 4350, Ph: +61 7 4688 1200, Fax: +61 7 4688 1199,
Email: brad.scholz@dpi.qld.gov.au.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought.The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: FS0491 . March 2001 . Reviewed: August 2003
