IPM - Microplitis demolitor and ascovirus: Important natural enemies of Helicoverpa

Helicoverpa (often called heliothis) is a serious pest of southern Queensland crops, particularly grain legumes, summer grains and cotton. There are two pest species of Helicoverpa in Australia that this publication refers to as 'helicoverpa': the native budworm, Helicoverpa punctigera ; and the cotton bollworm or corn earworm, H. armigera. The wasp Microplitis parasitises the caterpillars of both H. armigera and H. punctigera.

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What is Microplitis?

Microplitis demolitor is a small native wasp. It is one of the key beneficial insect species that attack helicoverpa caterpillars in Australia. Female wasps also play an important role in spreading ascovirus, a lethal disease of helicoverpa caterpillars.

Microplitis is a parasitoid

A parasitoid is an insect that kills its host to complete its life cycle. Parasitoids are usually described according to the life stage of the host that they attack. Microplitis wasps are called larval parasitoids because they lay their eggs in (parasitise) small helicoverpa caterpillars.

Microplitis cocoon attached to a dying helicoverpa caterpillar on a sorghum leaf. Finding these distinctive fawn cocoons next to dead or dying caterpillars is a key indicator that Microplitis wasps are active in a crop.

What is ascovirus and what is its link with Microplitis ?

Ascovirus lethally infects a range of caterpillar species. The ascovirus that infects helicoverpa is transmitted from caterpillar to caterpillar by female Microplitis, Heteropelma and Netelia parasitoid wasps. When a female wasp lays an egg (stings) inside an ascovirus-infected caterpillar she picks up the disease on her ovipositor (stinger). When she stings subsequent caterpillars, her contaminated ovipositor infects those caterpillars. The ascovirus prevents the wasp's offspring developing inside the caterpillar host. However, as the virus on the wasp's ovipositor wears off and decreases over time, the wasp can then produce viable offspring in uncontaminated caterpillars.

How important are Microplitis and ascovirus to Helicoverpa control?

A female wasp can parasitise approximately 70 helicoverpa caterpillars in her lifetime. Together Microplitis and ascovirus can have a significant impact on helicoverpa populations, especially when these populations are low and/or close to the economic threshold. The percentage of helicoverpa caterpillars killed by Microplitis parasitism and resulting ascovirus infection can exceed 75%, though 30-50% is more typical.

How is ascovirus different from the other Helicoverpa virus - NPV?

Helicoverpa NPV only kills Helicoverpa species. Ascovirus, on the other hand, infects other moth caterpillars in the Noctuid family (the same family to which Helicoverpa belongs). Some of these noctuids killed by ascovirus include pests such as armyworms (Spodopteraspp.).

Ascoviruses are not related to the commercially available NPVs (Vivus Gold^®, Gemstar^®). They belong to a separate virus family. A caterpillar infected with NPV will turn black, liquidise and splatter. A caterpillar infected with ascovirus is much harder to detect as being infected.

Interpreting mortality data

Some of the helicoverpa mortality caused by Microplitis is as a direct result of parasitism. However, direct parasitism is not the only way that Microplitis impacts on helicoverpa populations. As discussed, Microplitis is also the main carrier (vector) of an ascovirus disease that kills helicoverpa caterpillars. Understanding this important link between Microplitis and ascovirus is essential for interpreting the overall impact of Microplitis on helicoverpa in a crop.

The graph shows the overall impact of Microplitis against helicoverpa caterpillars in three unsprayed cotton fields at Warra, in southern Queensland, during the 1996-97 season. This graph shows that Microplitis was directly killing only up to 32% of the helicoverpa caterpillars collected. However, taking into account the caterpillars infected with ascovirus and Microplitis is shown as responsible for killing up to 75% of the second and third instar helicoverpa caterpillars in these unsprayed cotton crops-a significant contribution to the control of helicoverpa.

Percentage of helicoverpa larvae killed by Microplitis and ascovirus. Estimate the overall impact of parasitism and disease by adding the mortality caused by each. Data collected from unsprayed cotton at Warra, South-east Queensland during the 1996-97 season.

Microplitis lifecycle

Understanding the lifecycle of the Microplitis wasp is the key to understanding its role in helicoverpa control. From egg to adult, the Microplitis lifecycle takes about 12 days. This is made up of seven days from egg laying to forming a pupa (pupation), and then five days for pupal development.

Identifying Microplitis in the crop

Detecting and recognising the signs of Microplitis activity in a crop is important so that they can be monitored, and the impact of their parasitism factored into decision making.

Microplitis adults are small (3 mm) black-brown wasps, they are often seen flying slowly above the crop canopy in search of caterpillars (hosts). Microplitis wasps are active throughout the day, from shortly after sunrise until just after sunset. If Microplitis is active in a crop, sweep netting will often capture wasps that are in the plants.

 
Using a sweep net to monitor Microplitis activity in a crop.

What caterpillar stage does Microplitis attack?

Wasps have a preference for second instar helicoverpa caterpillars (4-7 mm). This is important for pest management because, in the absence of Microplitis or other natural enemies, most helicoverpa caterpillars that reach this age have a good survival rate and go on to cause crop damage. Third and fourth instar caterpillars are also suitable as Microplitis hosts, but are parasitised less frequently because they vigorously defend themselves, sometimes injuring or killing the wasp. Occasionally, twin parasitoids emerge from a single host and rarely (in about 1% of cases) the host caterpillar survives and pupates normally after the wasp has emerged.

Identifying parasitised caterpillars

In the field, you can identify parasitised helicoverpa caterpillars by performing a simple split test. Parasitised caterpillars will only grow to about 15 mm, so caterpillars smaller than this are potentially Microplitis hosts. To identify whether a caterpillar is parasitised, hold it across a forefinger with one thumb on the rear end of the caterpillar, and with the other thumb on the head. Gently stretch the caterpillar until the skin ruptures. A Microplitis larva developing within the caterpillar looks like a white maggot up to 4 mm long (more mature Microplitis larvae are browner in colour).

  
Split helicoverpa caterpillar showing Microplitis larva.

Clues to Microplitis activity levels

To determine whether Microplitis is active and present in significant numbers:

• look for adult waspsflying above the crop or use a sweep net to check in the crop
• collect caterpillars shorter that 15 mm and perform the split test on a number to estimate percentage parasitism
• look for visible signs of parasitoid activity, that is, the distinctive fawn cocoons (often next to a dead or dying caterpillar) and/or small pale caterpillars that might be infected with ascovirus (see Ascovirus symptoms)
• crop scouting data can also be an important trigger to check for Microplitis. Remember that Microplitis parasitises second instar caterpillars (4-7 mm). Therefore Microplitis activity should be suspected in situations when previous counts have shown the presence of eggs and very small caterpillars, but these do not seem to be developing through to third instar caterpillars and larger (that is, longer than 15 mm). Although this situation might also be due to other natural enemies, combined with the above observations, it is a good indication that Microplitis is exerting some control on the helicoverpa population.

Parasitised helicoverpa caterpillars cause much less crop damage before they die compared to unparasitised caterpillars. Experiments have shown that a caterpillar parasitised by Microplitis consumes about 90% less than a healthy caterpillar. This demonstrates the importance of parasitism on helicoverpa feeding behaviour. Because they do so little damage, do not include parasitised caterpillars in counts during crop scouting.

Ascovirus: Good for Helicoverpa management, not so good for Microplitis

Microplitis is the main vector of ascovirus and so plays a key role in increasing the percentage of caterpillars affected by this disease. However, high ascovirus levels can eventually cause Microplitis numbers to decline. How does this happen?

As ascovirus becomes more widespread in a helicoverpa population, female wasps become infected with ascovirus and pass the infection on to caterpillars that it parasitises. Unfortunately for Microplitis, when a parasitised caterpillar is killed by ascovirus, so is the Microplitis larva developing inside. As a result, high levels of ascovirus may lead to a decline in Microplitis wasp populations.

Ascovirus lifecycle

Ascovirus establishes in the helicoverpa population through spring-summer. The disease is transmitted from caterpillar to caterpillar by wasps (such as Microplitis). Ascovirus could be transmitted directly from one caterpillar to another by spitting - for example, when caterpillars encounter each other on the plant. Laboratory studies have also shown that ascovirus can be transmitted by cannibalism.

The way in which ascovirus survives winter is not clear. It could persist in low numbers of helicoverpa, or use alternative hosts such as Spodoptera larvae (e.g. cluster caterpillars and armyworms).

When ascovirus particles enter the caterpillar's body they multiply in tissue cells, eventually infecting the haemolymph (blood). This causes the haemolymph to change from clear to milky. The caterpillar stops eating, but may not die for several days or weeks, surviving in a lethargic state.

Some ascovirus-infected caterpillars stay in exposed situations and look as though they have been grazing lightly in the one place. Small 'windows' in the leaves next to infected caterpillars are a typical symptom.

Ascovirus symptoms

In cases where a Microplitis wasp has both parasitised a caterpillar and infected it with ascovirus, the symptoms seen are those of the disease rather than of the parasitoid. When ascovirus kills the caterpillar, it also kills the developing Microplitis larva. Caterpillars infected with ascovirus will generally stop eating within two days. They stop growing, but can live for weeks in a lethargic state before they die. Be aware that although ascovirus-infected caterpillars are smaller than similarly aged non-infected caterpillars, they may look otherwise healthy.

The two caterpillars in the photo are the same age. The caterpillar infected with ascovirus (on right) will stop eating and growing, and may be paler in colour, but can appear otherwise healthy.

Some other signs to look for:

• the blood of an ascovirus-infected caterpillar is white and creamy, whereas the blood of a healthy caterpillar is clear. Blood colour gives the best diagnosis in the laboratory and can be tested by splitting or pricking the caterpillar.
• small pale caterpillars in unusually exposed situations (e.g. on a leaf) that look as though they have been feeding in the same place for some time, making a window in the leaf, are probably infected with ascovirus
• because infected caterpillars can take a long time to die, ascovirus infection in the population can show up in crop scouting data over a series of checks as a number of small caterpillars (shorter than 8 mm) in the crop, with very few medium-large caterpillars coming through.

The white and creamy blood of an ascovirus-infected Spodoptera litura caterpillar (left) and the clear blood of an uninfected caterpillar (right). The symptoms are the same in helicoverpa caterpillars (Photo: I. Newton).

Can Microplitis be effective against large populations of Helicoverpa ?

As a general rule, Microplitis has greater impact on near-threshold helicoverpa populations than against populations in excess of the economic spray threshold for that crop. If we consider the example of a crop where the economic threshold is 2 caterpillars/m², 50% parasitism by Microplitis will reduce a population of 3 caterpillars/m²; to below threshold, but will not be effective on its own against a population of 10 caterpillars/m².

When is Microplitis most effective?

The overall effectiveness of Microplitis is related to the abundance of the wasp in the farming system. Microplitis is active in all crops attacked by helicoverpa, except chickpea because the plant's acid secretions deter predators and parasitoids. Microplitis numbers generally start off low in the spring, before building up and eventually peaking by late summer. In early spring, even low numbers of Microplitis can parasitise a significant proportion of the helicoverpa population because the helicoverpa numbers are also low (e.g. in spring mungbeans, sunflowers and sorghum).

Microplitis populations will increase during the season provided there are hosts to breed up on, and are not disrupted by insecticides.

Minimising insecticide disruption of Microplitis

Broad-spectrum insecticides will kill Microplitis. The cotton industry's IPM guidelines contain a table (page12 in pdf file) Beneficial insects - use them don't abuse them ( PDF 528 kB) with information on the impact of different insecticides on wasps (including Microplitis).

Spray timing

Even a biological insecticide, like NPV and Bt, can kill Microplitis larvae if the host caterpillar is killed before the parasitoid can complete its development. The cocoon (pupal stage) of Microplitis is less susceptible to insecticides than the adult and larval stages. It is not practical to try and time sprays to preserve Microplitis in crops where there is a range of life-stages present.

In sorghum, it is possible to time NPV sprays to conserve Microplitis because helicoverpa egg lay occurs over a relatively short period around flowering (10 days). To conserve Microplitis, NPV sprays should be applied at three days after 50% of heads in the field have completed flowering (that is, brown anthers to the base of the head). This timing allows most of the Microplitis larvae time to emerge before their caterpillar hosts are killed by the NPV infection.

Further information

• To obtain copies of DPI&F publications, contact the DPI&F Business Information Centre, open from 8 a.m. to 6 p.m. Monday to Friday telephone 13 25 23 for the cost of a local call wihtin Queensland, interstate callers 07 3404 6999 or email callweb@dpi.qld.gov.au  .
• DPI&F Summer and Winter Crop Management Notes on CD-ROM available from the Business Information Centre.
• Infopest - a DPI&F national database on CDROM, containing up-to-date information on all registered agricultural and veterinary chemicals, available from the Business Information Centre.
• Crop Insects: The Ute Guide - Northern Grain Belt Edition . 2000. ISSN 0727-6273. This publication can be purchased through the DPI&F Information Centre Toowoomba, telephone 07 4688 1415 or email informU@dpi.qld.gov.au  .
• Egg parasitoids of heliothis. 2000. QI00097.
• IPM - Understanding helicoverpa ecology and biology in southern Queensland : Know the enemy to manage it better.
• IPM - Using NPV to manage helicoverpa in field crops
• IPM - Parasitoids: Natural enemies of helicoverpa
• Heliothis Stateline. ISSN 1441-4244.
• The cotton pest and beneficial guide, Cotton Research and Development Corporation . ISBN 0 7242 6633 X.
• Cotton pest management guide (annual publication). NSW Department of Primary Industries, Australian Cotton CRC. ISSN 1442-8792.
• ENTOpak - A compendium of information on insects in cotton. Available from the Technical Resource Centre at the Australian Cotton Research Institute, Narrabri. Telephone: (02) 67991534.
• Information on up-to-date pesticide registrations is on the Australian Pesticides and Veterinary Medicines Authority (APVMA) website www.apvma.gov.au.
• The Australian Cotton CRC website cotton.crc.org.au has a variety of information related to integrated pest management including:
  • Impact of insecticides and miticides on predators in cotton.
  • Current insecticide resistance management strategy.
  • Integrated pest management guidelines for cotton production systems in Australia.

The DPI&F Entomology team is a leader in the science of managing insect pests and their natural enemies in broadacre farming systems. This work has been supported by the Australian Government Cotton Research and Development Corporation, GRDC, CRC and the Department of Primary Industries and Fisheries, Queensland. 

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