Dig-line i.

How does it work?

Diglyphus isaea is a widespread parasitoid of leafminer larvae. It is reported from a large number of host species, but commercially is of interest as a parasite of Liriomyza bryoniae, L. trifolii and L. huidobrensis, as well as of the Chrysanthemum Leafminer Phytomyza syngenesiae.

The adult wasps are very variable in size, and are a dark metallic green in colour. They search for host larvae of suitable size within leaf tissue, and sting them through the leaf cuticle. The sting paralyses the host larva within a short time, and the female wasp then lays one or more eggs alongside it. The hatching Diglyphus larvae feed externally on the paralysed, but still living, host. It is described as an ectoparasitoid. Parasitised larvae can be recognised by the lack of movement within the mine: they will often move away from the end of the mine, where a healthy larva would be actively feeding. Microscopic examination and dissection of the mine will show them to be flaccid and unresponsive. The host larva will discolour as the parasite develops, and because it is no longer feeding, the mine stops growing.

This is in contrast to the alternative parasitoids Dacnusa sibirica and Opius pallipes. These are endoparasitoids that lay eggs within the host without immediately killing it, so that the larva continues to feed and grow, and leaf damage continues. With these species, host death occurs later, when the host has pupated and emerged from the leaf.

The larva of Diglyphus continues feeding on the paralysed host until everything is consumed. It then moves away from the host carcass before pupating. The larva uses the waste products it has accumulated in its hind-gut (known as meconium) to construct a series of pillars, which support the mine around it and prevent this collapsing onto the delicate pupa. These pillars can clearly be seen upon close examination of the mine and give one of the clearest indications of parasitism.

The pupa itself is a clear pale turquoise in colour and can easily be seen within the mine with a hand lens or binocular microscope. A large host has been recorded as supporting up to five Diglyphus larvae and multiple larvae are very common. The amount of food available to each is therefore very variable and this is reflected in the range of sizes which that can be found in adult Diglyphus. Adult females of the related Diglyphus begini are known to be able to control the sex of the eggs they lay: the female will lay male eggs on small host larvae and female eggs on large host larvae. Thus the size range of available host larvae can affect the size and sex ratio of the subsequent adult population. This is important not only in production, but also in timing release of Diglyphus into a crop.

Adult females of all Diglyphus species will feed on host larvae in order to obtain a source of protein. This allows them to manufacture more eggs, and is important into maintaining fecundity and longevity and maximisinge offspring production.the production of offspring. Small host larvae larvae, which are not capable of supporting even a single parasite larva, will normally be killed for host feeding by adults.

At 20°C female Diglyphus larvae develop from the egg stage laying to the point of pupation in 9 days. They spend a further 8 days as pupae before emerging. The adult wasp characteristically emerges through a ragged, circular hole, which it bites in the upper surface of the mine. This is a further feature thatwhich can be used to indicate the extent of parasitism.

At 15°C total development from egg to adult takes 26-27 days, whilst at 25°C this is shortened to 10-11 days. Liriomyza bryoniae develops in 41, 27 and 17 days at 15°C, 20°C and 25°C respectively. Development of the parasitoid is thus much briefer than that of the host at all temperatures.

When and where should you use it?

Diglyphus isaea can be used in any crop where Liriomyza spp. or Phytomyza spp. causecauses problems, and where some damage to the leaves can be tolerated. Reduction in pesticide inputs associated with Integrated Pest Management and Integrated Crop Management often allows naturally occurring Diglyphus to invade crops, and these can contribute significantly to leafminer control.

In Northern Europe, it is often stated that Diglyphus will not function adequately before May, and that therefore it is essential to release Dacnusa sibirica or Opius pallipes into crops between January and early May. This is purportedly explained as being due to a lack of activity in Diglyphus isaea at low light levels.

There are, however, examples of perfect control of leafminer achieved by release of Diglyphus isaea alone in January. Timing of the release is probably the critical factor. Release of parasites when there are insufficient hosts, or when hosts are too small, will prevent establishment, and any Diglyphus adults produced in the next generation will be small and may be mainly males. Timing release so that there are sufficient host larvae of good size will ensure the production of plentiful adult females in the next generation, and so will thus help to ensure establishment and good control. See below for more detail.

How should you use it?

Optimum leafminer control with Diglyphus isaea depends upon careful monitoring of adult leafminer populations early in the life of the crop.

In tomatoes, place yellow sticky traps about 30 cm above the heads of the plants from planting. Leafminer pupae thatwhich have survived in the soil from the previous crop will begin to emerge at this time, and a proportion will be caught on traps. Count the number of adults caught each week. The first adults thatwhich are trapped do not require any action to be taken: a few mines will be seen on leaves but these are not a problem. The numbers of adults caught will soon decline. About one month later, the numbers of trapped adults will begin to increase, and fresh feed marks will be seen on leaves. When this second surge of adults is seen, order two shipments of Diglyphus for delivery one week and two weeks later. The rate of release should be between 0.25/m² and 0.5/m².

Eggs laid in these feed marks will hatch after 5-6 days, and larvae will begin to feed. The first eggs will have been laid before the increase was noted, and these will then be second and third instar larvae ideal for parasitism at the time the Diglyphus adults are released. A substantial number of these will be parasitised, and this ensures that a population of Diglyphus isaea is established in the crop.

Continue monitoring the numbers of adults caught on traps. It is probable that numbers will initially decline, but a second surge will occur as those larvae not parasitised by the initial Diglyphus releases emerge from the pupae. As this second surge occurs, make a further two releases of Diglyphus. Monitor parasitism following these releases. Continue to monitor numbers of trapped adults, and make a further two releases of Diglyphus if a third surge of adult leafminers occurs. Parasitism should be close to 90% by this time.

When should you not use it?

Do not use Diglyphus isaea as the sole form of control where large numbers of leafminer adults are present as the result of invasions, or where there are existing large populations of larvae in the leaves.

Many species of insects, including some Lepidoptera, also cause leaf mines, and some of these may not be suitable hosts. If in any doubt, check the species before using Diglyphus isaea.

What will it do?

Diglyphus isaea will control populations of leafminer below economic damage thresholds on tomatoes, cucumbers and peppers when used as directed. It will also control leafminers below damage threshold levels on ornamental plants where damage to the leaves does not affect the aesthetic value of the crop.

At high release rates, Diglyphus isaea can also reduce leafminer populations to non- damaging levels over several generations, but the cost of this treatment will be high.

What will it not do, and what are the control options?

Diglyphus isaea will not totally prevent development of mines on leaves. A high proportion of those thatwhich start to develop will be stopped while small, but the leaf damage will remain.

Diglyphus isaea will not protect from damage where large adult populations of leafminer exist before or immediately after release, nor will it immediately reduce the numbers of mines. Parasitised mines remain on the plant until the affected leaf is removed, so where visible damage to the leaf affects the value of the crop, Diglyphus isaea may not be the best option. The alternative parasitoids Dacnusa sibirica and Opius pallipes may be used instead of or in combination with Diglyphus isaea, but the comments on aesthetic damage above apply equally to these species.

The insecticides abamectin and cyromazine can rapidly reduce numbers of active leafminer larvae, and may be included as part of an overall control strategy. This will depend upon the status of registration of these compounds in the crop and country concerned.

Chemical compatibility

Cyromazine may be used without causing problems for Diglyphus, although the reduction in hosts will have an indirect effect on the population.

Abamectin is best used as an early or late season cleanup to reduce large existing populations.