Lucerne leafroller moth
Lucerne leafroller
The parent moth of the lucerne leafroller (Merophyas divulsana) has a wingspan of about 15 mm. The folded forewings, which give the moth a bell-shaped appearance, are light yellow with irregular dark markings, while the hindwings are pale brown. The moths deposit small eggs in clusters (like overlapping scales) on the upper surfaces of leaves.
The larvae are greenish with dark heads. They web and roll the terminal growth of the lucerne shoots and then reduce the leaves to skeletons from within the roll. As the grubs grow, the size of the roll increases. After three or four weeks, the larvae pupate within the leaf rolls and the adult moths emerge one week later.
High numbers of lucerne leafrollers are frequently present during the warm months. They are particularly common in well-grown lucerne shortly before, and during, flowering. Severe infestations are mostly associated with dry weather when the growth rate of plants is reduced. Damage during severe infestation reduces plant vigour, while loss of leaf tissue reduces hay quality.
Lucerne leafroller — larva and pupal webbing and rolling of leaf
Despite the common occurrence of the lucerne leafroller, routine spraying is generally uneconomical. Instead, spray only when the infestation is severe. Populations can be monitored reliably by the presence of leaf rolls.
It is generally best to begin spraying in hay-producing areas when 30% or more of terminals are rolled. Spraying may have to begin earlier if the quality of the crop is measured by its appearance.
If the crop is approaching maturity, early cutting or grazing should be considered. Although not as efficient as sprays, this saves money, conserves beneficial insects and may reduce the need for future treatment.
Jassids
Jassids, or leafhoppers, are small green to yellow torpedo-shaped, sap-sucking insects that are about 5 mm long. When disturbed, the adults jump or move sideways over the edges of the leaves. Jassids insert eggs into the plant tissues. The nymphs are similar to the adults but have no wings. The cycle from egg to egg requires two to three weeks in summer, allowing a rapid build-up in numbers.
Lucerne leafhopper/jassid adult
There are two species of jassids. Each is different in its effects. The vegetable jassid (Austroasca viridigrisea) is bright green and larger than the yellowish-green lucerne jassid (Austroasca alfalfae). The vegetable jassid has a wider range of hosts and is normally present in the fields far earlier in spring — summer than the lucerne jassid.
The vegetable jassid sucks from the cells in the leaves. Each feeding puncture kills some of the cells and the damage soon shows as a small white spot. Continued feeding results in the typical jassid stipple pattern on the leaves.
The lucerne jassid feeds from the sap-conducting system of the plant and does not produce obvious stippling. Instead, it causes yellowing of leaves and, during severe infestations, burning of leaf tips. Lucerne jassids have a more severe effect on the plant than vegetable jassids, and can stunt crop growth.
Growers can usually ignore the presence of vegetable jassids as they cause little, or no, economic damage except when in extremely high numbers. Lucerne jassids should be controlled when high populations are found.
Treatment should begin when more than 200 lucerne jassids are caught in 10 sweeps through the crop with a 350 mm diameter insect net. Unfortunately there is no better on-farm assessment method. Again, early harvesting is an option in pest management.
Whitefringed weevil adult — leaf damage
White fringed weevil
White fringed weevil (Graphognathus leucoloma) is a serious insect pest in lucerne in many districts, especially in the Moreton Region. Adults cause only minor leaf damage but their larvae, which are creamy white in colour and roughly resemble maggots, feed on taproots and secondary roots of lucerne plants. These larvae severely damage the root system and the productive life of lucerne stands is usually greatly reduced.
There is no insecticide registered to control this pest in Queensland. The recommended control is to rotate from lucerne into cereals or pure grass pastures, which are not the pest’s preferred hosts.
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