Insect Pests

Moisture plus warm conditions in autumn and early winter 2014, provided ideal conditions for insect pests. In some areas, high insect numbers coincided with emerging early sown crops, leading to significant crop damage. Some of these issues are reoccurring and predictable, while some were less predictable and likely to be one-off events.

Following are some of the insects which caused problems during 2014, along with brief notes on their habits and management.

Lucerne Flea

Lucerne Flea hatch from a summer diapause following periods of good soaking early rain. The trigger for hatching is 3 days of good moisture, followed by 11 days with average daily temperatures below 22oC. Lucerne Flea are more common on heavier soil types, as the females create a clay secretion which is used to protect over summering eggs.

Chemical control of Lucerne Flea is relatively effective using organophosphate insecticides such as Dimethoate and Omethoate. Given Lucerne Flea are often found in defined areas (relating to soil type), targeted spraying is preferred over blanket applications to preserve the beneficial insect population.

Preventative management is difficult. Lucerne Flea reproduce sexually and depending on moisture and temperature conditions, may go through three to five generations between autumn and spring. Each generation takes between three to five weeks, with females laying up to three batches of eggs during this time.

In many cases, spraying for Lucerne Flea is unnecessary. It is only when damage threatens the survival of emerging crops and pastures, or where stock feed is particularly short, that controlling Lucerne Flea is likely to be economic.

Red Legged Earth Mite

Over summering Red Legged Earth Mite (RLEM), hatch in autumn following exposure to adequate rainfall and approximately two weeks of cool temperatures. During winter, eggs are laid on the underside of leaves and hatch within 8 to 10 days. During spring, environmental conditions trigger females to produce over summering or diapause eggs. These are retained within the dead female body and are able to withstand the heat and desiccation of summer.

RLEM are relatively easy to control with insecticides. Preventative control is quite effective. This is achieved by a strategically timed insecticide application, aimed at killing females just prior to the production of over summering eggs. Timerite spraying uses climatic data to predict when RLEM will be producing over summering eggs. Timerite dates can be found by going to www.timerite.wool.com and entering a latitude and longitude location. In southern NSW, the Timerite date is around late September, early October.

Red Legged Earth Mite have a number of natural predators. Utilising Timerite spraying and avoiding unnecessary insecticide applications, will help to preserve predator numbers.

Bryobia Mite

Bryobia Mite are similar to Red Legged Earth Mite in the damage that they cause to crops. Where they differ is in their life cycle. Bryobia Mite have both over summering and over wintering eggs. They tend to be active in the warmer conditions of autumn and spring, plus all life stages can be present if there is sufficient green plant material available.

Bryobia Mite are more tolerant of insecticides. Control is best achieved by removing all green material well before sowing and by including an insecticide with this herbicide application. Higher rates of Omethoate are most effective for control.

Earwigs

There are two species of earwigs, the European Earwig a pest that feeds on developing crop seedlings and the Common Brown Earwig, or Native Earwig which feeds on caterpillars, Lucerne Flea and other insects. The Common Brown Earwig can be distinguished by an orange triangle behind its head.

Crops commonly affected by the European Earwig include canola and legume seedlings, including lucerne and clover.

The European Earwig appears to be spreading across southern agricultural areas, primarily in response to stubble retention systems which provide an ideal habitat.

Damage is characterised by chewing of cotyledons and true leaves. Often bare patches can be found in paddocks and closer inspection will reveal only the stumps of seedlings which have been eaten.

Control is ineffective with applied insecticides, as earwigs are nocturnal and live in nests below the surface or in clumps of organic matter. Best results are achieved through application of cracked grain baits treated with a mixture of Chlorpyriphos and vegetable or canola oil.

Prevention involves minimising the amount of residual dry matter on the surface, as well as cultivation to destroy their habitat. Application of Chlorpyriphos incorporated by sowing may provide preventative control, however this is not completely effective.

Weevil

The two main species of weevil that damage broad acre crops are Mandelotus Weevil and the Spotted Vegetable Weevil. Weevils will cause damage in emerging canola, lucerne and clover. Weevils feed on the foliage of the cotyledons and true leaves, hence damage presents as chewed or missing leaves. In severe cases bare areas are obvious and closer inspection will reveal only plant stems remaining, where all of the foliage has been removed.

Weevil damage can appear very quickly, with large areas of crop being eaten rapidly.

Like earwigs, weevils are more common where stubble retention and minimum tillage provide an ideal habitat. Until recently, the life cycle of weevils was poorly understood. The results of recent research suggests that weevils go through one generation per year. Adults begin emerging from the soil during early autumn, the adults lay eggs and larvae develop during winter / spring, then pupate during late spring early summer. The adult weevils over-summer at between 12 and 40 cm depth in the soil profile.

Based on the life cycle, it is likely that removal of organic matter and cultivation will be detrimental to their survival. There are limited products registered for weevil control, however in the past higher rates of Alpha-Cypermethrin have provided satisfactory results.

Aphids

There are a number of different aphid species, most of which are specific to the crops that they attack. The species that are most commonly an issue in southern NSW broad acre crops are; the Oat or Cereal Aphid which are found in cereal crops and the Cabbage Aphid, Turnip Aphid and Green Peach Aphid which are found in canola crops.

Aphids are sap sucking pests. Heavy infestations early in the season can cause yellowing and stunting of plants, with the sugary solution (honeydew) which they secrete also reducing plant growth. Late season infestations, particularly in canola are common and present as clusters of dense colonies on the flowering spikes. Such infestations are common in warm dry seasons and often coincide with drought stressed crops. At high levels, these infestations can lead to yield loss.

Aphids are also responsible for the spread of Barley Yellow Dwarf Virus in cereals and Beet Western Yellows Virus in canola. These are discussed in more detail in a separate article of this newsletter.

Preventative management of aphids revolves around removal of host plants or a green bridge. For high risk crops such as early sown oats, Wylah and Wedgetail wheat, or canola, application of an insecticide seed treatment may be effective.

Cutworm

Cutworm, which are the larvae of the Bogong Moth caused significant damage to emerging canola crops during 2014. The high numbers of Cutworm resulted from early rainfall and favourable conditions. Bogong Moths can lay eggs from early summer, but generally wait until favourable conditions during autumn. In 2014, the early break led to an increased number of moths laying eggs. In addition, the availability of food, moisture and mild conditions, favoured above average survival of larvae.

Based on this knowledge of the life cycle, it is reasonable to assume that the crop damage resulting from Cutworm during early 2014 was a one-off event. Therefore preventative strategies are unlikely to be required in the future. However, it does serve as a reminder of the sensitivity of germinating crops to insect attack and reinforce the need to monitor emerging crops on a regular basis.

Slugs

The two species of slugs that are responsible for crop damage are the Black Keeled Slug and the Grey Field Slug. Retained stubble (or other sources of organic matter) and soil moisture favour the existence of slugs, however unlike some other pests, slugs are often a reoccurring problem in a particular area.

Damage from slugs is characterised by chewing of leaves or complete loss of plants. Unlike weevil or earwig damage, it is not generally possible to find the stumps of chewed plants when slugs are the culprit. Damage is usually noticed when significant bare patches appear in paddocks.

Prevention or treatment of slugs involves spreading baits at or soon after sowing. Effective use of baits requires a bait that contains at least 3% Metaldehyde, with a uniform particle size to facilitate even spreading. Trials have shown that sowing baits in the crop row does not provide effective control.

Removal of stubble and organic matter through burning and cultivation has also been shown to reduce slug number through destruction of habitat. In addition, the removal of stubble makes identification of damaged areas easier, as the emerging seedlings are more visible when emerging on bare ground.