EXCELLENCE IN COCKROACH BAITING – PART 1: DESIGNING AN ATTRACTIVE BAIT MATRIX

Part 1 in our Active Insight series looking at how to get the best results with cockroach baiting.

Over recent years, cockroach baiting has become the primary method of control in urban cockroach management programs. Cockroach baiting offers a precise, targeted solution that employs significantly lower levels of toxicant, in keeping with the principles of Integrated (urban) Pest Management (IPM). It is also perceived as offering a lower risk approach to pest management.

Whilst many think of cockroach baiting as a relatively recent strategy, following the introduction of the first gel bait in Australia in 1997, baiting of cockroaches in fact dates back more than a century. The first commercial bait was sold in 1896 in the UK and USA, and consisted of phosphorus added to a sweetened flour paste. Prior to that, DIY baits consisting of one part plaster of Paris and 3-4 parts of flour were widely used.

Since then, a range of active constituents have been employed in cockroach baits in Australia, including boric acid, chlorpyrifos, hydramethylnon, abamectin, imidacloprid, fipronil and indoxacarb. A range of sophisticated bait formulations are also now available in the Australian market, including the now ubiquitous gel baits, a liquid micro- encapsulated bait, granular baits, and a dry flowable magnetic powder bait.

Compared to residual spray formulations, cockroach baits are generally less toxic and leave fewer residues, since they are applied in low doses directly to cockroach harbourages or areas of known cockroach activity. Consequently, they tend to be favoured in sensitive environments such as zoos, child-care facilities, electrical/computer areas, and food premises.

A further reported benefit is that baiting for cockroaches does not harm beneficials in the environment, thus enabling the conservation of oothecal parasitoids of cockroaches, such as nematodes and fungi (Suiter, 1997).¹ Though it is questionable whether this would present a sound marketing basis for baiting programs!

For a bait to be attractive when placed in urban areas with a number of competing food sources, it must contain nutrients that are both limited and unique in the environment. The nutritional value of foods has a very significant impact on cockroach development and reproduction. Given the relatively simplistic nerve structure of cockroaches, with the brain a supra-oesophageal nerve ganglion (cluster of nerves) formed by the fusion of three pairs of ganglia; it is surprising to learn that cockroaches are actually able to ‘self-select’ the nutrients they require to correct nutrient deficiencies that arise from feeding on unbalanced food sources.

Carbohydrates are mostly consumed during the first week after hatching, i.e. during the first stadium (instar), with lesser amounts required in each subsequent stadium. In contrast, protein is required during all life- stages, though in a lower proportion. When cockroaches were reared in an environment where they were able to self-select the nutrients required, they grew optimally by selecting the requisite nutrients for each nymphal development stage.

It is also reported that when nutrient deficiencies exist in a food source, German cockroaches (Blattella germanica) prefer to self-select an unbalanced food, provided it will compensate for the nutrients they were lacking, instead of a more balanced food (Raubenheimer and Jones, 2006).²

It is believed that cockroaches can in fact distinguish food of different nutritional value through a process known as specific nutrient learning. They learn which nutrients their body requires most and then seek these out. Associative learning has also been reported in the American cockroach (Periplaneta americana), where the insects were able to associate the smell of a food with the proteins present and required.

The consequences of this are most important in the development of a cockroach bait, since it becomes vital to have a food source within the bait that provides a complete and balanced diet containing all the nutrients required by the cockroaches in order to optimise the success of the baiting program.

Typically, three macronutrients are included in the matrices of a cockroach bait: carbohydrates, lipids and proteins, although it is essential to include further ingredients to provide a complete dietary food source. (Sadly many baits are based on sugars only which leads to less than optimal results in the field, as will be discussed later.)

Attractant and feeding stimulants are also important to draw the cockroaches to the bait and then to ensure they consume larger amounts of the bait and achieve increased uptake of the toxicant. Other additives are added to prolong the lifespan and retain moisture content in the baits.

In addition to considering the attractiveness of the bait and its nutritional value to the cockroaches, to effectively compete with other food sources, one needs to address the issue of feeding stimulation.

A food item can act as both an attractant and a feeding stimulant, but this is rarely the case. A substance that is attractive may not necessarily stimulate feeding. Similarly, an item that is a feeding stimulant may not attract cockroaches to a bait. Attractants function by drawing cockroaches towards the bait, and laboratory studies have shown that a high quality bait formulation can even draw cockroaches from upwind of the bait, away from corners and edges of walls, to a bait placed 25-30 cm away.

In contrast, feeding stimulants are required to induce and promote increased consumption of baits. Understandably the amount of the bait eaten at a single feed is very important in a control program, particularly when we consider secondary kill effects. This determines if sufficient toxicant is consumed to both kill the cockroach and have toxicant available for secondary transfer. This is because cockroaches that consume larger amounts of bait defecate and regurgitate more substances containing the toxicant.

Studies have shown some baits are more attractive to the cockroaches, and the cockroaches are drawn to them by preference. However, the cockroaches eat significantly less since they feed on these baits for a shorter time period. This is why the targeted addition of feeding stimulants is important.

Feeding stimulants are often species specific, though mixtures of these substances frequently show synergistic effects. This means that, by carefully combining different stimulants, we can greatly improve the performance of a cockroach bait.

The use of carefully designed bait stations can also improve bait performance. German cockroaches are often attracted to novel objects introduced into their environment. When the complexity of the introduced object is increased, for example by using a station with several entrances, the cockroaches spend more time exploring the station, and are most attracted to it when a food source is added.

In part two, we will look at cockroach foraging behaviour and how it impacts cockroach baiting programs.